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SINGLE-PHASE
INDUCTION MOTOR
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INTRODUCTION
 A single-phase induction motor is structurally similar
to a three-phase induction motor; the difference is
only in the stator winding arrangements. The stator
windings of a single-phase induction motor are
distributed, pitched and skewed to produce a
sinusoidal m.m.f. in space.
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CONSTRUCTION
 The construction of a single-phase induction motor is
similar to that of a three-phase induction motor. The
single-phase motor stator has a laminated iron core
with two windings arranged perpendicularly – one is
the main and the other is the auxiliary winding or
starting winding. The motor uses a squirrel-cage rotor,
which has a laminated iron core with slots in it. Singlephase induction motor is not self-starting and requires
some mechanism to assist it in the starting process.
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DOUBLE-FIELD REVOLVING
THEORY
 According to this theory, a pulsating field of a single-
phase motor can be resolved into two rotating field of
half its amplitude rotating in opposite direction at
synchronous speed.
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DOUBLE-FIELD REVOLVING THEORY
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DOUBLE-FIELD REVOLVING THEORY
 The torque–slip curve is shown below
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CROSS FIELD THEORY
 When a single-phase supply is given to the stator of a
single-phase induction motor, an alternating flux is
produced in the stator.
 This alternating flux will produce an induced e.m.f. in
the rotor windings, which is initially at standstill.
 No torque will be produced in the rotor
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CROSS FIELD THEORY
 If the rotor is rotated by giving some external force, an
e.m.f. will be induced in the rotor and thus a current
will flow through the rotor. This current will be
displaced by 90 degree (electrical) from the stator axis.
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STARTING OF SINGLE-PHASE
INDUCTION MOTOR
Some of the starting methods are,
i) Split-phase method
ii) Shaded-pole method
iii) Reluctance-start method
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Split Phase Method
The different split-phase techniques are:
i) Split-phase resistance-start motor.
ii) Split-phase capacitor-start motor.
iii) Capacitor-start – capacitor-run induction motor
iv) Permanent single capacitor induction motor.
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Split-phase resistance start motor
This method of starting is mostly used in motor
with low inertia load or continuous operating
load.
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Split-phase capacitor start motor
In this type of motor, an
electrolytic capacitor is
connected in series with the
auxiliary winding. Due to the
presence of the capacitor, the
auxiliary winding current will
now lead the applied voltage
and the main winding current
will lag the applied voltage.
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Capacitor-start – Capacitor-run
induction motor
These motors are used for
such applications where
large starting torque and
quiet operations are
required. These motors
produce constant torque
and have better efficiency
and power factor.
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Shaded Pole Method
A part of each pole is wrapped with low
resistance copper bands, which form a closed
loop (These copper bands are called shading
bands or shaded poles).
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Shaded Pole Method
• When a single-phase AC supply is given to the
stator of an induction motor, alternating flux
will set up a current in the shading bands.
• The flux in the shaded poles will lag the stator
flux. The result is similar to a rotating field
moving from un-shaded to shaded portion of the
pole. This will produce the starting torque.
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EQUIVALENT CIRCUIT OF A SINGLEPHASE INDUCTION MOTOR
Equivalent circuit at stand still based on double-field
revolving theory
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EQUIVALENT CIRCUIT OF A SINGLEPHASE INDUCTION MOTOR
Equivalent circuit at any slip
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Block Rotor Test
 Blocked rotor test is conducted on an induction motor.
It is also known as short circuit test or locked rotor test
or stalled torque test.
 From this test short-circuit current at normal
voltage, power factor on short-circuit, total leakage
reactance, starting torque of the motor can be found.
 The test is conducted at low voltage because if the
applied voltage was normal voltage then the current
flowing through the stator windings were high enough
to over heat the winding and damage them.
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No-load Test
 In this test, the rotor is made to rotate freely without
any load.
 Under this condition, the slip due to forward rotating
field will reach zero and the slip due to backward
rotating field will be 2.
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COMPARISON OF SINGLE-PHASE AND
THREE-PHASE INDUCTION MOTOR
 Single-phase induction motors are simple in construction,




reliable and economical for small power rating as compared
to three-phase induction motors.
The electrical power factor of single-phase induction motors
is low as compared to three-phase induction motors.
For the same size, single-phase induction motors develop
about 50% of the output as that of three-phase induction
motors.
The starting torque is low for asynchronous motors.
The efficiency of single-phase induction motors is less as
compared to that of three-phase induction motors.
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