AC Drive Theory

and Application
Application Guide AP04014005E
Effective May 2008

Benefits of Using AC Drives

AC drives have become very popular in recent years as it is recog-
nized that they provide a very efficient and direct method of control-
ling the speed of the most rugged and reliable of prime movers, the
squirrel cage motor. Eaton’s Cutler-Hammer AC drives provide many
economic and performance advantages in a wide variety of adjustable
speed drive applications.
The following are some of the benefits provided:
● High efficiency and low operating cost
● Minimal motor maintenance
● Controlled linear acceleration and deceleration provide soft
starting and stopping and smooth speed changes
● Multiple motor operation is easily accomplished
● Current limit provides for quick and accurate torque control
● Adjustable speed operation can be accomplished with existing
AC motors
● Improved speed regulation can be accomplished by slip
compensation
● AC motors are available in a wide variety of mechanical
configurations
● Flexibility of machine design due to the light weight and compact
size of AC motors
● IR compensation provides high starting torque easily and
economically
● AC motors are available in enclosures suitable for hazardous or
corrosive environments
● Fewer spare motors are required since the same motor can be
used for both adjustable speed and constant speed operations.
● Cutler-Hammer rugged and reliable designs ensure minimum
downtime expense
Introduction
● High speed operation can be economically accomplished using
Adjustable Frequency AC Drive System Description extended frequency operation
An adjustable frequency AC drive system consists of an ordinary ● Reverse operation is accomplished electronically without the need
three-phase induction motor, an adjustable frequency drive to for a reversing starter
control the speed of the motor and an operator's control station.
The most common motor used with an AF drive system is a standard
NEMA design B squirrel cage induction motor, rated for 230 or Basic Principles of AC Drive Operation
460 volt, 3-phase, 60 Hz operation. There are several classifications of adjustable frequency AC drives.
The adjustable frequency controller is a solid-state power conversion Some common types of drives are Variable Voltage Input (VVI) some-
unit. It receives 240 or 480 volt, 3-phase, 60 Hz power and converts it times called Six Step drives, current source input (CSI), pulse width
to a variable frequency supply which can be steplessly adjusted modulated (PWM) drives, Sensorless Vector drives, Field Oriented
between 0 and 60 Hz. The controller also adjusts the output voltage drives and Closed Loop Vector drives. The more common AC drives
in proportion to the frequency to provide a nominally constant ratio of are PWM, Sensorless Vector and Closed Loop Vector drives.
voltage to frequency as required by the characteristics of the motor. Figure 1 is a block diagram of a typical VVI drive. The AC/DC con-
The operator's station provides the operator with the necessary verter is an SCR bridge, which receives ac power from the input line
controls for starting and stopping the motor and varying the motor and provides adjustable voltage dc power to the dc bus. A voltage
speed. These functions can also be performed by a wide variety of regulator is required to preset the dc bus voltage to the level needed
automatic control systems. to provide the required output voltage amplitude to the motor.
 ac/dc Filter Inverter
ac Line Converter

Motor

Speed

Voltage Frequency
Regulator Control

FIGURE 1. TYPICAL WI DRIVE BLOCK DIAGRAM

The inverter uses either SCRs or transistors as solid-state switches

to convert the dc power to a stepped waveform output. The
amplitude of the dc bus voltage determines the amplitude of the
output voltage. Figure 2 shows typical output voltage and current Line-to-
waveforms for a VVI inverter. The voltage waveform is normally Neutral
referred to as a “six step” waveform. Voltage
Figure 3 is a block diagram of a typical PWM drive. It receives line
voltage and converts it to a fixed dc voltage using a 3-phase full wave
diode bridge. Since the dc bus is a fixed voltage level, the amplitude
of the output voltage is fixed. Modulating the output waveform using
IGBT inverter switches controls the effective value of the output
voltage. Figure 4 shows the output voltage and current waveforms
for the PWM inverter.

Line
Current

FIGURE 2. TYPICAL VVI VOLTAGE AND CURRENT WAVEFORMS

ac Diode Bridge
Line Filter Inverter
Rectifier

Motor

Speed
Reference

Voltage and
Frequency Control

FIGURE 3. BLOCK DIAGRAM OF A TYPICAL PWM DRIVE

2 EATON CORPORATION Cutler-Hammer AC Drive Theory and Application Application Guide AP04014005E Effective: May 2008
 Line-to
Neutral
Voltage

Line
Current

FIGURE 4. TYPICAL PWM VOLTAGE AND CURRENT WAVEFORMS

Principles of Adjustable Frequency If the voltage is held constant and the frequency is decreased, the
Motor Operation magnetic field strength would increase. This increases the iron losses
and would cause the motor to burn out.
Torque Speed Curves The operating speed of the motor is synchronous speed minus slip.
The operating speed of an AC induction motor can be determined by For a design B motor, slip is typically 3%.
the frequency of the applied power and the number of poles created Figure 5 is a speed/torque curve for a typical NEMA  design B motor.
by the stator windings. Synchronous speed is the speed of the There are several important points indicated on the curve.
magnetic field created in the stator windings. It is given by:
Locked rotor or stall torque is the amount of torque necessary to start
N = 120f / p the motor under full load conditions.
where: Pull out or breakdown torque is the amount of torque that will cause
n = speed in RPM the motor to pull out or stall.
f = operating frequency Full load torque is the amount of torque the motor is designed
p = number of poles to develop.
When the frequency is changed, the voltage must also be changed, The operating point is the point where the actual load is causing
based on the formula for reactance and Ohm’s Law. the motor to operate.
XL = 2 ⋅ π ⋅ f ⋅ L
Percent
Where L = inductance Torque
XL = reactance 200 Pull Out or
Breakdown
V = voltage Torque
Locked Rotor
Im = magnetizing current Operating
or
Stall Torque Point
V
Im = ------ 100 Load Torque (Friction)
XL

Combining the above equations yields: Slip

1 V
I m = ------------------------- ⋅ ---
2⋅π⋅f⋅L f 10 20 30 40 50 60 70 80 90 100
Percent Speed

For steady-state operation, a constant volts per hertz ratio must be

maintained. This is equal to the motor rated voltage divided by the Operating Speed
rated frequency.
Synchronous Speed
Example: 460 volt motor
60 hertz FIGURE 5. NEMA DESIGN B MOTOR SPEED/TORQUE CURVE

7.67 volts per hertz

For the magnetizing current to remain constant, the V/f ratio, or the
volts per hertz ratio, must remain constant. Therefore, the voltage
must increase and decrease as the frequency increases and
decreases.

EATON CORPORATION Cutler-Hammer AC Drive Theory and Application Application Guide AP04014005E Copyright 2008 3
Induction Motor Speed Control
IR Drop
Standard induction motors (NEMA design B) have approximately 3% Voltage
slip at full load.
If the drive only controls the output frequency, the motor speed will
deviate from the set speed due to slip.
For many fan and pump applications, precise speed control is not
needed. The motor slip can be:
● Ignored
● Compensated for by the drive based on motor current and a Motor Air Gap
programmed speed-torque characteristic of the motor Terminal Voltage
Voltage
● Compensated for by a control loop external to the drive. An
example would be a pump where a certain flow rate is desired.
The “flow control loop” tells the drive to either speed up or slow
down to reach the desired flow. The actual speed of the pump
has no importance.
Vector controlled drives need speed feedback of the rotor. For FIGURE 7. MOTOR EQUIVALENT CIRCUIT
Sensorless Vector, the rotor speed is calculated based on a model
of the motor stored in the drive. For Closed Loop Vector, a digital In a hypothetical example, lets assume that the optimum motor
encoder is added to the motor to provide actual rotor speed. terminal voltage is 460 V when we are operating at 60 Hz. If the
motor has a full load current of 40 amps and the internal resistance
is 1 ohm, then the IR drop would be 40 volts and voltage at the air
300 gap would be 420 V, or 7 V/Hz. If we then operate the motor at 6 Hz
and still require full load torque, the current must still be 40 amps
since current is proportional to torque. In this condition, if we require
7 V/Hz at the air gap, or 42 V and we still have an IR drop of 40 V, we
Percent Torque

200 must have a motor terminal voltage of 82 V (13.67 V/Hz).

This means that if we are required to produce full torque at low
speeds, we must have a significant V/Hz “boost” at low speeds.
100 Since the required boost voltage depends on individual motor and
load characteristics, some type of voltage boost adjustment is
usually provided.
0 In many cases, this voltage boost adjustment provides us with a
0 2 10 20 30 40 50 60 fixed voltage boost. If our motor load is always constant, this is no
Frequency - Hz problem. However, if we have a varying load, using a fixed boost can
produce undesirable results. In the example above, assume we are
FIGURE 6. FAMILY OF IDEAL SPEED/TORQUE CURVES using a fixed voltage boost to 13.67 V/Hz. If load is now cut in half, IR
drop, which is a purely resistive load, will also be cut in half. If we are
This curve is drawn for a motor operating at a fixed frequency. operating at 6 Hz and are applying 82 V to the motor terminals, we
Changing the frequency of the power applied to the motor changes now have an air gap voltage of 62 V (10.33 V/Hz). Because of this,
the slip/torque curve. we must set a fixed boost at some point that will give us adequate
starting torque without saturating the motor. This means that the
Figure 6 shows a family of ideal speed/torque curves drawn for a
motor is not producing optimum torque at low speeds. By using IR
motor operating from an adjustable frequency power source. As can
compensation instead of a fixed boost, we can provide improved
be seen, the value of slip is constant at any given operating torque
torque during low speed operations. This is accomplished by sensing
level, and the normal operating portions of the curves are a series of
motor current and automatically adjusting the voltage boost in propor-
parallel lines. When a motor is operated from an AC drive, it normally
tion to motor current.
never enters the dotted portion of the curve.
Volts per Hertz Regulation
In order to operate the motor with the desired speed/torque curve,
we must apply the proper voltage to the motor at each frequency.
As we have already seen, it is necessary to regulate motor voltage
in proportion to the frequency at a constant ratio. In reality, this
requirement for constant volts/hertz does not apply to the motor
terminals, but to a hypothetical point inside the motor. The voltage
at this point is called the air gap voltage. The difference between air
gap voltage and motor terminal voltage is the IR voltage drop as
shown in Figure 7.

4 EATON CORPORATION Cutler-Hammer AC Drive Theory and Application Application Guide AP04014005E Effective: May 2008
Soft Start
Constant V/Hz Constant V/Hz
Figure 8 shows torque/frequency and current/frequency at various (Constant Torque) (Constant hp)
operating frequencies. From these curves, we can see that when the Operating Range Operating Range
motor is operating in the normal operating portions (solid lines) of the
curves, motor current is directly proportional to motor torque. How-
ever, when we operate above 150% current, we can see the ratio of Intermittent Torque
150

Percent Torque and Horsepower

torque to current is significantly less than one. 140
130 High Efficiency Motor
120
600 110 Torque Horsepower
100 Hor
90 sep
Tor owe
80 qu r
e
70 1.15 S.F. Motor
500 60
Current 50
r
Percent Torque/ Percent Current

40 e
Torque pow
30 se
400 r
20 Ho
10
0
0 20 40 60 80 100 120
300 Operating Frequency (Hertz)

FIGURE 9. TYPICAL MOTOR PERFORMANCE CURVES

200 Figure 9 shows typical motor performance curves. These curves

show that by using a high efficiency motor or by oversizing the motor,
a wider constant torque speed range can be realized. Operation
100 above 60 Hz will also give us a wider speed range.
Load Characteristics
Most loads are divided into two categories:
0
0 2 10 20 30 40 50 60
● Variable Torque — centrifugal fans and pumps
Frequency - Hz ● Constant Torque — conveyors, hoists, etc.
FIGURE 8. SPEED/TORQUE AND SPEED/CURRENT CURVES
120
If the motor is line started, we can see from Figure 8 that the current
inrush will be approximately 600% of full load current. We can also
see that at the 2 Hz curve we are already on the solid portion of the 100
curve at start. This means that if a motor is started at 2 Hz or less we
will not require a high starting current. If we start at a low frequency 80
Percent Torque

and then increase speed by increasing frequency, the motor will

always operate on the solid portion of the curves and never require
more than 150% of rated current. 60

40
Motor Application and Performance
Motor Sizing 20
In sizing a drive, we must first match the torque/speed capabilities of
the motor to the requirements of the driven load. We can then match 0
the inverter to the motor. 0 20 40 60 80 100
Percent Speed
AC Drive Motor Torque vs. Speed Capability
When a drive is being used in a constant torque application, we must FIGURE 10. SPEED VS. TORQUE CONSTANT AND VARIABLE TORQUE
remember that as motor speed is reduced below base speed, motor
cooling will become less effective. The minimum speed allowable for The current drawn by an AC motor is proportional to the load torque.
continuous operation under constant torque conditions is effected by The above curves can also represent the motor load current versus
this limitation. speed (when supplied by an AC drive).

In a variable speed application, we do not have this limitation, as

motor load is lower at low speeds.

EATON CORPORATION Cutler-Hammer AC Drive Theory and Application Application Guide AP04014005E Copyright 2008 5
 220
6-Pole Motor Intermittent Torque

200

180

160

140 6-Pole Motor Continuous Torque

Percent Torque

120

100
4-Pole Motor Continuous Torque
80

60

40
Torque for 6-Pole motor is based on percent of rated torque of 4-Pole motor of the same
hp rating. Rating torque of 6-Pole motor is 1.5 times of rated torque of same hp 4-Pole motor.
20

0
0 10 20 30 40 50 60 70 80 90

Frequency (Hz) 1800 RPM 1800 RPM

4-Pole Motor 6-Pole Motor

FIGURE 11. TORQUE SPEED CHARACTERISTICS FOR EXTENDED MOTOR PERFORMANCE

Extended Motor Performance

We can obtain extended motor performance by operating a motor 120
above its base speed to 90 Hz. If an application was sized by using
an 1800 RPM motor, we could use a 1200 RPM motor of the same 110
size and operate the motor at 1800 RPM by increasing the maximum
frequency to 90 Hz (see Figure 11). We can see that the% torque 100
ratings are based on 100% torque equal to the rated torque of a
Percent Torque

4-pole motor. The rated torque of a 6-pole motor is 150% of the

90
rated torque of a 4-pole motor of the same rated horsepower.
The motor voltage is held constant between 60 Hz and 90 Hz; 80
therefore the available torque follows a constant hp curve.
This mode of operation increased the continuous and intermittent 70
torque available over most of the speed range. It increases the break-
away torque to 225%. Continuous constant torque speed range is 60
also increased. Because we have increased the frequency for any
given speed by 50%, we have reduced the possibility of clogging. 50
Since we have not increased the operating hp, it is usually not neces- 0 10 20 30 40 50 60
sary to oversize the drive to obtain extended motor performance. Frequency
Check the motor current to be sure.
FIGURE 12. MOTOR TORQUE DE-RATING
Operating Below Rated Motor Speed
Most motors have an internal cooling fan. Mechanical resonance may be present below the rated speed.
Operation below rated speed reduces the effectiveness of the fan. Continued operation at these speeds can effect the performance of
The motor may overheat. the driven equipment, and lead to premature failures.
This is usually not a problem for fan or pump applications, since the Most AC Drives allow certain speeds (frequencies) to be “skipped.”
load is very small at light loads. This avoids operating at the mechanical resonance speeds (also
called critical speeds).
For constant torque applications, conveyors, hoists, cranes, forced
cooling of the motor may be required.
The following shows typical torque derating for a fan cooled motor
operated below rated frequency.

6 EATON CORPORATION Cutler-Hammer AC Drive Theory and Application Application Guide AP04014005E Effective: May 2008
Operating Above Rated Motor Speed If motors are to be started and stopped separately, you must then
determine the highest intermittent current that will be required for
Speed/Torque Considerations the worst case combination of motors running and motors starting.
Most AC Drives can have output frequencies of 120 Hz or greater. Stopping individual motors may cause difficulty in some situations.
However, the output voltage is limited to the magnitude of the line If two or more motors are to be mechanically coupled together,
voltage. A drive supplied by 460 volts cannot output more than load-sharing requirements must be considered.
460 volts. Individual motor overload protection must be provided when using a
Therefore, as frequency is increased above 60 Hz, the output voltage multiple motor application.
remains constant, and the volts per hertz ratio decreases.
This reduces the motor torque. AC Drive Application
Below is a plot of AC Drive and Motor Torque versus Speed.
Matching the AC Drive to the Motor
PWM and Vector AC Drives are designed for use with any standard
120 squirrel cage motor. Sizing the drive is a simple matter of matching
the drive output voltage, frequency and current ratings to the
100 motor ratings.
Output Voltage and Frequency
80
Percent Torque

Most modern AC Drives are designed for use with various voltages
and frequencies. By adjusting the V/Hz properly, almost any 3-phase
60 motor can be used.
Output Current
40 AC drive full load currents are matched to typical full load motor
current ratings as listed in NEC Table 430-150. Usually an AC drive
20 can be matched to an AC motor by their hp ratings, however, actual
motor current required under operating conditions is the determining
factor. If the motor will be run at full load, the drive current rating
0 must be at least as high as the motor current rating. If the drive is
0 20 40 60 80 100 120 140 160 180 200 to be used with multiple motors, the sum of all the full load current
Percent Speed ratings must be used, and adding up the hp ratings of the motors will
usually not provide an accurate estimate of the drive needed.
FIGURE 13. SPEED VS. AC DRIVE + MOTOR TORQUE
Motor Protection
The thick line is the drive+motor torque curve. Motor overload protection must be provided as required by the appli-
The thin line is a typical speed torque curve for a centrifugal fan or cable codes. Motor protection is not automatically provided as part of
pump. No overspeed is possible for this type of load, since the load all AC drives. It may be provided as a standard feature on one model
torque exceeds the motor torque. or it may be an optional feature on another.
Operating above rated speed requires either: The best means of motor protection is a direct winding over tempera-
ture protection such as an over temperature switch imbedded in the
● A load with low torque, such as a unloaded crane
motor windings. Direct over temperature protection is preferred
● The motor to be oversized because overheating can occur at normal operating currents at
low speeds.
Mechanical Considerations
Most AC drives are equipped with electronic overcurrent protection,
Operating above the motor’s rated speed should be carefully such as I2t protection, similar to a conventional overload. Conven-
reviewed. tional overloads also may be used. In some modern drives, the I 2t
The NEMA MG-1 Standard gives typical overspeed capabilities of protection can be configured to protect the motor during low speed
induction motors. operation.
● Small motors can typically run at 200% speed In multiple motor applications, individual motor overload protection
● Large motors can typically run at 125 – 150% speed must be provided even where electronic protection is provided by the
drive. In some cases, short circuit protection may be required.
The mechanical vibration of a system will increase as speed
increases. The rotating equipment mounting, alignment and balance Motor Winding Damage
is more critical as the speed increases. The voltage output of AC drives contains voltage steps. In modern
Mechanical resonance may be present above rated speed. Some PWM drives, the dV/dt of a motor causes can cause very large
speeds (frequencies) may have to be skipped. voltage spikes. Voltage spikes of 1500 volts or more are typical for
a 460 volt motor.
Multiple Motor Operation
This can cause the end windings of a Non-Inverter Duty or standard
We can connect any number of motors in parallel across a single induction motor to fail.
AC drive. All motors will be operated at the same speed, since the
frequency to all the motors will be the same. With NEMA B motors, This problem gets worse as the cable length from the drive to the
motor speed will be matched within 3%, depending on load variations. motor gets longer. Corrective action is normally required for cables
longer than 150 feet.
If it is necessary to have exact speed matching, synchronous AC
motors must be used. Load side reactors, installed at the drive output terminals, will reduce
the voltage spikes at the motor terminals.
If an adjustable speed ratio is desired between the motors, individual
AC drives must be used. Most drive manufacturers have load side reactors available as
an option.
The simplest multiple motor application is where all motors are
started and stopped together and are permanently connected to the
drive. In this case, it is simple to size the drive to provide an output
current equal to the sum of the individual motors.

EATON CORPORATION Cutler-Hammer AC Drive Theory and Application Application Guide AP04014005E Copyright 2008 7
AC Drive Performance Current Limit
AC drives are equipped with current limit circuits. If current limit is
Operator Control and Interface Requirements
not provided, the overcurrent trip circuits will shut you down in the
A means must be provided to start and stop the drive and provide a event of an overload or attempting to accelerate too fast.
speed reference. This may be accomplished with a simple run/stop
switch and a speed potentiometer, or by more elaborate means. Regeneration Limit and Braking
Additional functions that may be required include reversing; lights During deceleration or in the event of an overhauling load, a motor
or relays to indicate drive status; and meters to indicate operating will produce braking torque.
speed, load, etc.
When a motor produces braking torque, it is operating as an induction
Speed Range generator. This means that the drive is being fed power from the
motor. When power is being fed into the drive, it cannot pass current
Speed range is usually determined by the characteristics of the
back out to the line. This means that this excess power is sent to the
motor, as the AC drive output frequency range is usually wider than
bus capacitors. If enough power is regenerated, the bus capacitors
the motor range.
will charge to the trip level for the drive. When this occurs, bus
Acceleration and Deceleration voltage will rise. If the voltage rises above a preset level, the drive
will trip.
Independently adjustable acceleration and deceleration rates are
usually provided with a drive. Actual field conditions determine the When the drive is provided with some type of dynamic braking cir-
optimum acceleration and deceleration rate of the drive. cuit, it will allow the motor to produce rated torque as braking torque.
Speed Regulation A full regenerative drive will allow the drive to feed this excess power
back onto the line.
As most AC drives do not use encoder feedback, speed regulation is
determined by the slip of the motor. Typical slip for a NEMA B motor
Cutler-Hammer is a federally registered trademark of Eaton Corporation. NEMA is the registered
provides for 3% regulation. Slip compensation circuits can be used to trademark and service mark of the National Electrical Manufacturers Association. National
improve this to about 1.0% regulation. In extreme cases, where very Electrical Code and NEC are registered trademarks of the National Fire Protection Association,
close speed regulation is essential, a motor encoder can be Quincy, Mass.
supplied to give 0.0l% speed regulation.

Eaton Corporation
Electrical Group
1000 Cherrington Parkway
Moon Township, PA 15108
United States
877-ETN-CARE (877-386-2273)
Eaton.com

© 2008 Eaton Corporation

All Rights Reserved
Printed in USA
Publication No. AP04014005E / Z7129
May 2008