Adjustable Speed Drives and Short Circuit Currents
Do all Adjustable Speed Drives contributed current to a line short circuit?
Howard G. Murphy P.E.
Per IEEE standard 141-1993 (Redbook), section 4.2.5, 'adjustable speed
drives can contribute current from the motors to a short circuit'.
This statement has led to confusion regarding adjustable speed drives and short
circuit currents. The statement is true for some adjustable speed drives such
as dc drives and Current Source Inverter (CSI) drives, but incorrect for PWM
adjustable frequency drives.
Examples of the questions that have been asked are:
Do PWM drives contribute backfeed current during a short circuit on the line
side of the drive? If so, what %FLA
of the motor load do they allow?
What are the drives' short circuit let through values? Are these maximum
permissable short circuit values stamped next to the drive?
Is the maximum rated short circuit number, stamped inside the drive, a value
for the interupting rating of the drive?
These are valid questions based on the statement made in IEEE standard
141-1993. However, understanding how the PWM type adjustable frequency drive
differs from the older technology used in dc drives and CSI drives will quickly
eliminate any concerns regarding the PWM drive as a source of current during a
short circuit condition of the incoming line.
In controlling ac motors, the PWM type ac drive buffers the ac line from the
characteristics of the motor. In transferring energy from the ac line to
themotor, the standard PWM drive, including vector controlled PWM drives,
allows power to flow in only one direction. Power is transferred to the motor,
but not from the motor to the ac line. If the drive is a line regenerative type
PWM drive (not normally used in standard motor control), then current can flow
from the motor to the ac line.
During operation of an ac motor, energy is stored in the motor. When a short
circuit condition exists on the ac line, the motor will temporarily act like a
generator and try to transfer energy back to the source. Without a PWM drive,
the energy can be transferred back to the ac line. However, with a PWM drive
between the motor and the ac line, the energy from the motor is circulated in
the output section of the PWM drive. The energy stored in the motor is
converted to a dc source which is stored in the internal filter capacitor of
the PWM drive. When too much energy circulates in the output section of the PWM
drive, an internal overvoltage condition can be created. Without any means to
dissipate the energy, the PWM drive senses the overvoltage condition and stops
operation. The diagnostic function within the PWM drive indicates the
overvoltage condition as an Overvoltage Fault and typically displays that
information to guide the user in understanding why the drive shut down. In a
line regenerative PWM drive, the energy stored as a dc source is transferred
back to the ac line in a contolled manner, limiting an excessive current
With standard PWM drives, no energy is transferred back to the ac line,since
the overvoltage condition forces the input rectifier section of the drive into
an off or non conducting state. The converter or rectifier section of a PWM
drive only allows power to flow towards the motor. With dc drives and Current
Source drives, the converter section allows energy to flow from the motor back
to the ac line. Essentially only a voltage source type drive such as the PWM
drive ensures that energy flow is from the ac line to the motor and not from
the motor to the ac line. In a regenerative PWM drives, the converter section
is modified to allow controlled current to be transferred back to the ac line.
Under a short circuit condition on the ac line, the amount of regenerated
current is limited to the rating of the drive.
When a short circuit condition occurs on the output of the drive, the short
circuit let through current is limited by the electronic current limiting
function of the drive. Typically, the maximum allowable short circuit current
is about 200% of the rating of the drive. The electronic current limit function
of the PWM drive interrupts excessive currents in microseconds. The rms heating
caused by the current into a drive is slightly more than the continuous rating
of the drive. Typically, an rms value of 150% or the rating of the drive would
be experienced. This value is far less than would typically be experienced if
the motor were operating directly across the line.
In practice, the impact of standard PWM drives during a line short circuit
condition can be ignored. For dc drives, Current Source Drives andline
regenerative PWM drives, the maximum contributing current should be only
slightly more than the overload rating of the drive. DC drives and Current
Source Drives contain current limit functions which will interrupt excessive
currents in milliseconds. In all cases, motors operated on adjustable speed
drives, will contribute less than would be experienced with the motor connected
directly on line. With the standard PWM drive, there is no contribution to
short circuit current when a short circuit condition exists on the incoming ac
It is important the remember that standards exists as guidelines to aid in the
design and operation of electrical systems. These standards often describe
conditions in general terms resulting in a conservative presentation of
technical information. Each installation is unique. The type of equipment
changes rapidly and its use within any electrical system will depend upon that
installation and the technology used within the equipment. Reviewing the
installation with the equipment supplier can result in fewer problems and lower
installation and maintanance costs.
Applying Variable Speed Drives
Howard G. Murphy P.E.
December 10, 1998
During the past 40 years, the variable speed drive market has changed
drastically. Mechanical and electrical speed drives, ie fixed speed motors with
adjustable pulleys and gear changers, eddy current clutches, etc. These, in
turn were replaced by electronic or solid state drives. From 1950 to 1970, the
success of the dc motordrives as the only dependable electronic variable speed
drive method created the desire for improved motor control and the start of an
ever growing variable speed motor market.
During the 1970’s, the ac drive, which controlled a standard 3 phase, ac
motor began to replace the dc drive as the only electronic method for obtaining
variable speed. In less than 10 years, the PWM (Pulse Width Modulated) ac drive
became the most common and widely use method for controlling the speed of an 3
phase ac motor. With the interest in reducing energy consumption and improving
the efficiency of motor operated equipment and processes, the demand and
utilization of the ac drives resulted in more than 30% of the fixed speed motor
market being converted with the application of variable speed ac motor drives.
Significant improvements resulted from these changes, including energy saving,
increased efficiency, and improved process control. Global awareness of these
improvement, has increased interest so that it is estimated that by the year
2008, more than 70% of the today’s fixed speed market will convert to
variable speed control using ac drives.
These changes have not been without problems. As the use of variable speed
drive increases, the knowledge of how to apply these drives will determine how
the success of these applications. There still exists much confusion about ac
drives and their impact on motors, distribution systems and how best to operate
these drives for a specific application. There are a number of different terms
used to describe the ac drive.AFD, VSD, VFD and Inverters all are used but have
the same meaning. The main purpose for the all ac drives is to control the
operation of the ac motor with regard to speed and torque. Much is still
mis-understood about the use and performance of the ac motor. Because of the
use of fixed speed ac motors, many types of motors exist. NEMA A, B, C, C and E
were designed to modify the starting characteristics of the motor. Synchronous
ac motors were design to improve the speed regulation of the ac motor. Most
applications have operated with fixed speed ac motors with limited problems.
Most problems were mechanical in nature and most solutions were limited to
replacing the motor with a larger motor or larger fuses when problems occurred.
Today, a lot of 'BUZZWORD' are used by suppliers in any attempt to
convince the user that their products are better suited to an application. For
the most part there are only subtle differences that exist between the
suppliers products. Most applications depend only on the characteristic of the
applied motor and the current rating of the ac drive. Accurate speed regulation
still depends on some type of feedback where the actual speed is detected.
Fortunately, most applications do not required precise speed control and as
such can use most of the ac drive product that are on the market. There are differences
in the quality of the product and differences in the ability of the supplier to
support and service their products, but in most cases, the user has many
choices for supplier and products which will result in thesuccessful use of an
ac drive and improvement for the process.
When it comes to applying ac drives, it is important that the user does not get
caught up in the 'MAGIC' image that some drive supplier attempt to
create for their products. When it comes to controlling motors, it is still
volts and amps that determine how the motor will perform. AC Drives can control
the applied voltage to the motor with a greater degree of accuracy that exists
when the motor is direct on line. Although ac drives are design with many
control parameters, ie programmable memory, there are very few parameters that
must be selected and programmed to operate the ac drive at it in its most
effective operating mode. The most important parameters are voltage. Since the
speed will vary from low to high speed, the voltages at each of these operating
point should be adjusted for optimum performance. At the highest or nameplate
rated speed, the voltage should be set to either the nameplate voltage value or
to a lower value if the motor is not fully loaded. The exact value for a
lightly loaded motor will depend on the motor and the characteristic of the
load. Most drive supplier can make a recommendation concerning this point. At
low speed, the voltage should be set to a value which allows the motor to
develop sufficient torque to overcome the load. Too little voltage will result
in not enough torque or too much current. Too much voltage will result in the
motor overheating if operation at this low speed continue for a period of time.
Fancy names for features of the ac drivelike vector control can not ensure
proper performance under all operating conditions. Most application can use the
ac drive 'right out of the box' and experience successful operation.
However, some applications may require the ac drive to be 'adjusted'
using the drive parameters to achieve successful operation.
There are five key points that should be considered when attempting to apply
variable speed drives. Since the most common variable speed drive today is the
ac drive, this paper will discuss those points as applied to ac motors and ac
The first point is optimizing electrical performance. Since performance depends
almost entirely on the characteristic of the ac motor, understanding the ac
motor will determine how successful the application will be when the motor is
used with an ac drive. The principal reasons for using an ac motor is to
convert electrical energy into mechanical energy to do some form of work.
With the use of ac motors, there are two components within the electrical
energy used. The most important is the real current the motor converts to
mechanical energy. The least important and undesirable is reactive current
which turns into heat. Until the PWM ac drive can into existence, little could
be done to eliminate or reduce the reactive current component. With the
introduction of the PWM ac drive, reactive current, which must exist in the
motor, can be eliminated or greater reduced as a burden for the distribution
system. PWM drives transfer real current from the ac line to the ac motor and
provide an electrical pathfor the reactive currents which must flow in the
motor. No longer does the distribution system have to carry the extra current
load. Since more than 20% of the current that flows in most distribution system
is reactive, reduction or elimination of the reactive current increases the
amount of real current, and resulting real work, that can be transferred by a
given distribution system. Applying PWM ac drives where fixed motor had be used
will decrease the demand on the distribution system.
A recent trend has been the change from standard efficiency motor to energy
efficient motors. Although this transition resulting in an improvement in
energy consumption, some unique and somewhat unexpected characteristics were
uncovered. Applying an energy efficient motor is an existing fan application in
many cases resulted in an increase in energy consumption. This was due to the
fact the energy efficient motors run slightly faster than standard efficient
motors. Due to the characteristic of a fan power curve, a slight increase in
fan speed results in a greater consumption of energy and higher electrical
costs. When the same energy efficient motor is used with an ac drive, the speed
of the motor can be controlled resulting in a decrease in energy consumption
and lower energy costs.
Understanding the nature of motor and the differences between standard and
energy efficient motors will aid in knowing how to correctly apply ac drives.
The most important aspects when comparing motors is their demand for current.
The energy efficient motor demands morecurrent
when starting across the line than the standard efficient motor. However, when
used with an ac drive, that disadvantage is eliminated. Inrush current when
starting a motor using an ac drive is less than full load amps. Another factors
is a slightly higher line voltage will result in a very hot energy efficient
motor. With an ac drive, the motor voltage can be adjusted to the correct value
to eliminate unnecessary heating in the motor. Of the many types of ac drives,
only the PWM drives offers a distinct advantage regarding eliminating or
reducing high current demands from the distribution system. Besides the near
unity power factor of the PWM ac drive, the reduction in reactive current
yields a much more effective means of controlling an ac motor. For example, a
460v, 100 HP motor would require about 124 amps from the distribution system if
operated direct on line. That same motor doing exactly the same work would only
require about 100 amps from the distribution system. This translates into lower
energy upstream losses in wires, transformer, etc. and may result in lower
utility charges for poor power factor.
The next point is the opportunity to reduce energy costs by eliminating
unnecessary losses in the application. In a variable volume fan application
where inlet dampers or output vanes are used to control the airflow,
substantial energy costs can be obtained with the application of ac drives. By
controlling the speed of the fan directly, flow can be controlled and pressure
losses across the fan blades can be reduced. In manyapplications, as much a 50%
reduction in energy costs have been obtained by using ac drives to control the
To control the flow by restricting the amount of air that can flow results in
pressure build up in the system. Removing the restrictive device, such as a
outlet damper or inlet vane and applying an ac drive to directly control the
speed of the motor and thus the fan results in reducing system pressures
contributing to substantial energy savings.
The same principles can be used in the control of pumps in many applications.
Replacing the throttling valves with ac drives, can reduce the pressure losses
that exist on the impeller blades of the pump. In both cases, eliminating the
high inrush current associated with starting a motor across the line are part
of the advantages that are obtained when using an ac drive to control the ac
motor that runs the fan or pump.
Another factor is eliminating high starting current losses. With standard
efficient motors, the lock rotor current is 6 to 8 times full load amps. With
energy efficient motors, the lock rotor current is 8 to 10 times full load
amps. With an PWM ac drive, the starting current is never more than full loads
amps and generally much less.
In many cases where an on-off method is used to control flow, merely selecting
a slightly lower operating speed and operating at that lower speed for a
slightly longer period of time will result in significant energy savings.
When applying ac drives, the response time compared to other means of flow
control is much faster. Short term high energydemands are eliminated thus
reducing short high energy costs. Regardless of the type of ac motor, standard
or energy efficient, the application of the ac drive improves performance of
the process while maximizing the efficiency of the system. The use of energy
efficient motor will further increase efficiency.
There are motor considerations to apply when using PWM ac drives. In most
cases, no concerns exist relative to heating insulation and torque capability
of the motor. In some applications, these factors must be considered. Most
supplier can advise the type of drive adjustment (parameter settings) that will
be useful in those applications. Generally, adjusting the voltage at various
operating speeds and adjusting the acceleration and deceleration parameters are
all that is required.
Addressing ac drive system concerns include a number of issues where many
unfamiliar terms are used. Harmonics, reflected waves, EMI, etc. For most
applications, these factors need not be considered. Most drive supplier provide
information and guidelines covering these topics.
Installation practices are most important and may have undesirable results if
recommendations are not followed. Power feeds should include a 4th wire to
provide a path for high frequency leakage currents that exist is all ac drive
installations. Although it is likely that a 3 wire power feed will appear to
work, currents circulating in undefined ground path may result in electrical
noise which could affect other electronic signal type equipment such as sensors
Signal andpower wires should never use the same conduit. Multiple motor
circuits should be kept to less than 5. Grounding should be based on high
frequency grounds for low ground to neutral voltages, not 60 Hz methods. All
PWM ac drives should have at least 3% series impedance between the voltage
supply and the drive. Branch circuit protection can use same methods as used
for fixed speed motor, however ac drives will sense current malfunction faster
and indicate those conditions but turning off and displaying the condition
which cause the shutdown.
During operation, the ac motor should always be started using the ac drive.
Motor starters should never be used to stop or start the motor. Time should be
allowed for the field in the motor to build so that full torque capability is
available. During running, acceleration and deceleration rates or times should
be selected so that the drive stays within its current rating. Too rapid an
acceleration will force an overcurrent condition and may cause the drive to
shut down. Use the drive to stop the motor, do not remove input. The drive must
have rated input voltage in order to maintain control over the motor. Any
energy stored in the motor must be absorbed if the motor is to be brought to a
rapid stop. This energy can cause drive to trip on overvoltage conditions if
the energy is not absorbed.
In general, most applications can use a standard or energy efficient motor with
an ac drive and improve energy saving and performance. Some applications need a
'little extra attention'. Applying ac drives can be madesimple if
care is taken regarding voltage from the source and current demanded by the motor.
Some thought about subtle changes to a process can yield large results.
Surviving Voltage Transients
Last Revision - August 5, 1997
Dealing with Power Factor Correction Capacitor Switching and AC Drives
There are two significant concerns regarding the existence of power factor
correction capacitors being located on the input of an AC Drive. One concern is
the additional current loading that may exist in the capacitor. The other concern
occurs when capacitor switching is used to adjust any variations in the power
factor of the motor load.
Power Factor Correction Capacitor Current
It has been stated that power factor correction capacitors and AC drives are
not good partners. The reason for this is that the current flowing through
power factor capacitors, when connected directly to the ac motor, is reactive
and is controlled by the reactive nature of the motor. The capacitors are sized
for kVAR or the rated power factor of the motor.
When capacitors are connected on the input of the AC drive, the power factor of
the motor is buffered or hidden from the ac line by the AC drive. Since
capacitor current is no longer controlled by the ac motor, the capacitor become
a voltage source for the ac drive and it’s load current becomes real not
reactive. This results in a higher current demand on the capacitor then would
be experienced when the motor and capacitor were connecteddirectly.
In some cases, where capacitors exist within an electrical distance of 250 ft
or less to the input of the AC drive, some additional heating of the capacitor
can be expected. The degree of heating will depend upon how much inductance
exists between the capacitor and the AC drive input. Inductance values
exceeding 50 microhenries will aid in reducing the peak current through the
capacitor and reduce some of the heating.
The best solution is to move the capacitors beyond the 250 ft distance or as
close to the transformer source as possible. Adding additional reactance between
the capacitors and AC drive will also reduce capacitors heating. Since the
input power factor of PWM AC drive is near unity, the use of capacitors with AC
drive controlled motors is not required. In cases where fixed speed motors and
AC drive controlled motors exist on the same distribution system, care should
be taken to insure sufficient electrical distance between any capacitors and
the input of the AC drive.
Power Factor Correction Capacitor Switching
All AC drive including PWM AC drive have an overvoltage withstand capability.
Because the power components within the AC drive have a voltage limit, that
limit must be controlled to insure reliability of the product. If no action
were taken as the input voltage increases, even for a moment, stress would
occur within the product causing premature failure. All PWM AC drive have this
voltage limit. Any AC drive manufacturer that states their product is immune
from overvoltage conditions or does not interrupt operationwhen an over voltage
transient occurs is either uninformed or a fool and likely both.
For a typical 460 volt, AC drive, the voltage limit is approximately 800 volts
DC. This voltage is obtained by rectifying the ac line. The nominal rectified
ac line is 650 VDC. For a 10% high line, the value would increase to 712 VDC
for a no load motor condition. The voltage limit of 800 VDC represents a 123 %
increase in the input nominal ac line. By design, the AC drive will monitor the
instantaneous rectified or dc internal voltage and terminate drive operation
any time the dc voltage exceeds the limit. This insures drive reliability by
removing stress from power switching components like transistors and IGBTs.
All switching components have a safe operating area. The safe operating area is
defined by voltage, current and time, too much voltage or current for more than
a few microseconds. To operating outside the safe operating area results in
shorter component life. Failure will occur if any power switching component is
operated outside the area. Failure may be instantaneous or may take hours to
occur. AC drive designs that do not interrupt operation when limits have been
exceeded will always result in premature failure. If the operation is not
interrupted when limits have been exceeded, then component stress will result.
When power factor capacitors are switched, voltage transients will occur. The
larger the amount of capacitance, the greater the transient. These voltage
transients act just like a voltage source. They will push current through
therectifier of an AC drive and cause the internal DC bus voltage to increase.
Usually, under full load operation of the ac motor, the extra
”energy” contained within the voltage transient will be absorbed by
the motor load. If the motor load is less than the rating of the AC drive, this
“energy” will be absorbed by the filter capacitor within the AC
drive. If sufficient “energy” is contained within the voltage
transient, the DC bus will reach the over voltage limit causing the AC drive to
interrupt operation. To minimize the interruption of AC drive operation when AC
drives are used in conjunction with switching of power factor correction
capacitors, additional hardware is required.
With AC drives larger than 5 HP, it is normal practice to include within the AC
drive a series reactor to slow down the current pushed into the AC drive by the
voltage transient. In AC drives 5 HP or smaller, the use of an internal series
reactor is not the practice. Market pressures insist on smaller and more
efficient packages. It is general practice to add external ac line reactors
when voltage transient conditions are expected. One method is to use the same
wire used to feed power to the input of small HP AC drives and create an air
core reactor by winding the feed wire into a coil, with a diameter of the width
of a hand, about 75 turns and tie wrap the coils. This is done in each input
phase. In many installations, these “handmade coils” will provide
sufficient inductance to slow the current and limit over voltage shutdowns.
Because there is no single method topredict the amount of “energy”
stored within the voltage transient, the amount of inductance required will
depend on 1) the amount of capacitance being switched, 2) the motor load, and
3) the nominal supply voltage.
All AC drives have an over voltage limit. Reliable designs interrupt operation
when voltage transients are greater than the over voltage limit. Installations
should minimize the occurrence of voltage transients and add inductance between
the voltage transient and the input of the AC drive. The impact of power factor
capacitors can be reduced by reducing the amount of capacitance being switched
at any given time. Switching capacitors in multiple groups rather than one
group will reduce the amplitude of the voltage transient. If the
“energy” contained within the voltage transient is larger than can
be absorbed by a line reactor, than the addition of a input transformer between
the capacitor and AC drive will be required.
You can download a self extracting compressed file PWRFACT.EXE containing a
copy of this paper in Microsoft Word 6.0 format. Run the downloaded file to
uncompress the file PWRFACT.DOC.
HARMONIC CURRENT SPECTRUM
Typical PWM Adjustable Frequency Drive with internal dc bus reactor
Description of full load current waveform using individual frequency components
The following table shows the approximate values for each frequency component
contained within the current pulses associated with the line current for a PWM
adjustable frequency drive with aninternal dc bus reactor. The components are
not true harmonics, only a representation of individual waveforms whose areas
equals the area under the original waveform or typical input power line current
It is important to keep in mind that the area under the input line current
pulse or waveform is equal to the real power being transferred from the
distribution system through the drive to the motor. With the PWM drive, are
current flows forward and each contributes some portion of real power that will
be used by the motor to perform work. T here are no negative sequence currents
as would be defined by a Fourier analysis of the waveform.
All current components contribute to the real power transferred. There is little
or no reactive component and as such no current injected into the system as
would be typical in linear, ie transformers and non linear load types which
contain a reactive component in the current waveform.
Other types of non linear equipment such as electric arc furnace controls, dc
drives, and current source inverters inject distorted reactive current into the
electrical system and as such may increase the current demand on a distribution
system. The following table does not describe the harmonic current spectrum for
other types of non linear equipment.
|Frequency Component (HZ) |Equivalent Harmonic |Theoretical % of Fundamental
|Typical % of Fundamental |Current Sequence |
|60 |1st |100 |95.5|Positive |
|300 |5th |20 |23.5 |Positive |
|420 |7th |14.29 |13.5 |Positive |
|660 |11th |9.09 |8.5 |Positive |
|780 |13th |7.69 |7.1 |Positive |
|1020 |17th |5.88 |5.4 |Positive |
|1140 |19th |5.26 |4.3 |Positive |
|1380 |23rd |4.35 |3.4 |Positive |
|1500 |25th |4 |2.8 |Positive |
|1740 |29th |3.45 |2.4 |Positive |
|1860 |31st |3.23 |2.1 |Positive |
|2100 |35th |2.86|1.8 |Positive |
|2220 |37th |2.7 |1.6 |Positive |
|2460 |41st |2.44 |1.2 |Positive |
|2580 |43rd |2.33 |1.1 |Positive |
|2820 |47th |2.13 |0.8 |Positive |
|2940 |49th |2.04 |0.4 |Positive |
|3180 |53rd |1.89 |0.25 |Positive |
|3300 |55th |1.82 |0.12 |Positive |
The preceding data is typical for a Pulse Width Modulated Adjustable Frequency
Drive with an internal dc link choke or reactor. It is also typical for a PWM
drive using ac line reactors rated for the output current of the drive and
containing at least 3% impedance. Line reactors with a larger rating than the
drive load must have a greater % impedance to limit the peak to rms value of
the input current pulses. The larger the peak to rmsvalue, the greater
the percent distortion of the waveform.
Calculation of rms current
The rms value of the input line current waveform is determined by calculating
the square root of the sum of the squares of all frequency components. The next
table shows how the total transferred rms current is calculated. Since the
current waveform is in phase with the applied line to line voltage, the power
transferred is positive or real power.
|Frequency Component (HZ) |Value |Value Squared |
|1st |.955 |0.9120 |
|5th |.235 |0.0552 |
|7th |.135 |0.0182 |
|11th |.085 |0.0072 |
|13th |.071 |0.0051 |
|17th |.054 |0.0029 |
|19th |.043 |0.0019 |
|23rd |.034 |0.0012 |
|25th |.028 |0.0008 |
|29th |.024 |0.0006 |
|31st |.021 |0.0004 |
|35th |.018 |0.0003 |
|37th |.016 |0.0003 |
|41st |.012 |0.0001 |
|43rd|.011 |0.0001 |
|47th |.008 |0.0001 |
|49th |.004 |-- |
|53rd |.0025 |-- |
|55th |.0012 |-- |
| |SQRT of sum --> |1.00 |
The value shown as the square root of the sum of the squares represents the rms
value for current. That value of 1.00 and is equal to 100% of the power
required to transfer power to the motor for full operation. Although the
current waveform can be separated into many individual frequency components,
each component contributes to the power that is transferred from the
distribution system to the motor. Some components may contribute some
additional heating to the distribution system, however that additional heating
is minor and will not cause overloading of a distribution system that is
designed to handle the same motor operating across the line or direct on line.
A key point is that PWM drives without an internal reactor or input ac line
reactors will contribute larger distortion values. To limit peak currents, a
minimum series impedance of 3% should be applied. If an existing distribution
system is tuned to a specific frequency, it is possible that one of the
frequency components contained within the current waveform may excite that
existing system. It is important to note that if the system is nottuned to a
specific frequency, the frequency components contained within the current
waveform can not cause resonance to occur. Since the current flow is into the
drive, no resonance can exist. The unidirectional characteristics of the PWM
converter prevent current from flowing back into the distribution system.
Applying the recommendation defined in IEEE-519 (1992) for current distortion
Changing the characteristics of the current waveform of a PWM drive with the
objective of meeting the recommendations defined in table 10.3 of IEEE-519
(1992) is reasonable if no additional costs or additional energy loss occurs.
Most knowledgeable utilities and consultants recognize that only more current
places a greater demand on the distribution system. Current distortion is not
more current. It is only a representation of the peak to rms value.
It is a fact that, compared to operating motors across the line, the current
demanded from the distribution system when PWM drives are used is significantly
less than the nameplate amps of the motor. Specifying IEEE-519 (1992) current
distortion recommendations insure greater costs and higher energy losses for
the sole reason of obtaining a 'pretty' waveform. Current distortion
is important when applied to some non linear motor equipment and electric arc
furnaces. Also voltage distortion limits are important, although some types of
voltage distortion, like flat topping, typically create no problems. Voltage
notching and distortion about the zero crossover point of the voltage waveform
can create problems with sometypes of power control.
What is important is that the peak of the current waveform not exceed the
capabilities of the distribution system. In most cases, the use of series
reactors solves that condition. Phase shifting the supply voltage also aids in
keeping the peak of the current waveform, with respects to the rms value, to a
reasonable value, however line to line voltage balance must be maintained to
insure that currents are equal. In all cases, series impedance must exist to
control the current peaks.
It is important to understand the characteristics of electrical equipment. To
apply current distortion and voltage distortion limitations as a means to
insure equipment operation only adds costs and losses to an electrical system.
It does not guarantee that elctrcial problems will be eliminated.
Understanding Variable Speed Drives
Part of DRIVESWORLD.COM
Last Revision - August 25, 2000 - H.G.Murphy P.E.
Performance and reliability of plant equipment often depend on proper
application of motors and drives. Drive types, installation parameters, and
environmental conditions must be considered when selecting a drive.
Adding variable speed control in motors often improves control and reduces
energy costs. However, cutting through the clutter of competing claims and
technical acronyms can be difficult. A better understanding of motor control
options can save time, money and frustration during installation and
maintenance and over the entire life cycle of the system.
Why variable speedcontrol?
How is variable speed control different from more traditional control methods,
such as electromechanical starters or solid state 'smart' motor
controllers? Each type is suitable for certain applications, and many
installations use multiple types of motor control. Electromechanical starters
are most useful for simple start and stop operations where the rapid change
from zero to full speed does not affect the process. Smart motor controllers
offer features such as soft-start, which are important for protection and
improving the process. Both types of starters are designed for constant speed
operation. Once the motor starts, it keeps running at the same speed until it
is turned off. Variable speed drives are best used when
The process requires variable speeds Adjustable speeds can optimize a process
to save energy and obtain utility company rebates, for example in many fan and
pump applications The application requires speed trimming and controlled
starting and stopping The operation requires high precision speed or torque
control The process requires coordination of speeds between sections
Variable speed drives are common in heating, ventilation and air conditioning
applications. In these applications, variable speed operation makes it simple
to slow down the fan when air demand is lower to reduce electricity use in a
facility. Another application is pump control, where variable speed control can
help protect equipment protect equipment during high or low volume operation.
Industrial applications are numerous and include conveyorcontrol, where
variable speed operation makes it simple to coordinate conveyor speed with
desired production rate.
Most electrical maintenance personnel are familiar with the basics of drive
technology. However, drive manufacturers tout their latest technologies with
confusing acronyms and terms. Here are some of the most common drive features,
from the basics to the latest technology, and some key factors in evaluation
AC and dc drives
There are a number of key differences between ac and dc drives. DC drive
technology typically incorporates Silicon Controlled Rectifiers (SCRs) to
transfer incoming ac power to control the applied voltage to the dc motor.
Although dc drives are often less expensive, initially, dc motors, which use
brushes, generally are more expensive and require more maintenance than ac
motors. However, dc drives can provide extremely precise and independent speed
and torque control. Using brushless dc motors eliminates many of the
maintenance and control concerns of dc motors, but they are generally more
expensive and complex than
comparable ac motors.
AC drives use newer technology and may cost more initially than dc drives.
However, the advantages of ac motors, including lower maintenance and repair
costs and higher energy efficiency, can provide a faster return on investment.
AC drives typically use Pulse Width Modulation (PWM) techniques to control the
voltage and frequency applied to the ac motor.
PWM drives use a diode bridge rectifiers or full conducting SCRs toconvert the
ac supply voltage to dc voltage. The dc voltage is then filter in a low pass
filter. The dc voltage is connected to the three terminals of an ac motor to achieve
the required voltage and frequency expected by the ac motor. The connection to
the ac motor are achieved using electronic switching devices. Many PWM drives
use Insulated Gate Bipolar Transistors (IGBTs) to control the switching
required for pulse width modulation. PWM drives typically can reduce most
harmonics to the motor by controlling the occurrence and duration of each
Newer vector controlled ac drives are now matching or exceeding the dc
precision and making inroads into many formerly dc only applications, such as
winding and roller operations.
Third generation IGBTs
The most recent advances in ac drive technology have gone hand in hand with
improvements in the size and performance of power switching devices known as
Insulated Gate Bipolar Transistors. Ideal for low level signal to power
control, ac drives use IGBTs to provide fast, accurate operation, electronic
signals to the motor, and quiet operation. Although some electrical loss had
been associated with the use of IGBT switching devices, drive manufacturers
have taken full advantage of this technology by upgrading drive packaging.
The ac drive was a demanding application for earlier IGBTs, However, what are
known as third generation IGBTs reduce losses and efficiently handle higher voltages.
These devices have faster switching speeds, and lower conduction drops. Losses
of the new devices are 39 watts at15 KHz which are 40% lower than earlier types
of IGBTs. In addition, the newer IGBTs reduce voltage overshoot by employing a
soft recovery free wheeling diode in the design. Improvement in both ac drives
and ac motors continue to take advantage of the higher performance power
technology. Drives with the latest IGBTs offer increased reliability and better
performance at a lower cost. At equipment voltages 460 volts or higher, use of
ac motors with improved insulation are required when long cable lengths exist
between the drive and the motor.
The most advanced vector controlled drives use field oriented control to make
the ac drive perform like a dc drive. They provide accurate speed and torque
control of ac motors. These drives are typically used for applications such as
winders and roller operations where precision torque control is critical.
Vector control methods let users apply efficient, low maintenance ac motors in
applications traditionally dominated by dc motors.
Describing all available vector controls is beyond the scope of this article.
Vector control drives control flux current and torque producing current
independently to control motor torque and speed. Indirect and direct means
feedback allow these drive to have varying levels of speed and torque accuracy.
All vector controlled drives are not the same. Faster current control loops
combined with precise control algorithms simplify installations and improve
performance by sampling motor parameter information.
Evaluation of a vector controlled drive requires a closelook at how well the
drive works with a particular type of motor. Some the question to ask the drive
Will it work with standard motors?
Must high performance motor be used to achieve stated efficiency levels?
Valid questions to ask your supplier to improve your chances of getting the
types of control required for your process. Many 'so called' vector
controlled drives fall short when a 'tough' application comes along.
Having different drives from many suppliers because the 'price is
right' creates a more difficult maintenance and repair situation. When a
vector controlled drive is required for an application, getting the drive that
performs the best will ensure that any and all applications can be controlled
with the same hardwired and simplify the programming requirements associated
with vector control drives.
With the increasing need for control and integration of automation devices in
facility information systems, the flexibility and ease of use communication
capabilities in adjustable speed drives is vital. A number of factors should be
considered when looking at communications, for example;
Will the be controlled by programmable controllers?
By a building management system?
Will it be linked on DeviceNet or Remote I/O networks?
Is there potential for future links to SCADA systems?
Are the drive's communications capabilities flexible enough to meet present and
Are communications links easily accessible and simple to install?
A major aspect of drive selection isa thorough evaluation of facility and
application parameters. Getting the most out of variable speed drives requires
looking not only at the drive, but also at the motor it will control, the
operating environment, and the way these factors will fit together. Following
are tips to consider;
Is the ampere rating on the drive comparable to the ampere and horsepower
rating of the motor?
Matching drive and motor horsepower is not always sufficient. It is important
to note amperage ratings of the motor and drive. For example, it may take a 30
hp drive to operate a 25 hp motor with higher current requirements.
Does the application have constant or variable torque loads?
Matching the drive to the load characteristics of an applications is a primary
consideration in selecting an adjustable speed drive. Constant torque loads are
among the most common. The drive can provide constant torque operation because
motor output horsepower is directly proportional to speed and rated torque is
available at any speed. Using power rated motors with adjustable speed drives
ensures the correct match between the drive, the motor and the application.
Constant torque loads are essentially friction loads. Constant torque is
required to overcome friction. Typical constant torque applications include
conveyors, extruders, and surface winders and other applications involving shock,
overloads, and high inertia loads.
Variable torque is often applied to help save energy. Common variable torque
applications include centrifugal fans, pumps and blowers. Control the
movementor flow of air or liquid can be achieved by indirectly restricting the
flow of air or liquid. By applying adjustable speed drives, the air or liquid
flow can be control directly by controlling the speed of the motor. Since
losses due to the flow of air or liquid decrease as the flow decreases, power
losses, associated with restricted control methods, can be eliminated by direct
control of the motor speed. Generally, these types of applications do not
require extra power for momentary peaks loads, therefore the overload capacity
of variable torque drives is suitable for most applications.
Environmental considerations are key to proper drive operation. It is important
to choose an enclosure appropriate for the environment. Some environmental
Is a separate control room available to provide the proper environment for this
Variable speed drives can be affected by the environment. Most drives are
available in highly protected cabinets minimizing the dangers.
What types of enclosures are available for the drive?
Drives typically are available in NEMA 1, 4, or 12 enclosures. Accessibility of
a separate room and other environmental issues affect the choice of enclosure.
How many motors are involved?
In multiple motor applications, drives can be packaged in motor control centers
(MCC) for increased flexibility and ease of maintenance. The MCC can include
drives, starters, and soft start devices.
How will your electrical system change when you install new drives?
A number ofelectrical issues must be considered. These issues include ensuring
proper grounding, shielding, and cabling, insulation ratings on motors, the
system's ability to handle nonlinear loads, and lead lengths from the motor to
the drive to avoid leakage current and voltage stress. All drives put voltage
stress on the motor. The drive manufacturer should be able to help show how to
install the drive and the motor to optimize performance and protection.
When installing and maintaining drives, it is important to understand the
differences between drives types and sizes, and the technology behind them.
Additionally, anyone involved with installing, applying, operating and
upgrading variable speed motor control needs to consider the drive as it
relates to the motor and the electrical system to ensure maximum benefits for
the variable speed solution.
What is meant by meeting the requirements of IEEE-519-1992?
Howard G. Murphy P.E.
First and foremost, IEEE-519-1992 describes the RECOMMENDED Practices and Requirements
for Harmonic Control in Electrical Power Systems. The scope of IEEE-519-1992 is
clearly stated as the intention of establishing goals for the design of
electrical systems that include both linear and non-linear loads. The document
describes the voltage and current waveforms that may exist throughout the
system and establish waveform distortion goals. It defines the interface
between sources and loads as the point of common coupling with observances of
the design goals to minimizeinterference between electrical equipment.
Secondly, it is the responsibility of any reputable equipment supplier to
provide their customers with equipment, at the best possible cost per
performance ratio, that will meet the known operating requirements of the
customer. Included with the purchase of that equipment are the less tangible
but equally important application experience that the supplier can share with
Lastly, the equipment supplier should be able to supply any necessary service
and application support directly associated with the performance of that
equipment and its impact on other electrical equipment utilizing the same point
of common coupling.
Specifically addressing the objective of meeting the requirements of
IEEE-519-1992, it can be stated that all variable speed drives (both AC and DC)
meet the requirements in most installations. With some types of variable speed
drives (DC and Current Source Inverters), it may be necessary to add an
isolation transformer or ac line reactors between the source and the drive to
prevent interference with other electrical equipment. The requirement for
isolation transformations or ac line reactors stems from the fact that those
types of power converter produce voltage distortion in the form of line
notching. Those types of converters also may contain higher levels of reactive
currents which may place additional stress on the components of the
distribution system when they are injected back into the system.
Typically, if the system capacity is greater than twice the demand,
voltagedistortion is minimal and no interference with other electrical
equipment is experienced. Often the recommended goals established within
IEEE-519 are exceeded without creating any interference problems. Because of
the diversity of the individual loads, the recommended goals defined within
IEEE-519 can not identify the demarcation line between an electrical system
that is problem free or a system that is subject to problems.
The principal goals established within IEEE-519-1992 are for voltage distortion..
The document clearly dedicates its volume to describing power converter types
where phase controlled rectifiers are used. The main applications for that type
of converter is in power control for arc furnaces, dc motor controllers and
current source drives. The converter used in PWM ac drives is mentioned briefly
in a few paragraphs of the 100+ page document. That fact should not go
It is generally accepted that PWM ac drives which contain internal bus reactors
or input ac line reactors do not create electrical interference with other
electrical equipment. Although some rare occurrences may exist, more than
500,000 installations where PWM ac drives are installed have not experienced
any interference problems. In the majority of these installations, the voltage
distortion goals defined within IEEE-519-1992 have not been exceeded.
In many or even most of the installations, the current distortion goals have
been exceeded (if current is measured at the input of the drive and not at the
utility). These facts clearly show that thedescription for current distortion
defined within IEEE-519-1992 is correctly stated as an aid in determining
In section 10.3 (Development of Current Distortion Limits), '.. The
objectives of the current limits are to limit the maximum individual frequency
voltage harmonic to 3% of the fundamental and the voltage THD to 5% for systems
without a major parallel resonance at one of the injected frequencies.' It
further goes on to state that; '..If individual customers meet the current
distortion limits, and there is not sufficient diversity between individual
customer harmonic injections, then it may be necessary to implement some form
of filtering on the utility system to limit voltage distortion levels.' It
can be clearly noted that achieving low voltage distortion is the principle
goal. Meaurements for current distortion are to help identify potential voltage
Meeting the requirements defined within IEEE-519 does not mean crossing every t
and dotting every i without applying common sense. It is a system evaluation
which requires going beyond the easy approach of putting in bigger and more
expense equipment that is not required. It is the responsibility of the
equipment supplier to take their best technical experience and share that
experience with their customers to provide the lowest cost, most efficient
solution to the application. If we are to efficiently use the limited energy
that is available to us at this time we must not implement the bigger, less
efficient, and more expense solutions when it isobvious that they are not
required for the proper performance and most efficient use of electrical
Finally, we must understand that we are dealing with electrical systems which are
dynamic. It may be required, in the future, that different solutions are
necessary. As the demands on the electrical system change, it is likely that
other solutions, that are more expensive today, may be required in the future.
There is every reason to believe that, when those solutions are needed, the
cost and reliability will be more favorable. IEEE-519 is also a dynamic
document. As technology changes, that document will change. As suppliers offer
new products, those products will help the customer meet their electrical needs
when those needs are required and they will likely be achieved at favorable
costs without an atmosphere of fear. If no interference between electrical
equipment exist in an installation, then the scope of IEEE-519 has been met. The
harmonic distortion percentage limits are guides to achieving that result.
Those who utilize IEEE-519 with that in mind clearly have met their
responsibility of designing and specifying on the basis of intelligence not
As a final note, It is recommended practice to consider the effect on power
factor correction capacitors that were applied to the system to improve or
counteract the lagging power factor of any motor operating across the line.
When applying drives, any capacitors on the system may have to be relocated or
electrically buffered to prevent an increased demand caused by forcing
thecapacitors to provide kVA power instead of the kVAR power that they were
designed to handle. Discussions with the equipment supplier can obtain a more
appropriate solution to a potential system problem.
Eliminating Voltage Notching on the Distribution System
Part of DRIVESWORLD.COM
Last Revision - August 25, 2000 - H.G.Murphy P.E.
When Silicon Controlled Rectifiers (SCR's) are used in electrical controls, it
is possible to experience line voltage distortion in the form of
'notches' in the waveform. The types of equipment that utilize SCRs
in converters or rectifiers, to change the ac line voltage to a dc voltage, and
thus experience notching include DC motor speed controls and induction heating
Line notches are just that - an irregularity in the voltage waveform that
appears as a notch as illustrated in Figure 1. They are typically present in
the waveform during SCR commutation. Commutation occurs when an SCR in one
phase is turned on to turn off an SCR in another phase. For this very small
duration of time, a short circuit is created between the two phases. With a
short circuit, the current increases and the voltage decreases. The decrease in
voltage is defined as a line notch.
The notch can appear at any point during the half cycle since the point of
commutation changes as the firing point of the SCR changes. Since the speed of
a dc motor is a function of the voltage applied to the motor, at low speed the
notch may appear near the end of the half cycle. Athigher speeds, the notch may
appear near the beginning of the half cycle. In the most severe cases, the
voltage is reduced to zero, creating an extra zero crossover or point where
voltage normally changes polarity. This extra zero voltage crossover causes the
Zero Voltage Crossing
During a normal cycle of sinusoidal voltage, the voltage crosses the
'x' axis, or zero, at 0 degrees and again at 180 degrees. During
normal conditions, there are two zero crossovers in each cycle. Some electronic
equipment is designed to be triggered on the zero crossover or when the voltage
is zero. This allows equipment to be activated without the surge currents or
inrush currents that would be present if switched while voltage was present.
Some equipment uses the zero crossovers for an internal timing signal. DC
drives use the zero crossovers to determine when to fire the SCR. When two dc
drives are operating on the same distribution system, they can disturb the
electrical system so that one dc drive operating at low speed will affect the
other dc drive that is operating at a higher or different speed. To prevent the
'power cross talk' between dc drives, it is normal practice to
install an isolation transformer ahead of each dc drive.
When notches are present, particularly in three phase equipment, we can
experience extra zero crossovers. Instead of two zero crossovers in each cycle
of voltage, we can actually experience four notches. These extra notches may
tell other equipment to 'turn on'. That means the equipment may turn
on at the wrongtime resulting in damage.
Eliminated voltage notching
Eliminating voltage notching requires that the source of the notching
(commutation) be isolated or buffered from other sensitive equipment using the
same distribution system. Considering the DC drive or other SCR controller as a
source of 'notch voltage' and the characteristics of the distribution
system, it is only required that other sensitive equipment not share the same
voltage source or a single voltage source. The simplest method to use to
eliminate the voltage notch is to isolate each piece of equipment with an input
transformer. If the impedance of the distribution system is low, generally dc
drives will not create a severe voltage notch that affects other equipment.
However, if the impedance of the distribution system is high (soft line) than
voltage notching will occur and likely impact other equipment. An alternate
method can use line reactors to reduce the voltage notch and reduce the
possibility of affecting other equipment. It is important to note that notches
created by dc drives will normally not affect the dc drive that is causing the
notching. It is other equipment that can be affected.
It is important that we understand and consider where other sensitive equipment
connects to that same voltage source. In order to protect the sensitive
equipment, we must reduce the notches before they get to that equipment. We
will do this through the creation of a simple voltage divider network.
If we add impedance, in the form of inductive reactance, in series with theSCR
controller, and between the controller and the point of other equipment connection,
(point B) then the notch voltage will distribute itself across the new
impedance (reactance) and the pre-existing line to source impedance. If the
added impedance (L3=L1=L2) is one-half as much as was already present, then 1/3
of the notch voltage is dropped across the new impedance and two thirds still
remains at the point of common connection with the other equipment.
If the new impedance is equal to the existing input impedance(L3 = L1 + L2),
then the notch distributes equally across the two impedances. One-half the
original notch voltage is now present at the point of common connection (B).
Notice that if the new impedance is added anywhere else but between the SCR
controller and the sensitive equipment, it will have minimal impact on the
notch voltage. Placing the reactance on the opposite side of point B offers no
improvement to the notching problem.
Use a 3% impedance reactorto solve three phase voltage notching problems.
Experience shows that 3% impedance is normally sufficient to reduce the notch
voltage, at the point of common connection with other sensitive equipment, to
about 50% or less of it's initial value (depth). This eliminates the multiple
zero crossings and typically solves the interference problems with neighboring equipment.
It is typically not recommended to use a 5% impedance reactor with SCR circuits
because the reactor not only reduces the depth of the notch, but it also
increases the notch width. Excessimpedance could increase the notch width
(time) too much causing problems in the controller itself. Increasing the notch
width may be seen as a loss of line voltage by some sensitive equipment. If the
impedance of the distribution system is low (1%), then a 3% impedance reactor
will result in a very small notch depth. Low impedance or stiff distribution
systems often have no notching problems. High impedance or soft distribution
systems may require a larger value of impedance to reduce voltage notching
problems. It is important to note that too much impedance can result in severe
loss of input voltage starving the equipment which can cause under voltage
Voltage notching can introduce harmonics on the distribution system. It is
important that distribution system be designed with the lowest possible
impedance. Prior to the use of power electronic equipment, designing an
electrical system with impedance served to reduce the short circuit current or
fault current in a system. With the use of power electronic equipment and its
self contained current limiting feature, the need for system impedances to
control fault current has been reduced.
DETECTING OVERVOLTAGE CONDITIONS WITH AFDs
Part of DRIVESWORLD.COM
Last Revision - November 15, 2000
Overvoltage conditions within Adjustable Frequency Drives (AFD) are determined
when the normal dc bus voltage increases to the dc bus overvoltage
'trip' design level. With 460Volt drives, this value is approximately
800vdc. This voltage ismonitored as as instantaneous value. That means
that if the voltage sensing circuitry detects an overvoltage, even a brief
period (< than 1 microsecond), the circuitry takes action to prevent further
increases in dc bus voltage. The types of action taken include, delay or
prevention of command speed decrease, interruption of output PWM voltage
switching, drive disable and shutdown.
Often, a braking chopper is added to a basic AFD to prevent, or limit,
overvoltage conditions. A braking chopper is an electronic switching circuit
which places an artificial load on the dc bus to dissipate energy. The design
voltage level for the braking chopper is below the overvoltage trip level and
usually has a sensing range about 10% below the overvoltage trip level. The
braking chopper can not determine what is causing the dc bus voltage to
increase but will attempt to maintain a maximum dc bus voltage level at its
Since momentary surges in the ac line are part of any electrical system, and
regen conditions are part of many motor control systems, it is difficult to
determine which source is creating the energy that results in on overvoltage
condtion on the dc bus. Keep in mind that ac line voltage sags are more
frequent than voltage surges. Various condition methods are employed with the intent
of determining the energy source. A brief description of input source and
motor/generator source are presented to aid in understanding where and how
energy results in overvoltage conditions.
The most obvious source of electrical energy is the incomingac line. Any time
the instantaneous peak value of input voltage rises above the nominal sinewave
value for an RMS voltage, a proportional increase can be expected on the dc
bus. Since AFDs are erroneously rated in RMS terms for their input supply
voltage, many view an increase in the RMS input voltage as the reason for
overvoltage problems, in reality, instantaneous peak voltages can occur without
a proportional increase in the RMS value. The series inductance between the
line source and the dc bus will determine IF a peak voltage results in an
overvoltage condition. In reality, AFDs should be rated in terms of RMS and
instantaneous peak voltage. Remember that an overvoltage condition occurs when
current flow into a filter capacitors of the dc bus is greater than the current
flowing out of the filter capacitors. Since the capacitor voltage is a function
of current flow versus time, any series inductance will affect whether a
specific voltage is achieved.
Some load characteristics are known that can create sufficient energy to cause
an overvoltage condition. The most common is the case where the ac motor is
rotating at a speed faster than its synchronous speed. Above synchronous speed
the motor becomes a generator. Above normal motor base speed, the motor will force
current from the motor back to the source. In the case of an AFD, this will
result in current flow into the filter capacitors of the dc bus. Whether a
overvoltage trip occurs depends on how much current and how long the current
flows from the motor to the drive. Keep in mind thatcurrent can flow from the
motor to the drive in a time frame observable due to obvious motor overspeed
and by not observable motor motion typically found in punch press operations
where the motor rotor is 'jerked' forward from its normal position by
the spring action of the compressed workpiece. Rapidly moving the motor rotor
through the motor field will create a 'pulse of energy' sufficient to
'push' current back to the drive.
So the question is asked, 'How do AFDs determine which source in causing
the overvoltage condition'. Additional questions regarding the same
determination when the AFD is equipped with a braking chopper.
The answers are the same, as to the methods employed, with the braking chopper
lowering the threshold or dc bus action voltage.
Before describing control methods used, it should be obviously that there are
likely to exist some set of conditions where determining the energy source
would be impossible. However, those conditions are rare.
It is unlikely that an AFDs will ever cause an overvoltage condition. The cause
is either the ac line or the motor load. It can be stated that a basic AFD can
only react to overvoltage conditions by terminating operation thus protecting
the AFD for component damage.
To minimize the occurance of AFD shutdown, application/installation
characteristics must be known and stored into the AFD as references against
which monitored conditions can be compared.
First, it is necessary to determine the line conditions before running the motor.
In fact, each time the drive is shutdown (with acline power still applied), the
line conditions should be stored.
Second, the drive should monitor the command conditions.
1) An increased speed command
prevents the motor from becoming a generator. So any overvoltage under that
condition will point to the ac line as the source.
2) A decrease in speed command can
cause the motor to become a generator. So the odds are any overvoltage is due
to motor regen.
1) A rapid, momentary reduction in
load current will indicate that an overvolage condition may soon follow.
Monitoring the dv/dt conditions of the dc bus, in combination with command
signals can serve as an indicator. For a fixed speed command, a high dv/dt in
combination with a fixed load current will indicate a ac line source change.
For the fixed speed command, a high dv/dt in combination with a reduction in
load current indicates a overspeed motor and/or increased line voltage.
However, if the previous monitored state, when the motor was drawing current,
indicated no dc bus voltage increase, then the ac line source can be ignored.
2) A period-value monitoring of the
dc bus voltage for a given motor load. Motor loads contain a load signature.
This can be used to predict abnormal load conditions which can be used in
combinations with dc bus voltage variations to determine whether the source for
a probable overvoltage condition.
Other control methods:
1) During setup of the system, the
time required to go from full speed to zero speed (Coast time) canbe determine.
The drive can store the 'coast time constant' and from that
information determine variations which might result in an overvoltage
2) During setup, the drive could
force a maximum allowable deceleration time to determine limits to be placed on
command functions. Although variations in loads may exist, the test should be
performed with the load at its maximum load inertia.
3) Since AFDs are designed to react
to overvoltage conditions with the intent being to stay within the safe
operating area of the power switching devices used, Determining whether to
shutdown the drive should depend on both maximum voltage and maximum current
values defined by the safe operating area. In those cases where dc bus voltage
limits have been reached but maximum current does not exist, shutdown action
should not be taken. Rather, switching action should force additional demand on
the dc bus to reduce the dc bus voltage level.
Although many overvoltage conditions result from a change in
a single source, accurately determining which source is creating the problem
required knowledge of both ac line, motor load conditions and command function
of the AFD. Operating within limits by changing command functions to prevent
overvoltage conditions provides the best, although not foolproof, protection
from overvoltage problems. It is impearitive that extreme variations in ac line
conditions be eliminated. Motor conditions can be controlled but only within
the capabilities of the AFD.
SYSTEM COMPATIBLITIY of AFDs
Last Revision - March 7, 1997
System Compatibility of AFDs 3/1/97
System compatibility refers to how well equipment performs when power line
abnormalities exist. There appears to exist a belief that process functions
should continue normally even if electrical power disappears. Momentary surges
and dips in the ac line are part of any electrical system. Expecting electronic
process control equipment(computers) and power electronic motor control
equipment(VSDs) to ignore the temporary loss of electrical power places one in
the same company as those who spend too much time watching 'StarTrek'
and believing there is enough energy stored in a 'Phasor' to destroy
the enemy. The System Compatibility paper presents an overview for system
compatibility and a realistic approach in dealing with momentary electrical
power quality events.
System Compatibility Testing 4/25/97
Interest in the System Compatibility of various types of electrical equipment
has become of major interest to a number of utilities. Testing organizations
are beginning to perform System Compatibility tests in hopes of establishing
design and application guidelines to minimize problems associated with the
momentary loss of electrical power. This paper comments on the suitability and
applicability of these types of tests and provides some insight into the
effectiveness of these types of tests.
Why PWM AFDs don’t cause Harmonic Problems
The Chances of Harmonic Problems are lessthan a Big Win in the Lottery
Howard G. Murphy P.E.
If the chances of having a Harmonic Problem with Adjustable Frequency Drives that
use Pulse Width Modulation and contain internal reactors or use input line
reactors is small, then why do many electrical specifications contain the
requirement to meet the guidelines of IEEE-519.
There are two reasons why electrical specifications include the IEEE-519
requirement. The first reason is fear of an unknown phenomenon called
Harmonics. The second reason is peer pressure. If your peers (including utility
“Experts”) tout the importance of Harmonics and refer to
“qualified” references, then it would look bad if your
specifications did not include the IEEE-519 requirement.
So what if the IEEE-519 requirement is included in your specification. Are
there any negatives? The answer is YES when the following conditions exist.
AFDs with a rating between 5HP and 1000HP
AFDs are specified as PWM with internal or external reactors (3% impedance
AFDs connected to a distribution system that use less than 90% of the source
The first negative for the project is that it will cost more than required. The
second, and most important, is that specifying IEEE-519 does not guarantee that
other electrical problems will not exist. The last negative is that equipment,
like harmonic filters, added to make the current waveform “pretty”
can create some of the problems that are explained in IEEE-519.
Harmonic Filter traps can act as “sinks” for harmonic currents
created by the normal operation of asystem. Turning on power, starting across
the line motors and load changes on across the line motors can create voltage
disturbances which will force currents into filters which were not designed to
handle these disturbances.
Perhaps a review of IEEE-519 would be in order. First the document explains
many of the characteristics of electrical distribution systems. It creates a
“bag of electrical bad guys” and dumps all Adjustable Speed Drives
Types (AC and DC) into that bag. It sets some numerical guidelines for all
types of drives without consideration for the major differences between those
In 1981, when the original version of IEEE-519 was released, the importance of
that document was to set guideline limits for total percent harmonic voltage
distortion (%THD) covering the use of Silicon Controlled Rectifiers in variable
voltage controllers like DC drives and arc furnaces. The phase control and
importance of balanced firing could and does place huge demands on electrical
systems. The possibility of harmonics problems with those types of controller
(converters) demanded that input transformers and line reactor were used.
During the 1980s, many imported Adjustable Frequency, PWM, AC drives were
introduced. These AFDs did not contain internal reactors and created
overloading and fuse failure problems. Where problems existed, the addition of
line reactors eliminated the problem. In response to these earlier AFDs, the
1992 version of IEEE-519 was released. This document carried through the
importance of limits for total percentharmonic voltage distortion, but added
guidelines which defined limits on percent injected current distortion. The
document did not clarify that injected current is reactive current or current
that is sent back into the distirbution system. Without that simple
clarification, percent distortion of the forward current or power producing current
was subjected to a demand that its shape be made to look like a sinewave.
It is the IEEE-519 limits for percent current distortion that has done a
disservice to the industry. There is no quantitative data which shows that
reducing the current distortion (making the waveform pretty) makes the system
operate better or more efficiently. The problem with IEEE-519 has become a
numbers game. The equipment used to measure percent current distortion compares
an ideal waveform with an actual waveform. There is no indication of how the
actual waveform impacts the electrical system. It is merely a comparison of a
pretty waveform versus a not so pretty waveform. Since IEEE-519 specifies that
'INJECTED' (reactive) current back into the distribution system
should be limited, trying to force the forward, power producing current to be
'pretty' is outside the recommendations of IEEE-519.
The present interpretation of the IEEE-519 guidelines presents the following
Given a garden hose capable of 100GPM, the guidelines state that using the hose
at 50GPM but wildly varying the flow from 30 to 80GPM will create problems.
Following the guidelines would require that a flow regulator be added so that
the flow couldnot change rapidly.
What would happen if the same limits were applied to the city water supply.
Restricting flow within some arbitrary limits would create a lot of unhappy
customers in the early morning when lots of flushing and showering occurs.
In the preceding case, the solution would be to regulated pressure. In the same
manner, it is important in electrical systems to regulate voltage. The
guidelines of IEEE-519 regarding % voltage distortion limits are important and
practical. However, to ensure voltage regulation, sufficient system capacity
must exist. When the system is too small, there is no way to regulate the
voltage or pressure when current or flow are rapidly changing. To set limits of
how rapidly flow or current can change does not yield the kind of performance
that customers require and demand.
Including IEEE-519 as part of the specification should still be considered.
However, some rational thinking should be employed when certain types of AFDs
are specified. To raise customers costs by blinding specifying 12 step AFDs or
harmonic filters where neither is required to obtain reliable and trouble free
operation is not only bad judgment, but bad engineering. The solution is to dig
into the IEEE-519 “bag of electrical bad guys” and deal with the
ones that may cause problems. Discard those which will not create problems. To
continue to specify IEEE-519, without clarification, would be like specifying
earthquake proof and watertight equipment because those possibilities exist.
Don’t let the measured percent harmonic number become“the
problem” you need to solve, use the percent harmonic measured number as
part of a data base to show how your electrical system is changing. You are
likely to reach the limits of the electrical system long before you encounter a
problem with harmonics.
The correct method for sizing any motor system is to pick an input source (i.e.
transformer) suitable for the ac motor operated across the line (suitable for
inrush). Using an AFD will reduce the total rms current and never allow typical
inrush currents. Do not use PF correction capacitors within 250 electrical feet
of the AFD input. If capacitors are required because of other fixed speed
motors on the system, place a line reactor between the capacitors and AFD
Power Quality and the AFD Overview
H.G. Murphy - March 1997
In the early 1990's, harmonics became the buzzword in the electrical power
quality arena. Electrical disturbances, causing equipment shutdowns and damage
created a growing list of uncomfortable users of electrical power. Concerns and
questions, about overloaded neutral conductors, stray voltages and overheated
transformers, were often met with unacceptable answers and limited solutions.
In an attempt to define these seemingly new power quality issues, the
electrical utility industry began to search for reasons and solutions that
would help their customers. Since the problem was not a new problem but an
increasing problem, it was assumed that newly introduced or installed equipment
could likely be thecause. As the utility began to investigate power usage,
another equipment problem was identified. As part of the energy conservation
interest, more ac motor controllers (Adjustable Frequency Drives) were being
installed. As the use of AFDs increased so did the complaints about AFDs
shutdowns due to low or high line voltage conditions. This increase in
complaints coupled with overloaded neutral conductors, stray voltages and
overheated transformer concerns resulted in overstating the impact of
Adjustable Speed Drives (ASDs) became an easy target as those who hoped to make
a 'buck' and those who did not have a glimmer of understanding about
the wide variation among ASDs began swinging the 'Fear all Harmonics
Banner'. A hasty renovation of the 1981 version of IEEE-519 was rushed
through to meet the 'next 5 year' review time target. Everything but
the 'kitchen sink' was dumped into the standard with little thought
about how the lack of details would have a significant impact on increasing the
cost of transferring energy and reduce the reliability of motor control
systems. Harmonic Voltage Distortion clearly and correctly defined within the
1981 version of IEEE-519 was quickly overshadowed by a 'new' set of
recommendation for limits on Harmonic Current Distortion.
With today’s AFD proliferation, there are more concerns about other
factors in power quality. EMI, RFI and Harmonics have become frequently used
terms. The good news is the BPWM (PWM with series inductance) drives contribute
little to the negative side of Power Quality. Thebad news is that lack of
information and misleading information results in many potential users missing
the many benefits than can be obtained by applying AFDs. Fear and ignorance
prevailed and are supported by “snake oil peddlers”. Customers need
knowledge to assist in decision making.
There are changes within power distribution systems to be reckoned with
however, most changes will result because of poor grounding, poor wire routing
practices and momentary loss of electrical power.
This paper will help in understanding the nature of the type of power line
disturbances that are encountered in many installations and will aid in
correcting the occasional power problem that occurs. Of all power line
disturbances, Harmonics is the least understood and most feared. This paper
will present a broad overview of harmonics.
To begin to understand harmonics, 5 questions need to be answered.
What are Harmonics?
What do Harmonics do?
What are major contributors of Harmonics?
How to determine if a harmonic problem exists?
How to address a harmonic problem.?
What are Harmonics?
Harmonics is a term used to describe the shape or characteristic of a voltage
or current waveform with respect to the fundamental frequency in an electrical
distribution system. Harmonics are not a thing but a way to define current or
voltage. With the intent of efficiently transferring power from the electrical
system to loads such as motors, eliminating any reactive component of current
is a practical goal. Further ensuring that the characteristic of anyreactive
component of current is limited to a single frequency is also important.
Multiple frequencies or harmonic content can increase the heating losses that
were previously definable with a single frequency waveform. If no reactive
component of current exists, then the characteristic of the real component of
current can be reduced to a concern for the peak and rms values that exist in
the current waveform .
If no harmonics exist, then the waveform is described as the fundamental
sinewave. A pure sinewave of voltage supplied by the electric utility can be
described using a rms value and a frequency. For example: a 480 VAC, 60 Hertz
sinewave will have an ideal shape with an inverted image about the 180 degree point
and mirrored images about the 90 degree and 270 degree points. Its peak value
is 1.414 times the rms value with a time base equal to 1/60 Hz or 16.67
Basically its shape can be described by a single frequency. Because it is a
single frequency, its impact on the electrical system can be determined using
simple rules like E x I = Watts. When the waveform can no longer be described
by a single frequency, the waveform must be separated into individual,
definable, frequency components. Then each frequency component or harmonic can
be dealt with separately. A harmonic is a waveform with a multiple of the
fundamental frequency. A voltage with a frequency of 120 Hz would be the 2nd
harmonic in a 60 Hz electrical system. A 180 Hz waveform would be the 3rd
harmonic in a 60 Hz electrical system. A 300 Hz waveform would be the5th
harmonic. The 5th harmonic is normally associated with the reactive current in
inductive circuits like motors and transformers. 5th harmonic current are
considered negative sequence currents that produce negative power and should be
filtered out of the system. Care should be taken to insure that current that
looks like 5th harmonics are truly negative before using trap filters.
What do harmonics do?
A complex waveform makes the calculation of power and watt loss difficult. One
way to make those tasks easier is to take apart the complex waveform and make
individual waveforms, each with a single frequency. The simple rules can be
applied to each frequency to determine how transformers, motors, capacitors,
wires, and other components in the distribution system will behave.
After each individual waveform is analyzed for power and watt loss, the results
can be recombined to obtain the total power and watt loss. Complex waveforms do
not create more losses. They will determine how the simple rules are applied.
Operating a 100 watt linear light bulb consumes 100 watts. Operating a non
linear 100 watt light bulb consumes 100 watts. The complex current waveform of
the non linear light bulb does not create more losses. It merely transfers the
current in a shorter period of time. This means higher peak current, not more
current since the time is shorter.. This may change the rules about rms heating
associated with the current.
One useful document available to qualify waveforms created by power converters
and rectifiers is IEEE-519. This documentwas intended to establish recommended
levels of voltage distortion, where utility power enters a facility.
Recommendations within IEEE-519 are not mandatory but are guides to good
electrical practices. Current distortion recommendations, for injected current,
have been presented in IEEE-519 as benchmarks whose purpose is to limit levels
of voltage distortion. Using current distortion percentages, for the real
component of current, as a gauge to prevent voltage distortion only leads to
adding waste in equipment cost and energy usage. When the displacement power
factor in near unity, the only features of current that need to be addressed
are the maximum rms value and the maximum peak value. Distributing the current
demand over the full voltage cycle is a useful method to use to minimize
Harmonics exist in all electrical systems. Within a 3 phase electrical system,
a 3rd harmonic voltage waveform exists simultaneously with the three phases of
voltage. The 3rd harmonic voltage is a natural part of the 3 phase system. In
all 3 phase systems, the sum of the current in the system must be equal to
zero. All current in one phase must find a return path through one of the other
phases. If a return path does not exist, current will not flow. In the event of
fault, current will flow from the phase through the fault and return through
the neutral conductor. This is an abnormal condition, so the neutral conductor
is not normally sized to handle the fault current on a continuous basis. The
neutral conductor only handles fault current until thefuse or circuit opens. It
is not uncommon to find that the neutral conductor equal or smaller than the
conductor size used for each phase. It is commonly understood and accepted that
the neutral conductor does not normally carry current in a 3 phase balanced
linear load system.
Unfortunately, times have changed and more non linear loads are being applied
to electrical distribution systems. Most non linear loads have been computers,
TVs, VCRs, and electronic lighting. These loads are referenced to the neutral
so it is not uncommon for currents to flow in the neutral because there is no
When the system is supplied by a delta source, some current could circulate in
the delta. When the system is supplied by a Wye source, then current will flow
in the neutral conductor. With 3 phase equipment like AFDs, no load current
will flow in the neutral because all current flows between phases. Leakage
current may flow in the neutral because of capacitive coupling that exist
between conductors and from motor windings to ground.
One of the concerns about what harmonics do is their impact on the neutral
conductor. It has been stated that the 3rd harmonic causes neutrals to become
overloaded. Neutrals overheat because they carry more current, not harmonics.
With balanced linear loads, in a 3 phase circuit, the neutral is not used as
the return conductor. With single phase non linear loads, the neutral is always
used as the return conductor. Balancing non linear loads, in single phase
systems, will not prevent the neutral from being used as thereturn conductor.
This means that the neutral of a distribution system must be sized to handle
the square root of the sum of the squares of the phase current.
Assume that each phase of a 3 phase system is providing non linear loads with 5
amps. The neutral must be sized to handle the square root of (52 + 52 + 52) or
the square root of 75 or 8.67 amps. The increased sizing of the neutral is
required because the non linear current pulses are displaced in time and can
not find a return path through the loads on other phases. This results in 6
current pulses in the neutral conductor. It is real current that is flowing
through the neutral, not 3rd harmonic current. It may look like the 3rd
harmonic but it is displaced in time from the real 3rd harmonic.
These same pulses of current flow between phase to phase in a three phase
system. Therefore the neutral is not required to carry any current with a 3
phase load. Only single phase loads that use the neutral as the return
conductor required oversizing of the neutral conductor. Some most electrical
distribution systems provide power for single phase as well as 3 phase load,
increasing the size of the neutral conductor is mandatory.
The pulses of current in a 3 phase system, with a 3 phase non linear load,
compresses the time when current flows to about 2/3 of the normal time. The
type of 3 phase load is important regarding the shape of the pulse. In
converters used with DC motor controllers, arc furnaces and Current Sources AC
drives, the control of speed or current is accomplished by reducing
theconduction time of the converter by phasing back the Silicon Controlled
Rectifiers (SCRs) used in the converter. By phasing back the SCRs, the shape of
the pulse will become very narrow or extremely compressed so that a high value
exists for a short period of time each cycle. These types of converters also
have a displacement power factor that is low. This means that a high value of
reactive current can exist. The existence of reactive current plus the harmonic
nature of the current waveform means that some hot spot heating may exist in
transformers built to 60 Hz rules. Transformers that have a K factor rating
greater than 1 are built differently. These transformers are designed to let
the heat caused by load currents exit the transformer more effectively.
Typically flat or rectangular wire is used instead of round wire. An improved
winding configuration is used to reduce hot spot heating.
When the converter type of a full wave, 3 phase bridge operating without
phasing back the SCRs or power diodes, then a near unity displacement power
factor exists. When the voltage and current in phase, there is not reactive
current injected back onto the distribution system. Since current flows while
the voltage is at its peak value, it takes less current to produce power than
would be the case if current was flowing when the voltage was at a lower value.
In general, a transformer sized to handle an ac motor operating across the
line, will not experience any additional loading when an adjustable frequency
PWM drive is used to control the speed of themotor. When a reactor exists,
either externally or internally, the peak of the current pulse is reduced to
within a reasonable value of the rms current required to transfer power to the
motor load. Without a minimum of 3% reactance in series with the flow of power,
the peak value of the current pulse can create nuisance problems. The pulses of
current can place more stress on line fuses. The fuse element will respond to
peak current and will fatigue over time. Magnetic circuit breakers may nuisance
trip because of these higher peak currents. Although the rms value of the
current has not changed, the 60 Hz rms rules for component behavior will
Power factor correction capacitors can be affected by the pulse currents
associated with non linear loads. The capacitor is a friendly component willing
to provide all the current demanded by the load. Since its impedance is low, it
does not restrict how much or how rapidly current leaves the capacitor on its
way to the load. The capacitor can not differentiate between real current and
reactive current. The characteristic of the load determines the type of
current. Although correctly sized to handle the reactive component of current,
power factor correction capacitors can not take on the additional requirement
for the real component of current.
Power factor capacitors are designed and sized to handle reactive currents.
Reactive current is current that is of a different polarity that the supply
voltage. When the voltage polarity changes, the power factor capacitor begins
to supply the load current.With most non linear loads, reactive currents are
supplied by the non linear control device. The use of power factor capacitors
and non linear load types like PWM AFDs are not well matched. The near unity
power factor of PWM type AFDs eliminates the need for power factor capacitor
located on the input of the AFD. Power factor capacitors can still be used at
the inputs of line operated motors as long as there is inductance between the
capacitor and the input of the AFD. Some capacitor manufacturers recommend a
minimum of 250 electrical feet between the capacitors and the input terminals
of an ASD..
Essentially, power factor capacitors function as kVA devices rather than as
kVAR devices, when used in AFD circuits. This means that fuses protecting
capacitors will likely nuisance blow and capacitors may get hot.
What are major contributors of Harmonics?
Harmonics or complex current waveforms are caused by any reactive or inductive
load and by any product that uses a rectifier to convert AC voltage to DC
voltage. Single phase equipment like TVs, VCRs, computers, electronic lighting
all convert AC to DC. Three phase equipment like Electric Arc Furnaces,
electric heaters, DC drives, Welders, Uninterruptable Power Supplies, AFDs all
convert AC to DC. These types of products are referred to as Power Electronic
equipment. Their characteristics vary with the type of equipment. The major
contributor to harmonics or peak currents would be equipment that using SCRs as
voltage control devices in a high current or high torque applications where the
reactivecomponent of current is high.
When comparing loads, do not assume that single phase loads are too small to be
of concern. With AC to DC converters, the demand current occurs around the peak
or high point of the voltage sinewave. A thousand 100 watt lights consumes 100
kW of power. If the lights are non linear type loads, then the peak of the
currents will add directly. This can cause a dip in the voltage waveform. If
the dip in the voltage waveform drops below the threshold voltage for
electronic lights, flickering will occur.
How to determine if a harmonic problem exists?
In most applications with PWM AFDs, there will be no harmonic problems. With
other converters loads such as arc furnaces, dc drives, current source drives
and other high reactive current loads, harmonic problems may exit Some of the
following problems may indicate a harmonic condition but may also indicate line
voltage unbalance or overloaded conditions.
Nuisance input fuse blowing or circuit breaker tripping
Power Factor Capacitor Overheating, or fuse failure
Overheating of supply transformers
The problems listed above will be more common on single phase systems.
Problems that are not harmonic problems are:
Overheating of motors
Overcurrent tripping of AFDs
Interference with AM radio reception
Motor failures or insulation breakdown
Wire failure in conduits
Harmonic problems are rare. There have been no documented harmonic problems
with PWM AFDs that use a series reactor in the dc bus or in the input ac line.
There have been many problems blamed on harmonicsbut investigation resulted in
identifying some other cause. There have been overheated neutrals but this has
been with single phase, non linear loads where the neutral conductor was sized
to the 60 Hz, linear load rules.
How to address a harmonic problem?
If a true harmonic problem occurs, the solution is to change the
characteristics of the distribution system. Changing inductance, resistance and
capacitance will change the way the distribution system behaves. Adding filters
is sometimes recommended. This method should be done with great care since
filters will act as a preferred path for all currents in the system. The filter
could easily be damaged by outside sources that are added later or were not
identified while attempting to solve the problem. It should be noted that tuned
filters are normally used to trap or divert negative sequence (negative power
producing) currents. Magnetizing current for transformers and motors can create
5th and 11th harmonic currents that are negative sequence currents.
In severe cases, it would be good practice to insert tuned filters to create
alternate paths for these currents. The converter or 6 pulse rectifier circuits
of DC drives, Current Source Drives, and Arc Furnaces can also contribute to
5th and 11th harmonics that, under some cases contain negative sequence
currents. This type of equipment, not PWM type AFDs, may cause problems if a
problem is going to occur. With PWM type AFDs, all currents on the input are
positive power producing currents. It would not be good practice to insert
tuned filtersto divert positive power producing currents from their intended
load, the motor.
A safe assumption is that harmonic problems, with PWM type AFDs, do not exist.
Peak current problems may exist. The only solution to peak current problems is
to reduce the current peak with inductance or to time shift individual peak
currents so that they do not add directly. Using multiple voltage sources with
different phase shift will reduce the accumulation of peak currents from non
Derating the supply source as the non linear load content increases will
prevent overheating and overloading problems. With DC drives, the input
transformer should be derated by at least 30%. Using PWM type AFDs without line
reactor or a dc bus reactor would require a 20% derating of the input
transformer. A 10 % derating would be recommended when using a PWM type AFD
that uses either ac line reactors or contains a dc bus reactor.
There are many more details covering the topic of harmonics. For this overview
the important thing to remember is that injected current harmonics do not exist
with PWM AFDs and should not be the issue, rms and peak current are the issue.
Using Harmonics as the only method to describe the shape of the current or
voltage waveform only leads to confusion. Real current can cause instantaneous
voltage changes, however the reactive component of current increases the demand
on the distribution system. This greater demand will result in more voltage
distortion. How much the voltage distortion impacts the other components on the
distributionsystem will depend on the design of those components. Derating
components may be required on some systems. Relocation of other components may
be required. For the majority of installations, there will be no harmonic
related problems. The greatest problem will continue to be the problem of
overcoming the fear and lack of understanding that consultants have of
harmonics. Specification will continue to carry the requirement for meeting
IEEE-519. Fortunately, in most installations, the recommended voltage
distortion limits defined in IEEE-519 can be meet. Using PWM adjustable
frequency drives results in no injected or reactive currents reducing the
losses in the distribution system and increasing the efficient use of
electrical power for the control of ac motors.
Understanding Neutral versus Ground
Last Revision - April 3, 1998
Understanding Neutral versus Ground when applying RFI filters in electrical
The grounding of electrical equipment is probably one of the lest understood
aspects of electricity. As the characteristics of electrical equipment changes
from linear to non-linear, the nature of grounding expands from the task of
insuring the safety of personnel to insuring that one type of electrical
equipment does not interfere with other types of electrical equipment. One
point for confusion rests with the often interchanged terms of Neutral and
Ground. Many articles have been written concerning the problems with 3rd
harmonics overloading the neutralconductor. Many articles have been written
concerning the problem of electrical ground noise. Even with all these
articles, there still exists confusion concerning whether equipment should be
connected to the neutral or connected to a ground.
It may be possible that a simple rule would clarify the differences between
Neutral and Ground.
It can be stated that Neutral can be grounded, but Ground is not neutral.
A Neutral represents a reference point within an electrical distribution
system. Conductors connected to this reference point (Neutral) should,
normally, be non current carrying conductors, sized to handle momentary faults
(short circuits) occurring in electrical equipment. However, with the
introduction of non linear loads, such as computers, electronic lighting, TVs,
VCRs and other switchmode power conversion equipment, the requirements for the
neutral conductor has changed (increased).
A Ground represents an electrical path, normally designed to carry fault
current when a insulation breakdown occurs within electrical equipment. (Note:
Breakdowns can be forced by connecting (dropping) a metal tool or conductive
material from a voltage potential to the steel structure within a facility.)
Connections to the electrical path (Ground) are made convenient for the
installation of electrical equipment. Some current will always flow through the
ground path. This current will come from a number of normal sources. Capacitive
coupling and Inductive coupling between power conductors and the ground path
(conductive conduit, conductive structuremembers, etc) are the greatest sources
of ground path current.
Among the many types of distribution systems, the 3 phase, 4-wire, 480/277 V
system used in commercial centers and large buildings is very common. It is
used since it enables 3 phase ac motors to operate at the 480 V level while 120
V fluorescent lighting operates with the primary of a stepdown lighting
transformer connected to the 277 V (line to neutral) potential. The secondary
(120 V side) of the lighting transformer has one of its terminals connected to
ground. This grounding procedure is done to reduce the possibilities of shocks
due to an internal fault in the transformer. The grounded terminal of the 120 V
lighting supply is often referred to as the lighting neutral. This IS NOT the
true Neutral of the distribution system. Although this point within the
lighting system is grounded, it is unlikely that a short in any lighting
equipment on that branch will ever see current returned to the true neutral
within the distribution system.
In a 3 phase low voltage distribution system, the preferred installation should
consist of a five wire system. That 5 wire system would consist of, 3 phase
conductors, a neutral conductor and a separate ground conductor. In normal
practice, the ground conductor is often the building ground consisting of the
metallic building structure. Although this type of ground is usually suitable
for 60 cycle leakage and fault currents, it is not suitable for leakage
currents that exist when non linear loads such as computers, electronic
lighting, variablespeed drives and other equipment using internal switch mode
power supplies and other types of conversion rectifiers are used. The current
caused by non linear electrical equipment consisting of low amperage high
frequency currents. These currents are often measured incorrectly by 60 Hertz
sensors and mistakenly interpreted as higher amperage 60 Hertz values. This
occurs as a higher voltage develops across the sensor whose impedance increases
as the frequency increases.
The previously mentioned equipment creates rapid changes in voltage and current
while transferring energy from the distribution system to the equipment load.
These changes cause currents to flow through capacitive paths that exist
between phase conductors and between any phase conductor and ground conductors.
These currents have high frequency characteristics which results in a phenomena
not unlike the results from a small radio transmitter. Very little power is
required to create magnetic field which can transmit a Radio Frequency
Interference types of electrical noise to other equipment. Usually other
equipment, in metallic enclosures is not affected by these small radio-type
signals, however, some equipment circuitry may be affected. The typical
solution is to add RFI filters in the incoming power lines to the equipment
causing the condition. These RFI filters, like other electrical equipment,
require grounding. The normal grounding practice is to connect the RFI filters
to same ground point used by the equipment causing the condition. This practice
is suitable when thatground point has a high frequency low impedance path to
the equipment creating the condition. It would not be necessary to install a
separate conductor back to the Neutral reference point in the electrical system
as long as a high frequency, low impedance path exists from the gounding point
to the equipment ground.
Unfortunately, too many variables exist within any grounding system. However,
if standard grounding practices are replaced with high frequency grounding
practices (shielding and low impedance ground paths), it is unlikely that
electrical ground noise problems will occur. Radio Frequency Interference types
of electrical noise will always create problems for AM radios, which are
designed as very sensitive electrical noise detectors. Fortunately, most types
of electrical equipment are designed, and tested, to be insensitive to RFI
generated noise. The general practice should include discussions with equipment
suppliers to determine if and what types of electrical interference affect
their products. Suggestions regarding installation practices should exist and
may be suitable in eliminating any potential problems.
Susceptibility to Current Distortion
Last Revision - March 20, 1998
Effects of Current Distortion on electrical equipment
There seems to be concerns about the effects of current distortion on
electrical equipment. This is partially due to the fact that incorrect
information about the dangers of current distortion are being provided to users
of electricalequipment. Some of this misleading information is due to a lack of
knowledge on the part of the provider of the information and some is due to
methods intended to frighten users in purchasing equipment which reduces
current distortion. Equipment that is not required for proper performance of
The fear of Current Distortion stems from a lack of knowledge of basic
electricity. Current, in the forms resulting from most nonlinear motor loads,
cause RMS heating. More current causes more heating. Distorted current, with
the same RMS value as undistortion current causes the same heating. Some would
profess that distorted current causes more heating than undistorted current
with the same RMS value. This is completely false. The worst results is that
heat losses may shift to different locations, but the total system heating is
Another Myth about Current Distortion is that equipment can be damaged by large
percentages of Current Distortion. Percent Current Distortion does not define
the RMS value of the current. It merely defines the RMS versus the crest or
peak. The fact is that Current Distortion is a result of a variable demand
load. This variable demand load causes current to flow from a source to the
load in a non-sinusoidal fashion. If the RMS demand from the load is the same
as if a sinusoidal load were present, the results are equivalent. No additional
RMS current will be present. Some would profess that electrical sources such as
Power Factor Correction Capacitors and Transformers will be damaged by CurrentDistortion.
This is also FALSE. Capacitors and Transformers will be damaged when the
current that must be handled exceeds the value for which they were designed.
Some profess that Current Distortion consists of Harmonic frequencies which
cause more heating in electrical equipment. They take the basic facts and twist
them to fit the misinformation presented to the user.
If it is true that higher frequencies will cause more heating than it must be
true that harmonics which are higher frequecies will cause more heating
therefore you can only protect yourselves from the dangers of current
distortion by purchasing this distortion protector product.
What they fail to explain is that the higher frequencies have insufficent
energy to result in significant additional heating. The overall RMS value is
often no greater than a non distorted current waveform.
Another Myth is that Current Distortion should be measured at the input
terminals of the equipment under question. This is FALSE and professed by some
to convince the user that their products are not only better but must be used
to meet the recommendation of guides such as IEEE-519. Current distortion will
not and can not directed affect any piece of properly sized equipment on the
electrical distribution system. Current distortion can caused Voltage
distortion. It is the Voltage distortion which provides current for the loads
of the electrical system. In some cases, the Voltage distortion is of a type
which affects other electrical equipment. This is not usual but can occur when
the electrical system issmall compared to the connected loads. Voltage notching
is an example of the type of distortion which affect equipment.
Sizing the electrical system to provide power to its loads requires some
thought. Sizing the loads to the exact kVA capability of the source is not a
good practice. For example, with a 100 kW motor operating across the line, it
would be bad practice to supply the motor with a 100 kVA transformer. A better
practice would be to supply a 150kVA (minimum) transformer to insure that
sufficent power was available to support the locked rotor currents required to
allow the motor to achieve full speed. Power factor correction capacitors sized
to handle the reactive requirements (kVAR) of the 100 kW motor would not be
required if the motor were operated on a PWM, variable frequency drive. The
capacitors would be required to handle the kVA required not the kVAR
requirements if they remained connected to the input side of the variable
frequency drive. The capacitors would be too small to handle the kVA
In general, the percentage value of Current Distortion is no indicator as to
whether the electrical system is properly sized or loaded. It is the Voltage
distortion, or more correctly, the type of Voltage distortion, not the
percentage which determines if a problem might exist. Large values of percent
Current distortion can not cause problems unless the total RMS demand on the
electrical system exceeds 90% of the electrical system capability, on a continuous
basis. If generator souces are used, the generator should be sized atleast 200%
more than the peak demand of the connected load. Utilities typically use a
value of 20% current distortion as an indicator to determine when Voltage
distortion may reach a value where problems may occur. However, it is the type
of Voltage distortion (actual waveshape) not the percentage of Voltage
distortion which may result in harmonic problems. Typical PWM adjustable
frequency drives are not affected by Current distortion or Voltage distortion.
SYSTEM COMPATIBILITY of ADJUSTABLE FREQUENCY DRIVES
Howard G. Murphy P.E.
Last Revision - March 22, 1997
The characteristics of Adjustable Frequency Drives, as an electrical system
component, are not well understood. They have been blamed for almost every
electrical problem that can not be explained. It is true that AFDs have
characteristics not typical of more conventional components like motors,
starters, and transformers. Fortunately, the most common AFDs of the
1990’s are very easy to understand. By far, the largest installed base of
AFDs is the Pulse Width Modulated, voltage source type. The unique features of
the PWM, AFD can be defined and with those definitions, successfully applied
for process improvements and for energy savings.
For simplicity, four categories will be explained.
As a source for the ac motor (Energy Transfer component).
As a load for the distribution system ( Energy Consumption component).
As a load for the ac motor (Energy Absorption component).
As a source for the distribution system(Energy Storage component).
Energy Transfer component
As a source for the ac motor, the task of the AFD is to connect to the terminal
of the motor a voltage potential that can provide a positive or negative
polarity. The positive polarity will allow current to flow into the motor from
the AFD. The negative polarity will allow current to flow from the motor to the
AFD. When polarity changes, in a symmetrical fashion, the rate at which the
change occurs becomes the applied frequency for the connected voltage
Since the function of the AFD is to control 3 phase ac motors, a successful
operation depends on the ac motor experiencing a voltage source that provides
the following characteristics.
A symmetrical waveform (mirrored imaged)
A balanced waveform (+/- 2%)
A low impedance source (less than 1%)
A 3 phase source (120 degree displacement)
A controlled volts/hertz relationship through the speed range. The area under
the voltage curve must be fixed to the frequency. A 480V, 60Hz source and a
240V, 30Hz source have the same volts per hertz relationship. The relationship
is 8 volts per hertz.
When connecting an ac motor directly to the ac supply line, the utility supplies
a voltage that meets the listed characteristics. The resulting current waveform
will, for the most part, produce a symmetrical waveform. The torque (or work)
that the motor is able to produce is proportional to that current waveform. For
a fixed terminal voltage (supplied by the utility), the rms value of the
current will indicate how much work the motor isperforming.
When connecting an ac motor to an AFD, the resulting current waveform is also
symmetrical. As the polarity of the terminal voltage changes, the frequency of
the current waveform will change. The current produces the magnetic fields
whose interaction produces the motor torque and results in the rotation of the
motor (rotor). Changes in motor rotation stop when one of two factors occurs.
The first, and most important, occurs when there is insufficient voltage to
produce additional current that produces the torque necessary to change
rotational speed. The second occurs when the rotational speed of the rotor
matches the speed of the rotating magnetic field (applied frequency).
There is no significant difference between the current in the motor operated
directly across the ac line or connected to the output terminals of an AFD. The
real current circulating in the motor always comes from the utility or power
source connected to the input terminals of the AFD. The AFD merely connects the
input source to the terminals of the motor. The AFD does not produce power or
store power. The AFD is simply a connecting device that allows a limited amount
of power to flow from a source to a load. The motor circuit carries real and
reactive current. Unlike the motor operating directly across the line, the AFD
operated motor does not allow the reactive current to flow in the incoming
distribution system. The reactive portion of the motor current will flow in the
output (inverter) section of the AFD. Only real current flows from the incoming
distributionsystem through the AFD and into the motor. This single
characteristic of PWM, voltage source AFDs means that there will always be
fewer system losses when operating an ac motor with an AFD compared to an ac
motor operated directly across the line.
Energy Consumption component
As a load for the distribution system, the PWM, voltage source AFD will permit
only a limited amount of energy transfer. Unlike other types of loads, like
motors and transformer, the short circuit conditions that can exist with AFDs
are much, much less than typically found in distribution systems. Protection
from short circuit demands is normally controlled by fuses or other circuit
protection devices. An important feature of the AFD is its current limiting
As a connecting device, the AFD can interrupt the flow of current in
microseconds. This means that no high rms values of fault current will ever
exist. The connecting semiconductor switch operates as a extremely fast
recloser for values of current within the AFD rating. The semiconductor switch
operates as a manually resettable circuit breaker when the instantaneous value
of current exceeds the trip limit of the AFD. It is rare that fuses or other
protective devices would function faster than the electronic current limiting
that exists in AFDs.
As a result of the current limiting feature, the short circuit current
let-through is well below the typical values defined by branch circuit
protection found with fuses and circuit breakers. In practice, and for
simplicity, branch circuit protection for AFDs isselected as if the circuit
were a standard motor circuit. This is very conservative, but less confusing
than attempting to redefine local electrical codes. When input fuses are
specified by the AFD supplier, those fuses have, most often, been recommended
to conform to some testing criteria such as UL.
When the AFD, as a load, is compared to the ac motor, electrical parameters
such as power factor and surge currents are more favorable than expected. A
typical ac motor may have a power factor of 0.8. The AFD controlling that same
motor would have a power factor much closer to unity. That difference in power
factor means that less current will flow in the distribution system. Less
current means lower system losses or greater system efficiency. The losses in
the AFD are similar to the losses experienced by the voltage drop across the
contacts of a motor starter. As the current increases, the voltage drop will
increase. As the current decreases, the voltage drop will decrease. In some
applications where power reduces as speed reduces (fans), the efficiency of the
system can improve significantly.
Energy Absorption component
As a load for the ac motor, the AFD is expected to absorb some energy when the
mechanical energy is converted by the motor to electrical energy. This occurs
when the motor rotates faster than the applied frequency. The motor, now acting
as a generator, will force energy (current) back to the AFD. Since the standard
AFD will transfer power in one direction only, the energy supplied by the motor
must be absorbed or that energywill cause the AFD to shut down on an over
Three factors must be considered when the motor is operating as a generator.
The first is that the motor will operate as a voltage source, supplying current
to the AFD as long as the rotational speed of the motor is greater than the
synchronous speed of the motor. The second factor is that no generator action
can occur unless there is a magnetic field in the motor. Unlike a dc motor
which has a separately excited field, the ac motor depends on the operation of
the AFD to maintain the field. If the AFD sends no supply voltage to the motor,
then the motor can not send energy back to the AFD. The third factor is that
the AFD will rectify the ac voltage being supplied by the motor and store that
voltage in the filter capacitor (dc bus) of the AFD. Since the AFD can not send
that motor voltage back to the distribution system, rectification will continue
until the voltage being stored in the AFD filter capacitor exceeds the over
voltage limit. The amount of energy that can be stored is very small, so the
limit is reached very rapidly. When the limit is reached, the AFD will shut
This shut down will result in the AFD being disconnected from the ac motor.
Since the ac motor is disconnected, the magnetic field of the ac motor will
disappear and the motor can no longer operating as a generator. The motor will
coast to a stop unless the AFD fault is cleared and the AFD is restarted. To
prevent a shutdown of the AFD when the motor is operating as a generator
(regeneration), the voltage beingstored on the filter capacitor can be
controlled by discharging that stored energy through a power consuming path.
Typically, an electronically switched resistor is connected across the filter
capacitor of the AFD (Dynamic Braking Module). When the voltage on the
capacitor reaches a present value, the resistor is connected across the filter
circuit. When the voltage drops below a lower present value, the resistor is
disconnected from the filter circuit. This action can continue until there is
no more energy to dissipate and the voltage on the filter capacitor remains
below the present value.
Energy Storage component
As a source for the distribution system, the standard PWM, AFD is prevented
from sending energy back to the distribution system. This is due to the 3
phase, full wave bridge rectifier on the input circuit of the AFD. In the event
of a fault on the input to the AFD, no energy can flow from the AFD to the
distribution system. Unlike other power converters like dc motor controllers,
Current Source Inverters and arc Furnaces, the PWM, AFD can only transfer
energy from the input supply to the motor.
Per IEEE standard 141-1993 (Redbook), section 4.2.5, adjustable speed drives
can contribute current from the motor to a short circuit . Based on the
preceding statement, it would be nature that the following questions would be
raised. What percent FLA of the motor load would adjustable speed drives allow?
What are the drive’s let through values? Are these maximum permissible
short circuit values indicated on the drive’s label? Is themaximum rated
short circuit number and interrupting rating for the drive indicated on the
Other power converters can feed energy back to the distribution system, however
that energy can be controlled and in most cases can only contribute a very
small portion of energy to the fault condition when comparing to the energy
that can be provided by transformers and line operated motors. In all cases
where AC drives or DC drives are applied, the value of rms current that is
transferred from the ac supply to the motor or from the motor to the ac supply
will be limited to the maximum ratings of the AC drive or DC drive.
Of VSDs, only DC drives and Current Source Inverters (AC Drives) have the
natural capability to feed current back to the ac line and thus contribute to
shorts on the ac line. However, in the case of short circuit on the line side
of the drive, it is most likely that these drives would detect that condition
and terminate operation.
In terms of maximum permissible short circuit values, only the DC drive lacks
the capability of electronically controlling the instantaneous value of the
current. The DC drive is typically a line commutated, 3 phase rectifier which
connects the ac line to the armature of the dc motor. With a 60 Hz supply, the connections
from the ac line to the motor can be commutated (turned off) every 8.3
milliseconds or approximately one-half cycle. Since the dc motor is providing
the current, the question of how much current will the dc motor contribute to
the line short will depend on the motor’s abilityto increase in armature
current during that 8.3 milliseconds. In the event of a short on the ac line,
the dc motor can continue to send current out of the motor back to the shorted
line as long as the field of the dc motor is energized and the motor is
rotating. Due to the variations that would exist, it would be difficult to
predict a maximum value. In practice, the characteristics of the incoming fuses
are used to define the maximum let through value. The I^2T rating of the fuse
defines when the fuse should open.
In Current Source AC drives (CSIs), the current path can be controlled
electronically. The inverter or output section of the CSI can interrupt the
flow of current and limit the let through current. Once again, it is likely
that the CSI will detect the fault condition on the ac line and terminate
operation, thus limiting how much current flows back to the ac line. Like the
DC motor, the ac motor used with a CSI drive must maintain it’s magnetic
field if it is to send current back to the ac line. Unlike the dc motor which
has a separately excited field, the ac motor depends on the CSI drive to
electronically maintain its magnetic field. A major difference between the DC
drive and the CSI drive is the large reactor that exists between the ac line
and the ac motor in the CSI drive. This link reactor will prevent current from
changing rapidly and will limit the current that the CSI drive can contribute
to the ac line fault. In practice, it is normal to use the I^2T value of the
line side fuses to define the maximum let through current.In actual operation,
the value of current that can be contributed by a CSI drive is less than the
FLA of the motor and will not be sustained for more than one half cycle of the
With PWM, voltage source AC drives (AFDs), no energy can be returned to the
distribution system unless the AFD is a special line Regenerative type drive.
In the event of a short circuit on the ac line, the input supply voltage to the
drive is reduced to a single phase supply. Since the motor is buffered from the
conditions on the ac line by the filter section of the drive, it continues to
operate in a controlled manner. Current will not increase in the motor since
the motor believes that nothing has happened to it’s voltage supply to
cause any changes. If motor current does not increase, than there will be no
change reflected to the incoming supply line and thus no contribution to the
short circuit condition on the ac line.
Some types of adjustable speed drives can contributed to the fault current on a
ac line short. PWM adjustable (frequency) speed drives do not contributed to ac
line shorts. The typical PWM, AFD is designed to transfer power from the ac
line to the ac motor. In doing so, improvements in power factor result which
reduce the value of total current that the distribution system must handle.
Energy flow is a one way process so PWM AFDs can not add energy to faults that
occur on the distribution system. By optimizing the voltage applied to the
motor for many applications, a general reduction in total energy consumption
willoccur. In many applications, modifying the operating parameters of the
application can result in overall energy savings.
SYSTEM COMPATIBILITY of ADJUSTABLE FREQUENCY DRIVES
Howard G. Murphy P.E.
Last Revision - April 25, 1997
This paper is an evaluation of the System Compatibility Report (July 1996)
released by the Electric Power Research Institute, Power Electronics
Application Center as an approach to meet the objectives of the project SC-610:
Performance Criteria for System Compatibility of Adjustable Speed Drives Used
in Low Voltage AC Power Systems.
The principle objectives defined within the report are: Establish and define
baseline squirrel cage induction motor/ASD performance versus electric utility
expectations of product capability.
Based on the resulting documentation of ASD capabilities and expectation, the
project goal was to facilitate a dialog between electric utilities and ASD
product manufacturers which could result in enhanced product system
compatibility and therefore improved reliability to end-users.
The tests covered within the context of the report concentrate on ASD
characterization relating to that portion of the system more directly related
to the interests of the sponsoring electric utilities. This report did not
address issues relating to the suitability of motors used with ASDs for topics
such as reflected wave and motor bearing issues or for electrical noise
The categories for which tests performed within thecontext of the report
consisted of the following:
This test measured the following parameters of the AFD test unit(full load at
Input phase to phase voltages
Input phase currents
Input Voltage % Total Harmonic Distortion
Input Power factor
Input Line Current Harmonics
This test measured the following parameters of the AFD test unit(full load at
Input Current % Total Harmonic Distortion
Line Voltage Unbalance
This test measured the following parameters of the AFD test unit(full load at
Input phase currents with input voltage unbalances of 0.6%, 1.3% and 2.4%
Input Current % Total Harmonic Distortion with same unbalanced input voltage
Low Steady-State Input Voltage
This test measured the following parameters of the AFD test unit with the ac
motor operating at full load at base speed with 5% and 10% steady state voltage
DC Bus Voltage
% Slip Full load
Ride-through response for 6 cycles and 30 cycles voltageLL sag
This test measured the following parameters of the AFD test unit(75% load at
DC Bus voltage
Response to Capacitor Switching Voltage Transients
This test monitored AFD response to transients on the input voltage source
caused by the connecting of power factor correction capacitors to the system.
An evaluation for each test is detailed below.
This test was primarily intended to verify the each AFD operated. The numerical
values resulting from the test provided no useful information since no ideal or
expected values were defined which could be used as a comparison.
Input Line Current Harmonics
This test was performed using a soft and stiff voltage source. The numerical
values resulting from the test provided no useful information since no ideal or
expected values were defined which could be used as a comparison. The results
indicated that the source characteristics had a large influence on the shape of
the current waveform and that additional series impedance, such as reactors,
would reduce the peak value of a current pulse. The test provided little
information concerning injected (reactive) current waveforms or forward power
producing current waveforms. The test supported the misconception that any
distortion of the current waveform classifies as harmonic current distortion
whether injected back into the electrical system or not.
Line Voltage Unbalance
This test showed that a line voltage unbalance will result in an input current
unbalanced. It also indicated the importance of series impedance as a method of
correcting some level of unbalance. The numerical values resulting from the
test provided no useful information since no ideal or expected values were
defined which could be used as a comparison.
Low Steady-State Input Voltage
This test showed that the AFD aids in minimizing the effect on the motor when
the electrical system is subjected to input voltagereductions. The numerical
values resulting from the test provided some useful information since general
knowledge exists for the impact on motor when exposed to a reduced line
Ride-through response for 6 cycles and 30 cycles voltageLL sag
This test was primarily intended to verify how the AFD responded when exposed
to a temporary reduction in input voltage. The criteria of 50% motor speed was
used as the limit or evaluation point. The numerical values resulting from the
test provided no useful information since the 50% operating motor speed limit
does not relate to the needs of the process or to the behavior of other system
components. Using the speed of the motor as the single criteria for evaluation,
assumed that motor speed or the lack of it determines a successful process.
There was no consideration of safety considerations or other functions within
the process. The single judgment point was loss of production due to a
significant loss in motor speed. Based on the concerns of sponsoring electric
utilities about their potential liability to electrical power users for loss
production in the event of a power outages, it is obvious that this test was
conducted in a narrowly focused manner.
Response to Capacitor Switching Voltage Transients
This test showed the AFD response to transients on the input voltage source
caused when capacitors are connected to the ac line to correct power factor.The
numerical values resulting from the test provided no useful information since
no ideal or expected values were defined which could be used as acomparison.
The System Compatibility Report (July 1996) provided an excellent example
describing the results of many hours of organizing, equipment construction,
testing and report generation. However, the report provided little useful
information for the electric utilities or for AFD users who may have the
occasion to see the report or hear third-hand tales about the report.
It is likely, however, that some unscrupulous individuals may use this report
as a qualified resource to promote self interests which will ultimately hurt or
at least mislead utilities and AFD users to make inaccurate decisions about equipment
selection and installations. There is only one thing worst than bad
information, and that is misinformation or technically limited propaganda.
There are always concerns about using new technology in the control process.
Provided partial information or recommendations to potential users about what
to consider when evaluating the use of new technology makes any decision more
difficult when results do not agree with the recommendations provided. Full
information is required in order to provide credible recommendations. Anything
less is potentially harmful
Application of Transformers and Reactors to AFDs
H.G. Murphy P.E. Last Revision - May 1, 1997
AC Line Reactors and DC Link Reactors
Pulse Width Modulated AC drives have become the standard for 3 phase ac motor
speed control. The PWM drive is used in the majority of applications. With a
greater use of the PWMdrive, some questions have been raised concerning their
influence on the electrical distribution system. Where large numbers of PWM
drives have been installed, there have been some problems with the Power
Quality of the electrical service.
A solution to many of those problems has been to add ac line reactors to the
input of the PWM drive. This solution has overlooked the fact that some PWM
drive have internal dc link reactors that perform in a manner which, generally,
eliminates the need for additional ac line reactors. The majority of small
horsepower PWM drives do not contain internal link reactors. When an internal
reactor exists, the addition of ac line reactors can reduce both the efficiency
and performance of the PWM drive and ac motor.
The following diagrams show the differences between a basic PWM drive with
external ac line reactors and a BPWM or buffered PWM drive containing an
internal link reactor.
DISPLACEMENT POWER FACTOR for PWM type AC drive is near unity. All current
flowing from the Utility supply into a PWM type drive is real current. There
are no negative or zero sequence current components.
Normally energy from the ac line is transferred to the ac motor in the form of
a sinewave of current. That current wavefrom contains real and reactive
components. The reactive component is responsible for distorted current that is
injected back into the distribution system. It is the injected or reactive
current that cause the supply voltage to increase. The reactive current may
cause harmonic type problems.
Becauseenergy from the ac line is transferred, through an ac drive, to the ac motor
in the form of a pulses of current, there is concern that DISTORTION POWER
FACTOR will create harmonics that may cause problems. With a PWM ac drive the
input current contains only a real component. There can be problems caused by
the peak current of the pulse of current if that value exceeds the capability
of the electrical system. It is important to reduce the value of the peak
current. This is accomplished by inserting 3 to 5 % impedance in series with
the power flow.
The use of an internal dc link reactor or choke provides the 3 to 5% impedance
to reduce the peak of the current pulse. An equivalent method would be to
insert individual 3 to 5% reactors or chokes in each ac line. An ac line
reactor must be larger since the current in 3 phases must be controlled. Each
phase element of the ac line rector works part time since current does not
exist in a continuous manner. With an internal dc link reactor or choke is
applied in the dc link section of the PWM drive, that reactor can be used full
time to control current since, at full rating, the link reactor will maintain
and control some continuous value of current. AC drives that use internal link
reactors are called Buffered Pulse Width Modulated Drives or BPWM. Using BPWM
type ac drives yields high POWER FACTOR without adding an additional input ac
line reactor. According to the DRIVEPOWER Technology Atlas (ESOURCE Inc.1993),
above 5% impedance there is little reduction is the current distortion. The
most significantreduction in distortion and improvement in power factor occurs
with series impedance up to 3%. The internal dc link reactor provides more
series impedance with less space and at greater efficiency.
HARMONICS and IEEE 519
Thomas A. Short
Prepared for the Electric Council of New England
September 17, 1992
• Importance of Understanding Harmonics in Today's Systems
• Harmonic Effects
• Sources of Harmonics
o Static Power Converters
• IEEE 519
o Guidelines for Individual Customers
o Guidelines for Utilities
o Time Duration Limits
o Manufacturer's Guidelines
• Other Sources of Information
In an ideal power system, the voltage supplied to customer equipment, and the
resulting load current are perfect sine waves. In practice, however, conditions
are never ideal, so these waveforms are often quite distorted. This deviation
from perfect sinusoids is usually expressed in terms of harmonic distortion of
the voltage and current waveforms.
Power system harmonic distortion is not a new phenomenon - efforts to limit it
to acceptable proportions have been a concern of power engineers from the early
days of utility systems. At that time, the distortion was typically caused by
the magnetic saturation of transformers or by certain industrial loads, such as
arc furnaces or arc welders. The major concerns were the effects of harmonics
on synchronous and inductionmachines, telephone interference, and power
capacitor failures. In the past, harmonic problems could often be tolerated
because equipment was of conservative design and grounded wye-delta transformer
connections were used judiciously.
Distortions of the fundamental sinusoid generally occur in multiples of the
fundamental frequency. Thus on a 60 Hz power system, a harmonic wave is a
sinusoid having a frequency expressed by:
where n is an integer.
Figure 1 illustrates the fundamental frequency (60 Hz) sine wave and its 2nd,
3rd, 4th, and 5th harmonics.
Figure 1. Fundamental Frequency (60 hz) Sine Wave and Harmonics: 2nd Harmonic
(120 Hz); 3rd Harmonic (180 Hz); 4th Harmonic (240 Hz); and 5th Harmonic (300
Figure 2 shows how a distorted wave can be broken into its harmonic components.
The distorted wave is composed of the fundamental combined with wave 3rd and
5th harmonic components.
Figure 2. Distorted Wave Composed by the Superposition of a 60 Hz Fundamental
and Smaller Third Harmonic and Fifth Harmonics.
Harmonics are often characterized by a harmonic distortion factor (DF) defined
The distortion factor can be used to characterize distortion in both current
and voltage waves. Total harmonic distortion factors can be specified for a
range of harmonics such as the second through the eleventh harmonic. A
distortion factor can also be given for a single harmonic or small range of
harmonics. The total harmonic distortion (THD) is the distortion factor
including all relevant harmonics (typically taken as thesecond through the
Importance Of Understanding Harmonics In Today's Systems
As mentioned earlier, harmonic distortion problems are not new to utility and
industrial power systems. In fact, such distortion was observed by utility
operating personnel as early as the first decade of this century. Typically,
the distortion was caused by nonlinear loads connected to utility distribution
Today, however, additional methods for dealing with harmonics are necessary for
three main reasons:
1. The use of static power converters has recently proliferated.
2. Network resonances have increased.
3. Power system equipment and loads are more sensitive to harmonics.
The introduction of reliable and cost-effective static power converters has
caused a very large increase in the number of harmonic-generating devices and
has resulted in their dispersion over the entire power system. The term
'static power converter', as used in this text, refers to a
semiconductor device that converts power of one frequency into power of another
frequency. The types of converters most frequently used in industry are the
rectifier, converting ac power to dc, and the invertor, converting dc power to
Moreover, the harmonic problem is often aggravated by the trend in recent years
to install capacitors for power factor improvement or voltage control. Since
the capacitor installation is in parallel with the inductance of the power
system, as shown in Figure 3, a resonant condition will exist at a frequency
where L represents the inductance of the power system, and C represents
the capacitance of the capacitor installation.
Figure 3. Excitation of a Parallel Resonant Circuit
If a harmonic current is injected (from a static power converter, for example)
at a frequency near the resonant frequency, a high oscillating current can flow
that may in turn cause capacitor fuse blowing and high harmonic voltages.
In addition to the increase in harmonic generators and network resonances,
electric systems and loads have become no less, and in some cases even more,
sensitive to harmonics. There are a number of areas of new and continuing
1. Computers, computer-controlled machine tools, and various types of digital
controllers are especially susceptible to harmonics, as well as to other types
2. Harmonics can cause damaging dielectric heating in underground cables.
3. Inductive metering can be adversely affected by harmonics.
4. Capacitor bank failures are frequently caused by harmonics.
5. Less conservative designs for rotating machines and transformers aggravate
heating problems caused by harmonics.
6. Harmonics can be especially troublesome to communication systems.
Today's harmonics problems may have more serious and widespread consequences
than in the past. System planners and designers should be able to recognize and
avoid or mitigate such problems.
The effects of harmonics are divided into three general categories:
1. effects on the power system itself2. effects on consumer load
3. effects on communication circuits
On the power system, harmonic currents are the main culprit, causing equipment
overheating and thermal loss-of-life. This may be a concern for motors or
transformers. The impact is worse when network resonances amplify harmonic
currents. Harmonics may also interfere with relaying and metering to some
Harmonics can also cause thyristor firing errors in converter and SVC
installations, metering inaccuracies, and false tripping of protective devices.
The performance of consumer equipment, such as motor drives and computer power
supplies, can be adversely affected by harmonics. In addition, harmonic
currents flowing on power lines can induce noise on nearby communication lines.
Harmonic voltage distortion may cause equipment insulation stress, particularly
in capacitors. When harmonics cause the voltage impressed on the capacitor bank
to be distorted, the peak voltage may be high enough to cause a partial
discharge, or corona, within the capacitor dielectric. This may eventually
result in a short circuit at the edges of the foil and failure of the capacitor
High harmonic currents also cause fuse blowing in capacitor banks. This results
in a loss of reactive power supply to the system which may cause other
Sources Of Harmonics
Harmonics are caused by nonlinear loads attached to the power system. Nonlinear
loads draw nonsinusoidal current. Resistors, inductors, and capacitors are
linear devices. When a resistive load isapplied to an AC power system, it draws
sinusoidal current. When an inductive load is applied, it too draws sinusoidal
current although it is phase shifted compared to the resistive load. There are
many types of nonlinear loads which cause harmonics. The largest source of
harmonics are converters. Converters range from a huge 1000 MW invertor station
for an HVDC line to a 75 W rectifier found in a television. Other nonlinear
sources of harmonics include arcing devices such as arc furnaces, transformer
magnetizing impedance, and fluorescent lights. The harmonic current caused by
the nonlinear sources can cause harmonic distortion in the system voltage which
may cause problems for other devices. Figure 4 shows measured current waveforms
and harmonic spectrums for several common harmonic sources.
Static Power Converters
The largest application of static converters is in adjustable-speed drives for
motor control. These static drives are now used in all types of industrial
motors, providing higher efficiencies, better speed control, and more
maintenance-free operation than other, more conventional motor drives.
Converters use solid state switching devices to convert power from one
frequency to another (usually between AC and DC). These switching devices may
be diodes, thyristors, GTO's, or many other power electronics devices.
Figure 4. Example waveforms from several common sources.
A single-phase, full-wave rectifier is shown in 9 to illustrate why the
switching devices cause harmonics. Full-wave rectifiers arevery common in all
sorts of electronics (TVs, computers, stereos, etc). The voltage across the DC
load is shown in Figure 5. The diodes act to flip the negative half of the sine
wave over. The capacitor tries to hold the voltage at the peak. Two times per
cycle, the capacitor is charged up, and this is the only time the rectifier
draws current from the system. Therefore, the load current is drawn in short
pulses as shown in Figure 6.
Figure 5. Single-Phase, Full-Wave Rectifier
Figure 6. AC Current and Voltage Across the Load in a Full-Wave Rectifier
Figure 7. AC Current Drawn by Consumer Equipment.
Power system problems that were associated with harmonics began to be of
general concern in the 1970s, when two independent developments took place. The
first was the oil embargo, which led to price increases in electricity and the
move to save energy. Industrial consumers and utilities began to apply power
factor improvement capacitors. Capacitors reduce MVA demand from the utility
grid systems by supplying the reactive power portion of the load locally. As a
result, losses are reduced in the industrial plant and the utility network. The
move to power factor improvement resulted in a significant increase in the
number of capacitors connected to power systems. As a consequence, there has
been an equally significant increase in the number of tuned circuits in plant
and utility networks.
The second involved the coming of age of low voltage thyristor technology. In
the 1960s, thyristors weredeveloped for dc motor drives and then extended to
include adjustable-speed ac motor drives in the 1970s. This resulted in a
proliferation of small, independently operated converters usually without
harmonic mitigation techniques employed.
Even with relatively low levels of harmonic currents, a resonant circuit can
cause severe problems of voltage distortion and telephone interference. A
parallel resonant circuit can amplify harmonic current levels to a point where
equipment can fail. Series resonant circuits can concentrate the flow of
harmonic currents in specific lines or feeders to a point where telephone
interference is a major problem.
The increase in the use of static converters, both in industrial control
equipment and in domestic applications, combined with the increase in use of
power factor improvement capacitors, created widespread problems. Because these
problems have been so extensive, it has proven necessary to develop analytical
techniques and guidelines for equipment application and harmonic control. This
segment discusses those guidelines and their significance in system design.
American standards regarding harmonics have been laid out by the IEEE in the
519 Standard: IEEE Recommended Practices and Requirements for Harmonic Control
in Electric Power Systems. There is a combined effect of all nonlinear loads on
utility systems that have a limited capability to absorb harmonic current.
Further, utilities are charged with the responsibility to provide a high
quality supply in terms of voltage level and waveform. IEEE 519 recognizesnot
only the absolute level of harmonics produced by an individual source but also
their size relative to the supply network.
It should be noted that IEEE 519 is limited to being a collection of
Recommended Practices that serve as a guide to both suppliers and consumers of
electrical energy. Where problems exist, because of excessive harmonic current
injection or excessive voltage distortion, it is incumbent upon supplier and
consumer to resolve the issues within a mutually acceptable framework.
The purpose of IEEE 519 is to recommend limits on harmonic distortion according
to two distinct criteria, namely:
1. There is a limitation on the amount of harmonic current that a consumer can
inject into a utility network.
2. A limitation is placed on the level of harmonic voltage that a utility can
supply to a consumer.
Guidelines for Individual Customers
The primary limit on individual customers is the amount of harmonic current
that they can inject into the utility network. The current limits are based
upon the size of the consumer relative to the size of the supply. Larger
customers are restricted more than smaller customers. The relative size of the
load with respect to the source is defined as the short circuit ratio (SCR), at
the point of common coupling (PCC), which is where the consumer's load connects
to other loads in the power system. The consumer's size is defined by the total
fundamental frequency current in the load, IL, which includes all linear and
nonlinear loads. The size of the supply system is defined by the level
ofshort-circuit current, ISC, at the PCC. These two currents define the SCR:
A high ratio means that the load is relatively small and that current limits
will not be as strict as limits that pertain to a low ratio. This is
demonstrated in 1, which lists recommended, maximum current distortion levels
as a function of SCR and harmonic order. The table also identifies total
harmonic distortion levels. All of the current distortion values are given in
terms relative to the maximum demand load current. The total distortion is in
terms of total demand distortion (TDD) instead of the more common THD term.
Table 1 shows current limits for individual harmonic components as well as
total harmonic distortion. For example a consumer with an SCR between 50 and
100 has a recommended limit of 12.0% for TDD, while for individual odd harmonic
components with orders less than 11, the limit on each is 10%. It is important
to note that the individual harmonic current components do not add up directly
so that all characteristic harmonics cannot be at their individual maximum
limit without exceeding the TDD.
Table 1. IEEE 519 Current Distortion Limits.
For conditions lasting more than one hour. Shorter periods increase limit by
|Harmonic Current Limits for Non-Linear Load at the Point-of-Common-Coupling
with Other Loads, for voltages 120 - 69,000 volts |
|Maximum Odd Harmonic Current Distortion in % of Fundamental Harmonic Order |