THREE PHASE & SINGLE PHASE

INDUCTION MOTOR

□Introduction of three phase induction motor and

its working principle
□Introduction of Single phase induction motor and its
operating mechanism
□Application of induction motor(1-phase and 3-phase)

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 3-PHASE INDUCTION
MOTOR
● Like any other electric motor, a 3-phase induction
motor has a stator and a rotor.
● The stator carries a 3-phase winding (called stator
winding) while the rotor carries a short-circuited
winding (called rotor winding). Only the stator
winding is fed from 3-phase supply.
● The rotor winding derives its voltage and power
from the externally energized stator winding through
electromagnetic induction

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 Construction
● An induction motor has two main parts
⚫ 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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 ⚫ 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
(wound-rotor) » 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
(squirrel-cage)
● Two basic design types depending on the rotor design
● squirrel-cage
● wound-rotor

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 Department of Applied Science, BVCOE New Delhi Subject: ELECTRICALS
18 CIENCE, Instructor:Ashima Airan
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 Application Of A Induction Motor

● Leath machine , Compressors , centrifugal pump where

cost is most important factor

● Crane , hoist , lift and where high starting torque is

required

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 Single Phase Induction Motor
● It is more convenient to use single phase ac motors
instead of d.c. motors. Practically single phase a.c.
motors are used in most of the applications.
⚫ Construction:
□ A single phase induction motor has two main parts
namely stator (the one which is stationary) and rotor
(the one which is rotating).
□ The stator winding is connected across a single
phase
a.c. supply. The ac supply produces a changing
magnetic field in the airgap between the stator and
rotor.
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07/11/2024 Department of Electrical and Electronics Engineering, BVCOE New Delhi
Subject: ELECTRICAL TECHNOLOGY , Instructor: NEEVATIKA VERMA
 Principle of operation:
□A single phase ac supply is connected to the stator winding. This forces an alternating
current through the stator winding. This current produces an alternating flux in the
air gap between stator and rotor.
□The alternating flux passes over the rotor conductors and induces an emf into it due
to transformer action.
□Due to this induced voltage, a current starts flowing through the rotor conductors.
This current will then produces its own flux called as rotor flux
□The main flux produced by the stator winding interacts with the rotor flux to
produce the torque.
□The rotor flux gets produced due to the principle of induction hence it is called
as induction motor.
□But single phase induction motor are not self starting, like DC motors.

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 Types of single phase induction motor:
□Some of the methods used to make an induction motor self starting. Based on these
techniques, the single phase induction motor are classified as follows:
1. Split phase induction motor
2. Capacitor start induction motor
3. Capacitor start, capacitor run induction motor
4. Shaded pole induction motor.
R.esistive Split Phase
Induction Motor
□ The construction of split phase induction
motor is
as shown in fig.(1).
□ This motor consists of two winding namely the
main winding and the auxillary (starting)
winding.
□ The main winding is highly inductive while
starting winding is resistive
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 Principle of operation:
□The current flowing through the main winding (Im ) lags behind the V by 900 since the
main winding is highly inductive.
□The current flowing through the starting winding (Ist) is almost in phase with the
supply voltage V as this winding is resistive.
□The fluxes produced due to these currents will be placed 900 with respect each other.
And the resultant of these fluxes will be a rotating magnetic field. Due to the RMF
a non-zero starting torque acting in one direction will be produced.
□The centrifugal switch connected in series with the starting winding gets automatically
open circuited when the motor speed reaches about 70% to 80% of the synchronous
speed.
□After that the motor rotate only on the main winding. Thus under the
running condition the auxillary winding remains out of the circuit.
□The direction of rotation of split phase motor can be reversed by reversing the
terminals of either main winding or starting winding. The direction of rotation changes
due to the reversal in direction of rotating magnetic field.

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 Applications:
□The starting torque of this motor is poor. So it is used in following applications:
1. Fans and blowers
2. Washing machines
3. Centrifugal pumps

Capacitor Start Induction Motor

□ The construction of this motor is as shown in fig.(1.a).
□ As shown in fig, the starting winding connected in series with the capacitor draws a
leading current while the main winding continues to draw the lagging current.
□ Due to this the fluxes produce a rotating magnetic field which result in the rotation
of the motor.
□ The current (Im) through the main winding will lag behi9nd the source voltage as the
main winding is inductive. But the angle (Ist) through the starting winding leads the
supply voltage by some angle due to the presence of capacitor. Hence the angle
between the fluxes produced by Im and Ist will be large as shown in fig.(1.b).

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 Due to this large angle, the starting torque produced by the capacitor start motor
produces a larger starting torque as compared to that produced by the split
phase induction motor.
As soon as the speed reaches 75% to 80% of the maximum speed, the centrifugal
switch is automatically open circuited and the starting winding alongwith the capacitor
goes out of the circuit.
The induction motor will then be running only on the flux produced by the
main winding

Fig.(1): Capacitor Start Motor

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 Applications:
□Due to high starting torque, the capacitor start or capacitor start capacitor run
motors are used in the following applications:
1. Grinders
2. Compressors
3. Conveyers
4. Fans and air conditioners
5. Refrigerators

Shaded Pole Induction Motor

□ Fig.(3), shows the construction of a shaded pole induction motor.
□ Every stator pole is divided into two parts by keeping a small slit in the pole
face and the smaller portion is covered with a thick short circuited copper wire
called shading band.
□ When stator winding carries current, the main pole produces a flux øm.
□ This flux links with the shading band and this band acts as a shorted secondary
winding, stator winding being its primary. Circulating currents induced tn the
band produced nother flux øs.

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 .

□ At instant t1 current is increasing. The induced emf tries to oppose it. Thus
flux øs opposes it and resultant flux is in unshaded part.
□ At instant t2 current is almost constant. Induced emf and flux øs are
negligible. Resultant flux is almost at the center of the pole. Thus it
has shifted its position.
□ At instant t3 current is decreasing. The induced current and flux øs try to
oppose this decrease. Resultant flux lies in shaded part.
□ This action continues and the resultant field rotates from unshaded part
to shaded part. Hence rotor also rotates in the same direction.
□ The direction of rotation cannot be reversed unless position of shaded ring
is changed from one part of pole to another part.
□ Such motor develops low starting torque and it has a low power factor

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 .

Fig.(3): shaded pole induction motor

Applications:
1. Table fans
2. Blowers
3. Washing machines
4. refrigerators

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 SYNCHRONOUS MOTOR
• A synchronous motor runs under steady state condition at a fixed speed
called the synchronous speed.

• The synchronous speed depends only upon the frequency of the applied
voltage and the number of poles in the machine.

• The speed of the synchronous motor is independent of the load as long

as the load is within the capability of the motor.

• If the load torque exceeds the maximum torque that can be developed by
the motor, the motor simply comes to rest.
 Principal Of Synchronous Motor
• Synchronous motor is a doubly excited machine i.e two
electrical inputs are provided to it.

• It’s stator winding which consists of a 3 phase winding is

provided with 3 phase supply and rotor is provided with DC
supply.

• The 3 phase stator winding carrying 3 phase currents produces 3

phase rotating magnetic flux.

• The rotor carrying DC supply also produces a constant flux.

• At a particular instant rotor and stator poles might be of same

polarity (N-N or S-S) causing repulsive force on rotor and the
very next second it will be N-S causing attractive force.
• Due to inertia of the rotor, it is unable to rotate in any direction due to
attractive or repulsive force and remain in standstill condition. Hence
it is not self starting.
• To overcome this inertia, rotor is initially fed some mechanical input
which rotates it in same direction as magnetic field to a speed very close to
synchronous speed.
• After some time magnetic locking occurs and the synchronous motor
rotates in synchronism with the frequency.
 Methods of starting of Synchronous motor

1. Start with variable frequency: In this method the frequency of the

source voltage is gradually increased and hence the speed also
increases gradually and the motor reached its synchronous speed.

2. Pony motor start: Synchronous motors are mechanically coupled with

another motor. It could be either 3 phase induction motor or DC shunt
motor. DC excitation is not fed initially. It is rotated at speed very close
to its synchronous speed and after that DC excitation is given. After
some time when magnetic locking takes place supply to the external
motor is cut off.
3. Damper winding: In case, synchronous motor is of salient pole type,
additional winding is placed in rotor pole face. Initially when rotor is
standstill, relative speed between damper winding and rotating air gap flux
is large and an emf is induced in it which produces the required starting
torque. As speed approaches synchronous speed, emf and torque is reduced
and finally when magnetic locking takes place, torque also reduces to zero.
Hence in this case synchronous motor is first run as three phase phase
induction motor using additional winding and finally it is synchronized
with the frequency.
 Application of Synchronous Motor

1. Synchronous motor having no load connected to its shaft is used for

power factor improvement. Owing to its characteristics to behave at
any electrical power factor, it is used in power system in situations
where static capacitors are expensive.

2. Synchronous motor finds application where operating speed is less

(around 500 rpm) and high power is required. For power requirement
from 35 kW to 2500 KW, the size, weight and cost of the
corresponding three phase induction motor is very high. Hence these
motors are preferably used. Ex- Reciprocating pump, compressor,
rolling mills etc.