2003 Microchip Technology Inc. DS00887A-page 5
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Capacitor Start/Capacitor Run AC
Induction Motor
This motor has a start type capacitor in series with the
auxiliary winding like the capacitor start motor for high
starting torque. Like a PSC motor, it also has a run type
capacitor that is in series with the auxiliary winding after
the start capacitor is switched out of the circuit. This
allows high overload torque.
FIGURE 7: TYPICAL CAPACITOR
START/RUN INDUCTION
MOTOR
This type of motor can be designed for lower full-load
currents and higher efficiency (see Figure 9 for torque-
speed curve). This motor is costly due to start and run
capacitors and centrifugal switch.
It is able to handle applications too demanding for any
other kind of single-phase motor. These include wood-
working machinery, air compressors, high-pressure
water pumps, vacuum pumps and other high torque
applications requiring 1 to 10 hp.
Shaded-Pole AC Induction Motor
Shaded-pole motors have only one main winding and
no start winding. Starting is by means of a design that
rings a continuous copper loop around a small portion
of each of the motor poles. This “shades” that portion of
the pole, causing the magnetic field in the shaded area
to lag behind the field in the unshaded area. The
reaction of the two fields gets the shaft rotating.
Because the shaded-pole motor lacks a start winding,
starting switch or capacitor, it is electrically simple and
inexpensive. Also, the speed can be controlled merely
by varying voltage, or through a multi-tap winding.
Mechanically, the shaded-pole motor construction
allows high-volume production. In fact, these are usu-
ally considered as “disposable” motors, meaning they
are much cheaper to replace than to repair.
FIGURE 8: TYPICAL SHADED-POLE
INDUCTION MOTOR
The shaded-pole motor has many positive features but
it also has several disadvantages. It’s low starting
torque is typically 25% to 75% of the rated torque. It is
a high slip motor with a running speed 7% to 10%
below the synchronous speed. Generally, efficiency of
this motor type is very low (below 20%).
The low initial cost suits the shaded-pole motors to low
horsepower or light duty applications. Perhaps their larg-
est use is in multi-speed fans for household use. But the
low torque, low efficiency and less sturdy mechanical
features make shaded-pole motors impractical for most
industrial or commercial use, where higher cycle rates or
continuous duty are the norm.
Figure 9 shows the torque-speed curves of various
kinds of single-phase AC induction motors.
Rotor
Main
Winding
Input
Power
Start Cap
Centrifugal Switch
Start Winding
Run Cap
Shaded Portion of Pole
Unshaded Portion of Pole
Copper Ring
Supply Line
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DS00887A-page 6 2003 Microchip Technology Inc.
FIGURE 9: TORQUE-SPEED CURVES OF DIFFERENT TYPES OF SINGLE-PHASE
INDUCTION MOTORS
THREE-PHASE AC INDUCTION
MOTOR
Three-phase AC induction motors are widely used in
industrial and commercial applications. They are
classified either as squirrel cage or wound-rotor
motors.
These motors are self-starting and use no capacitor,
start winding, centrifugal switch or other starting
device.
They produce medium to high degrees of starting
torque. The power capabilities and efficiency in these
motors range from medium to high compared to their
single-phase counterparts. Popular applications
include grinders, lathes, drill presses, pumps,
compressors, conveyors, also printing equipment, farm
equipment, electronic cooling and other mechanical
duty applications.
Squirrel Cage Motor
Almost 90% of the three-phase AC Induction motors
are of this type. Here, the rotor is of the squirrel cage
type and it works as explained earlier. The power
ratings range from one-third to several hundred horse-
power in the three-phase motors. Motors of this type,
rated one horsepower or larger, cost less and can start
heavier loads than their single-phase counterparts.
Wound-Rotor Motor
The slip-ring motor or wound-rotor motor is a variation
of the squirrel cage induction motor. While the stator is
the same as that of the squirrel cage motor, it has a set
of windings on the rotor which are not short-circuited,
but are terminated to a set of slip rings. These are
helpful in adding external resistors and contactors.
The slip necessary to generate the maximum torque
(pull-out torque) is directly proportional to the rotor
resistance. In the slip-ring motor, the effective rotor
resistance is increased by adding external resistance
through the slip rings. Thus, it is possible to get higher
slip and hence, the pull-out torque at a lower speed.
A particularly high resistance can result in the pull-out
torque occurring at almost zero speed, providing a very
high pull-out torque at a low starting current. As the
motor accelerates, the value of the resistance can be
reduced, altering the motor characteristic to suit the
load requirement. Once the motor reaches the base
speed, external resistors are removed from the rotor.
This means that now the motor is working as the
standard induction motor.
This motor type is ideal for very high inertia loads,
where it is required to generate the pull-out torque at
almost zero speed and accelerate to full speed in the
minimum time with minimum current draw.
500
400
300
200
100
20
40
60
80 100
Speed (%)
Torque (% of Full-Load Torque)
Capacitor Start and Run
Changeover of Centrifugal Switch
Capacitor Start
Split-Phase
PSC
Shaded-Pole
2003 Microchip Technology Inc. DS00887A-page 7
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FIGURE 10: TYPICAL WOUND-ROTOR
INDUCTION MOTOR
The downside of the slip ring motor is that slip rings and
brush assemblies need regular maintenance, which is
a cost not applicable to the standard cage motor. If the
rotor windings are shorted and a start is attempted (i.e.,
the motor is converted to a standard induction motor),
it will exhibit an extremely high locked rotor current –
typically as high as 1400% and a very low locked rotor
torque, perhaps as low as 60%. In most applications,
this is not an option.
Modifying the speed torque curve by altering the rotor
resistors, the speed at which the motor will drive a
particular load can be altered. At full load, you can
reduce the speed effectively to about 50% of the motor
synchronous speed, particularly when driving variable
torque/variable speed loads, such as printing presses
or compressors. Reducing the speed below 50%
results in very low efficiency due to higher power
dissipation in the rotor resistances. This type of motor
is used in applications for driving variable torque/
variable speed loads, such as in printing presses,
compressors, conveyer belts, hoists and elevators.
TORQUE EQUATION GOVERNING
MOTOR OPERATION
The motor load system can be described by a
fundamental torque equation.
EQUATION 3:
For drives with constant inertia, (dJ/dt) = 0. Therefore,
the equation would be:
EQUATION 4:
This shows that the torque developed by the motor is
counter balanced by a load torque, T
l
and a dynamic
torque, J(dω
m
/dt). The torque component, J(dω/dt), is
called the dynamic torque because it is present only
during the transient operations. The drive accelerates
or decelerates depending on whether T is greater or
less than T
l
. During acceleration, the motor should sup-
ply not only the load torque, but an additional torque
component, J(dω
m
/dt), in order to overcome the drive
inertia. In drives with large inertia, such as electric
trains, the motor torque must exceed the load torque by
a large amount in order to get adequate acceleration.
In drives requiring fast transient response, the motor
torque should be maintained at the highest value and
the motor load system should be designed with the low-
est possible inertia. The energy associated with the
dynamic torque, J(dω
m
/dt), is stored in the form of
kinetic energy (KE) given by, J(ω
2
m
/2). During deceler-
ation, the dynamic torque, J(dω
m
/dt), has a negative
sign. Therefore, it assists the motor developed torque T
and maintains the drive motion by extracting energy
from the stored kinetic energy.
To summarize, in order to get steady state rotation of
the motor, the torque developed by the motor (T)
should always be equal to the torque requirement of
the load (T
l
).
The torque-speed curve of the typical three-phase
induction motor is shown in Figure 11.
Slip Ring
External Rotor
Resistance
Brush
Wound Rotor
TT
l
– J
dω
m
dt
ω
m
dJ
dt
+=
where:
T = the instantaneous value of the
developed motor torque (N-m or lb-inch)
T
l
= the instantaneous value of the load torque
(N-m or lb-inch)
ω
m
= the instantaneous angular
velocity of the motor shaft (rad/sec)
J = the moment of inertia of the motor –
load system (kg-m
2
or lb-inch
2
)
TT
l
J
dω
m
dt
+=
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DS00887A-page 8 2003 Microchip Technology Inc.
FIGURE 11: TYPICAL TORQUE-SPEED CURVE OF 3-PHASE AC INDUCTION MOTOR
STARTING CHARACTERISTIC
Induction motors, at rest, appear just like a short cir-
cuited transformer and if connected to the full supply
voltage, draw a very high current known as the “Locked
Rotor Current.” They also produce torque which is
known as the “Locked Rotor Torque”. The Locked
Rotor Torque (LRT) and the Locked Rotor Current
(LRC) are a function of the terminal voltage of the motor
and the motor design. As the motor accelerates, both
the torque and the current will tend to alter with rotor
speed if the voltage is maintained constant.
The starting current of a motor with a fixed voltage will
drop very slowly as the motor accelerates and will only
begin to fall significantly when the motor has reached
at least 80% of the full speed. The actual curves for the
induction motors can vary considerably between
designs but the general trend is for a high current until
the motor has almost reached full speed. The LRC of a
motor can range from 500% of Full-Load Current (FLC)
to as high as 1400% of FLC. Typically, good motors fall
in the range of 550% to 750% of FLC.
The starting torque of an induction motor starting with a
fixed voltage will drop a little to the minimum torque,
known as the pull-up torque, as the motor accelerates
and then rises to a maximum torque, known as the
breakdown or pull-out torque, at almost full speed and
then drop to zero at the synchronous speed. The curve
of the start torque against the rotor speed is dependant
on the terminal voltage and the rotor design.
The LRT of an induction motor can vary from as low as
60% of FLT to as high as 350% of FLT. The pull-up
torque can be as low as 40% of FLT and the breakdown
torque can be as high as 350% of FLT. Typically, LRTs
for medium to large motors are in the order of 120% of
FLT to 280% of FLT. The PF of the motor at start is
typically 0.1-0.25, rising to a maximum as the motor
accelerates and then falling again as the motor
approaches full speed.
RUNNING CHARACTERISTIC
Once the motor is up to speed, it operates at a low slip,
at a speed determined by the number of the stator
poles. Typically, the full-load slip for the squirrel cage
induction motor is less than 5%. The actual full-load slip
of a particular motor is dependant on the motor design.
The typical base speed of the four pole induction motor
varies between 1420 and 1480 RPM at 50 Hz, while the
synchronous speed is 1500 RPM at 50 Hz.
The current drawn by the induction motor has two com-
ponents: reactive component (magnetizing current)
and active component (working current). The magne-
tizing current is independent of the load but is depen-
dant on the design of the stator and the stator voltage.
The actual magnetizing current of the induction motor
can vary, from as low as 20% of FLC for the large two
pole machine, to as high as 60% for the small eight pole
machine. The working current of the motor is directly
proportional to the load.
Sample Load Torque Curve
Pull-up Torque
Full Voltage Start Torque
Full Voltage Stator Current
Pull-out Torque
Current (% of Motor Full-Load Current)
LRC
LRT
Rotor Speed (% of Full Speed)
10%
20% 30% 40%
50%
60%
70%
80%
90% 100%
7 x FLC
6 x FLC
5 x FLC
4 x FLC
3 x FLC
2 x FLC
1 x FLC
2 x FLT
1 x FLT
Torque (% of Motor Full-Load Torque)
2003 Microchip Technology Inc. DS00887A-page 9
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The tendency for the large machines and high-speed
machines is to exhibit a low magnetizing current, while
for the low-speed machines and small machines the
tendency is to exhibit a high magnetizing current. A
typical medium sized four pole machine has a
magnetizing current of about 33% of FLC.
A low magnetizing current indicates a low iron loss,
while a high magnetizing current indicates an increase
in iron loss and a resultant reduction in the operating
efficiency.
Typically, the operating efficiency of the induction motor
is highest at 3/4 load and varies from less than 60% for
small low-speed motors to greater than 92% for large
high-speed motors. The operating PF and efficiencies
are generally quoted on the motor data sheets.
LOAD CHARACTERISTIC
In real applications, various kinds of loads exist with
different torque-speed curves. For example, Constant
Torque, Variable Speed Load (screw compressors,
conveyors, feeders), Variable Torque, Variable Speed
Load (fan, pump), Constant Power Load (traction
drives), Constant Power, Constant Torque Load (coiler
drive) and High Starting/Breakaway Torque followed by
Constant Torque Load (extruders, screw pumps).
The motor load system is said to be stable when the
developed motor torque is equal to the load torque
requirement. The motor will operate in a steady state at
a fixed speed. The response of the motor to any
disturbance gives us an idea about the stability of the
motor load system. This concept helps us in quickly
evaluating the selection of a motor for driving a
particular load.
In most drives, the electrical time constant of the motor
is negligible as compared to its mechanical time con-
stant. Therefore, during transient operation, the motor
can be assumed to be in an electrical equilibrium,
implying that the steady state torque-speed curve is
also applicable to the transient operation.
As an example, Figure 12 shows torque-speed curves
of the motor with two different loads. The system can
be termed as stable, when the operation will be
restored after a small departure from it, due to a
disturbance in the motor or load.
For example, disturbance causes a reduction of ∆ω
m
in
speed. In the first case, at a new speed, the motor
torque (T) is greater than the load torque (T
l
). Conse-
quently, the motor will accelerate and the operation will
be restored to X. Similarly, an increase of ∆ω
m
in the
speed, caused by a disturbance, will make the load
torque (T
l
) greater than the motor torque (T), resulting
in a deceleration and restoration of the point of
operation to X. Hence, at point X, the system is stable.
In the second case, a decrease in the speed causes
the load torque (T
l
) to become greater than the motor
torque (T), the drive decelerates and the operating
point moves away from Y. Similarly, an increase in the
speed will make the motor torque (T) greater than the
load torque (T
l
), which will move the operating point
further away from Y. Thus, at point Y, the system is
unstable.
This shows that, while in the first case, the motor
selection for driving the given load is the right one; in
the second case, the selected motor is not the right
choice and requires changing for driving the given load.
The typical existing loads with their torque-speed
curves are described in the following sections.
FIGURE 12: TORQUE-SPEED CURVE – SAME MOTOR WITH TWO DIFFERENT LOADS
0
Torque
Torque
X
T
T
Y
ω
m
ω
m
T
l
T
l
0
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DS00887A-page 10 2003 Microchip Technology Inc.
Constant Torque, Variable Speed Loads
The torque required by this type of load is constant
regardless of the speed. In contrast, the power is
linearly proportional to the speed. Equipment, such as
screw compressors, conveyors and feeders, have this
type of characteristic.
FIGURE 13: CONSTANT TORQUE,
VARIABLE SPEED LOADS
Variable Torque, Variable Speed Loads
This is most commonly found in the industry and
sometimes is known as a quadratic torque load. The
torque is the square of the speed, while the power is the
cube of the speed. This is the typical torque-speed
characteristic of a fan or a pump.
FIGURE 14: VARIABLE TORQUE,
VARIABLE SPEED LOADS
Constant Power Loads
This type of load is rare but is sometimes found in the
industry. The power remains constant while the torque
varies. The torque is inversely proportional to the
speed, which theoretically means infinite torque at zero
speed and zero torque at infinite speed. In practice,
there is always a finite value to the breakaway torque
required. This type of load is characteristic of the trac-
tion drives, which require high torque at low speeds for
the initial acceleration and then a much reduced torque
when at running speed.
FIGURE 15: CONSTANT POWER
LOADS
Constant Power, Constant Torque Loads
This is common in the paper industry. In this type of
load, as speed increases, the torque is constant with
the power linearly increasing. When the torque starts to
decrease, the power then remains constant.
FIGURE 16: CONSTANT POWER,
CONSTANT TORQUE
LOADS
High Starting/Breakaway Torque
Followed by Constant Torque
This type of load is characterized by very high torque at
relatively low frequencies. Typical applications include
extruders and screw pumps.
FIGURE 17: HIGH STARTING/
BREAKAWAY TORQUE
FOLLOWED BY
CONSTANT TORQUE
Torque
Power
Speed
Torque
Power
Speed
Torque
Power
Speed
Torque
Power
Speed
Torque
Speed
2003 Microchip Technology Inc. DS00887A-page 11
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MOTOR STANDARDS
Worldwide, various standards exist which specify vari-
ous operating and constructional parameters of a
motor. The two most widely used parameters are the
National Electrical Manufacturers Association (NEMA)
and the International Electrotechnical Commission
(IEC).
NEMA
NEMA sets standards for a wide range of electrical
products, including motors. NEMA is primarily associ-
ated with motors used in North America. The standards
developed represent the general industry practices and
are supported by manufacturers of electrical equip-
ment. These standards can be found in the NEMA
Standard Publication No. MG 1. Some large AC motors
may not fall under NEMA standards. They are built to
meet the requirements of a specific application. They
are referred to as above NEMA motors.
IEC
IEC is a European-based organization that publishes
and promotes worldwide, the mechanical and electrical
standards for motors, among other things. In simple
terms, it can be said that the IEC is the international
counterpart of the NEMA. The IEC standards are
associated with motors used in many countries. These
standards can be found in the IEC 34-1-16. The motors
which meet or exceed these standards are referred to
as IEC motors.
The NEMA standards mainly specify four design types
for AC induction motors – Design A, B, C and D. Their
typical torque-speed curves are shown in Figure 18.
• Design A has normal starting torque (typically
150-170% of rated) and relatively high starting
current. The breakdown torque is the highest of all
the NEMA types. It can handle heavy overloads
for a short duration. The slip is <= 5%. A typical
application is the powering of injection molding
machines.
• Design B is the most common type of AC
induction motor sold. It has a normal starting
torque, similar to Design A, but offers low starting
current. The locked rotor torque is good enough to
start many loads encountered in the industrial
applications. The slip is <= 5%. The motor effi-
ciency and full-load PF are comparatively high,
contributing to the popularity of the design. The
typical applications include pumps, fans and
machine tools.
• Design C has high starting torque (greater than
the previous two designs, say 200%), useful for
driving heavy breakaway loads like conveyors,
crushers, stirring machines, agitators, reciprocat-
ing pumps, compressors, etc. These motors are
intended for operation near full speed without
great overloads. The starting current is low. The
slip is <= 5%.
• Design D has high starting torque (higher than all
the NEMA motor types). The starting current and
full-load speed are low. The high slip values
(5-13%) make this motor suitable for applications
with changing loads and subsequent sharp
changes in the motor speed, such as in
machinery with energy storage flywheels, punch
presses, shears, elevators, extractors, winches,
hoists, oil-well pumping, wire-drawing machines,
etc. The speed regulation is poor, making the
design suitable only for punch presses, cranes,
elevators and oil well pumps. This motor type is
usually considered a “special order” item.
FIGURE 18: TORQUE-SPEED CURVES OF DIFFERENT NEMA STANDARD MOTORS
Design B
Design C
Design D
Design A
20
40
60
80
100
Speed (%)
Torque (% of Full-Load Torque)
300
200
100
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DS00887A-page 12 2003 Microchip Technology Inc.
Recently, NEMA has added one more design –
Design E – in its standard for the induction motor.
Design E is similar to Design B, but has a higher
efficiency, high starting currents and lower full-load
running currents. The torque characteristics of Design
E are similar to IEC metric motors of similar power
parameters.
The IEC Torque-Speed Design Ratings practically
mirror those of NEMA. The IEC Design N motors are
similar to NEMA Design B motors, the most common
motors for industrial applications. The IEC Design H
motors are nearly identical to NEMA Design C motors.
There is no specific IEC equivalent to the NEMA
Design D motor. The IEC Duty Cycle Ratings are
different from those of NEMA’s. Where NEMA usually
specifies continuous, intermittent or special duty
(typically expressed in minutes), the IEC uses nine
different duty cycle designations (IEC 34 -1).
The standards, shown in Table 1, apart from specifying
motor operating parameters and duty cycles, also
specify temperature rise (insulation class), frame size
(physical dimension of the motor), enclosure type,
service factor and so on.
TABLE 1: MOTOR DUTY CYCLE TYPES AS PER IEC STANDARDS
No. Ref. Duty Cycle Type Description
1 S1 Continuous running Operation at constant load of sufficient duration to reach the thermal
equilibrium.
2 S2 Short-time duty Operation at constant load during a given time, less than required to reach
the thermal equilibrium, followed by a rest enabling the machine to reach a
temperature similar to that of the coolant (2 Kelvin tolerance).
3 S3 Intermittent periodic duty A sequence of identical duty cycles, each including a period of operation at
constant load and a rest (without connection to the mains). For this type of
duty, the starting current does not significantly affect the temperature rise.
4 S4 Intermittent periodic duty
with starting
A sequence of identical duty cycles, each consisting of a significant period of
starting, a period under constant load and a rest period.
5 S5 Intermittent periodic duty
with electric braking
A sequence of identical cycles, each consisting of a period of starting, a
period of operation at constant load, followed by rapid electric braking and a
rest period.
6 S6 Continuous operation
periodic duty
A sequence of identical duty cycles, each consisting of a period of operation
at constant load and a period of operation at no-load. There is no rest period.
7 S7 Continuous operation
periodic duty with electric
braking
A sequence of identical duty cycles, each consisting of a period of starting, a
period of operation at constant load, followed by an electric braking. There is
no rest period.
8 S8 Continuous operation
periodic duty with related
load and speed changes
A sequence of identical duty cycles, each consisting of a period of operation
at constant load corresponding to a predetermined speed of rotation,
followed by one or more periods of operation at another constant load
corresponding to the different speeds of rotation (e.g., duty ). There is no rest
period. The period of duty is too short to reach the thermal equilibrium.
9 S9 Duty with non-periodic
load and speed variations
Duty in which, generally, the load and the speed vary non-periodically within
the permissible range. This duty includes frequent overloads that may
exceed the full loads.
2003 Microchip Technology Inc. DS00887A-page 13
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TYPICAL NAME PLATE OF AN
AC INDUCTION MOTOR
A typical name plate on an AC induction motor is
shown in Figure 19.
FIGURE 19: A TYPICAL NAME PLATE
TABLE 2: NAME PLATE TERMS AND THEIR MEANINGS
ORD. No.
<Name of Manufacturer>
<Address of Manufacturer>
286T
1.10
415
60
01/15/2003
95
B
1N4560981324
HIGH EFFICIENCY
42
42
1790
CONT
F
TYPE
H.P.
AMPS
R.P.M.
DUTY
CLASS
INSUL
FRAME
SERVICE
FACTOR
VOLTS
HERTZ
DATE
NEMA
NOM. EFF.
NEMA
DESIGN
3 PH
Y
4 POLE
Term Description
Volts Rated terminal supply voltage.
Amps Rated full-load supply current.
H.P. Rated motor output.
R.P.M Rated full-load speed of the motor.
Hertz Rated supply frequency.
Frame External physical dimension of the motor based on the NEMA standards.
Duty Motor load condition, whether it is continuos load, short time, periodic, etc.
Date Date of manufacturing.
Class Insulation Insulation class used for the motor construction. This specifies max. limit of the motor winding
temperature.
NEMA Design This specifies to which NEMA design class the motor belongs to.
Service Factor Factor by which the motor can be overloaded beyond the full load.
NEMA Nom.
Efficiency
Motor operating efficiency at full load.
PH Specifies number of stator phases of the motor.
Pole Specifies number of poles of the motor.
Specifies the motor safety standard.
Y Specifies whether the motor windings are start (Y) connected or delta (∆) connected.
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DS00887A-page 14 2003 Microchip Technology Inc.
NEED FOR THE ELECTRICAL DRIVE
Apart from the nonlinear characteristics of the induction
motor, there are various issues attached to the driving
of the motor. Let us look at them one by one.
Earlier motors tended to be over designed to drive a
specific load over its entire range. This resulted in a
highly inefficient driving system, as a significant part of
the input power was not doing any useful work. Most of
the time, the generated motor torque was more than
the required load torque.
For the induction motor, the steady state motoring
region is restricted from 80% of the rated speed to
100% of the rated speed due to the fixed supply
frequency and the number of poles.
When an induction motor starts, it will draw very high
inrush current due to the absence of the back EMF at
start. This results in higher power loss in the transmis-
sion line and also in the rotor, which will eventually heat
up and may fail due to insulation failure. The high
inrush current may cause the voltage to dip in the
supply line, which may affect the performance of other
utility equipment connected on the same supply line.
When the motor is operated at a minimum load (i.e.,
open shaft), the current drawn by the motor is primarily
the magnetizing current and is almost purely inductive.
As a result, the PF is very low, typically as low as 0.1.
When the load is increased, the working current begins
to rise. The magnetizing current remains almost con-
stant over the entire operating range, from no load to
full load. Hence, with the increase in the load, the PF
will improve.
When the motor operates at a PF less than unity, the
current drawn by the motor is not sinusoidal in nature.
This condition degrades the power quality of the supply
line and may affect performances of other utility
equipment connected on the same line. The PF is very
important as many distribution companies have started
imposing penalties on the customer drawing power at
a value less than the set limit of the PF. This means the
customer is forced to maintain the full-load condition for
the entire operating time or else pay penalties for the
light load condition.
While operating, it is often necessary to stop the motor
quickly and also reverse it. In applications like cranes
or hoists, the torque of the drive motor may have to be
controlled so that the load does not have any
undesirable acceleration (e.g., in the case of lowering
of loads under the influence of gravity). The speed and
accuracy of stopping or reversing operations improve
the productivity of the system and the quality of the
product. For the previously mentioned applications,
braking is required. Earlier, mechanical brakes were in
use. The frictional force between the rotating parts and
the brake drums provided the required braking.
However, this type of braking is highly inefficient. The
heat generated while braking represents loss of
energy. Also, mechanical brakes require regular
maintenance.
In many applications, the input power is a function of
the speed like fan, blower, pump and so on. In these
types of loads, the torque is proportional to the square
of the speed and the power is proportional to the cube
of speed. Variable speed, depending upon the load
requirement, provides significant energy saving. A
reduction of 20% in the operating speed of the motor
from its rated speed will result in an almost 50%
reduction in the input power to the motor. This is not
possible in a system where the motor is directly
connected to the supply line. In many flow control
applications, a mechanical throttling device is used to
limit the flow. Although this is an effective means of
control, it wastes energy because of the high losses
and reduces the life of the motor valve due to
generated heat.
When the supply line is delivering the power at a PF
less than unity, the motor draws current rich in harmon-
ics. This results in higher rotor loss affecting the motor
life. The torque generated by the motor will be pulsating
in nature due to harmonics. At high speed, the pulsat-
ing torque frequency is large enough to be filtered out
by the motor impedance. But at low speed, the pulsat-
ing torque results in the motor speed pulsation. This
results in jerky motion and affects the bearings’ life.
The supply line may experience a surge or sag due to
the operation of other equipment on the same line. If
the motor is not protected from such conditions, it will
be subjected to higher stress than designed for, which
ultimately may lead to its premature failure.
All of the previously mentioned problems, faced by both
consumers and the industry, strongly advocated the
need for an intelligent motor control.
With the advancement of solid state device technology
(BJT, MOSFET, IGBT, SCR, etc.) and IC fabrication
technology, which gave rise to high-speed micro-
controllers capable of executing real-time complex
algorithm to give excellent dynamic performance of the
AC induction motor, the electrical Variable Frequency
Drive became popular.
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