THREE-PHASE
INDUCTION MOTOR
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 Three-phase induction motors are the most common
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and frequently encountered machines in industry
It can be considered to be the cheapest motor.
It is rugged and requires less maintenance.
It is simple in design.
It gives reliable operation.
Its efficiency is very high.
It is easy to control
It runs at constant speed from zero to full load
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CLASSIFICATION OF INDUCTION
MOTOR
Depending on the rotor construction, induction motor
can be classified into two categories:
 Squirrel-cage induction motor.
 Slip-ring induction motor or wound rotor induction
motor.
Depending on the number of phases it can be classified
as:
 Single-phase induction motor
 Three-phase induction motor
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CONSTRUCTION
The three basic parts of an AC motor are the
rotor, stator, and enclosure.
 A stationary stator
 consisting
of a steel frame that supports
a hollow, cylindrical core
 core, constructed from stacked
laminations, having a number of evenly
spaced slots, providing the space for the
stator winding
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CONSTRUCTION
 A revolving rotor
 composed
of punched laminations, stacked to
create a series of rotor slots, providing space for
the rotor winding
 one of two types of rotor windings
 conventional 3-phase windings made of
insulated wire similar to the winding on the
stator
 aluminum bus bars shorted together at the ends
by two aluminum rings, forming a squirrel-cage
shaped circuit.
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CONSTRUCTION
Squirrel cage type:
Rotor winding is composed of copper bars embedded in the
rotor slots and shorted at both end by end rings
Simple, low cost, robust, low maintenance
Wound rotor type:
Rotor winding is wound by wires. The winding terminals can be
connected to external circuits through slip rings and brushes.
Easy to control speed, more expensive.
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PRINCIPLE OF OPERATION
When a 3 phase stator winding is connected to a 3 phase
voltage supply, 3 phase current will flow in the
windings, which also will induced 3 phase flux in the
stator. These flux will rotate at a speed called a
Synchronous Speed, ns. The flux is called as Rotating
magnetic Field.
Synchronous speed is given by the expression
Where p = is the number of poles, and
f = the frequency of supply
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PRINCIPLE OF OPERATION
 This rotating magnetic field cuts the rotor
windings and produces an induced voltage
in the rotor windings
 Due to the fact that the rotor windings are
short circuited, for both squirrel cage and
wound-rotor, and induced current flows in
the rotor windings
 The rotor current produces another
magnetic field
 A torque is produced as a result of the
interaction of those two magnetic fields
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SLIP
The rotor speed of an Induction machine is different from
the speed of Rotating magnetic field. The % difference
of the speed is called slip.
Where;
ns = synchronous speed (rpm)
nr = mechanical speed of rotor (rpm)
If the rotor runs at synchronous speed, s = 0. If the
rotor is stationary, s = 1
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Effect of slip on rotor frequency
Where fr is the
rotor
frequency, Er2 is
the rotor emf,
Effect of slip on rotor induced e.m.f.
R2, X2 and Z2 is
the rotor
Effect of slip on rotor resistance,
resistance,
reactance and impedance
reactance and
impedance
respectively.
f is the supply
frequency, E2 is
the standstill
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e.m.f.
TORQUE EQUATION OF A THREEPHASE INDUCTION MOTOR
The torque of a three-phase induction motor
depends on the following factors:
 Rotor current (I2)
 Power factor of the rotor circuit
 Flux which links with the rotor (φ)
or
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Torque Equation for Induction
Motor in Running Condition
Where,
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TORQUE SLIP CHARACTERISTICS
 When slip s = 0, Nr = Ns. Under this condition, the
motor stops. So the torque (T) at this value of s is zero.
This shows that the torque slip characteristics starts
from the origin.
 For smaller values of slip, the torque is directly
proportional to the slip.
 For larger values of slip, the torque is inversely
proportional to slip.
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TORQUE SLIP CHARACTERISTICS
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RATIO BETWEEN FULL-LOAD
TORQUE AND MAXIMUM TORQUE
Where Tfl is the full load torque Tmax
is the maximum torque sfl is the slip
at full load and s is the slip at
maximum torque.
m
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RATIO BETWEEN STARTING
TORQUE AND MAXIMUM TORQUE
Where Tst is the starting torque Tmax
is the maximum torque, s is the slip
at maximum torque.
m
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EFFECT OF ROTOR RESISTANCE ON
TORQUE–SLIP CHARACTERISTIC
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Equivalent Circuit of Induction
Motor
Equivalent circuit with Rotor open
circuited
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Equivalent Circuit of Induction
Motor
Equivalent circuit with Rotor short
circuited
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Equivalent Circuit of Induction
Motor
Equivalent circuit with electrical
equivalent of mechanical load
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No-load Test
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No-load Test
• The motor is allowed to run freely.
• This test determines the rotational and
core loss of the motor .
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Block Rotor Test
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Block Rotor Test
• In this test, the rotor is blocked from
rotating.
• This test gives the total winding
resistance and leakage reactance of the
motor.
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STARTING OF THREE-PHASE
INDUCTION MOTOR
There are several methods of starting squirrel-cage
induction motors, which include:
i) Direct on-line starting.
ii) Autotransformer starting
iii) Star–delta starting
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Direct on-line (DOL) starter
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Autotransformer starter
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Star-delta starter
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SPEED CONTROL
From the stator side, the speed of an induction motor can be
controlled by the following methods:
i) V/f method of controlling the speed of induction motor
ii) Speed control by changing the supply voltage.
iii) Speed control by changing the number of poles.
iv) Speed control by changing the rheostat connected with
the stator terminals.
From the rotor side the speed of an induction motor can be
controlled by the following methods:
i) Speed control by changing the rheostat connected with the
rotor terminals.
ii) Cascade control
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BRAKING
The braking methods for induction motor are as follows:
i) Regenerative braking of induction motor
ii) Braking by plugging
iii) Dynamic braking
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