Induction Motor

7/10/2013 PRB/EEE/SCE 1

EE 2302 ELECTRICAL MACHINES - II

TEXT BOOKS

1. D.P. Kothari and I.J. Nagrath, ‘Electric Machines’, Tata McGraw Hill Publishing Company Ltd 2002.2. P.S. Bhimbhra, ‘Electrical Machinery’, Khanna Publishers, 2003.

REFERENCES1. J.B. Gupta, ‘Theory and Performance of Electrical Machines’,

S.K.Kataria and Sons, 2002.2. K. Murugesh Kumar, ‘Induction and Synchronous Machines’,

Vikas Publishing House Pvt Ltd, 2002.3. V.k.Mehta, ‘ Principle of Electric Machines’, S.Chand & Co.

UNIT - III

THREE PHASE INDUCTION MOTOR

POLY PHASE INDUCTION MOTOR –

denotes two or more phases

27/10/2013 PRB/EEE/SCE

Asynchronous Motor – Rotating Transformer

7/10/2013 PRB/EEE/SCE 3

HistoryInventorsRMF concept – Galileo Ferraris – 1885Later by Nikola Tesla – 1887US patents – Nov. 1887Technical Paper – A new system of AC Motor and transformer – May 1888 (AIEE- American Institute of Electrical Engineering)

Licensed in 1888- First Manufacturer- GEC – General Electric Company

Reason behind the name

Three phase induction motors convert three –phase AC

electrical energy into mechanical energy.

The stator winding is fed from 3-phase AC supply.

The rotor winding derives its voltage and power from the

externally energized stator winding through electromagnetic

induction principle and hence the name.

i.e., there is no electrical connection from the

rotor to any source of supply.


Of all electric motors, three-phase induction motor

is the most extensively used in industries, domestic

applications and in other applications.

All over the world , about 80% of total energy

conversion from electrical to mechnical is carried

out by three-phase induction motors.


Advantages of three-phase induction motors

Simple and extremely rugged construction.

Low cost and reliability.

Requires minimum maintenance.

High efficiency ( full load efficiency varies from 85% to 94%)

and good power factor (full load power factor varies from 0.85

to 0.92 lagging in nature)

It has self-starting torque.


Disadvantages of three-phase induction motors

Speed decreases with increase in load.

Its starting torque is inferior to D.C shunt motor.

It is nearly a constant speed motor and its speed cannot be

changed easily.


CLASSIFICATION

1. Single-phase induction motor

2. Three phase induction motor

According to its rotor construction

Slip ring OR phase

wound rotor

Squirrel cage rotor

Single cage Double cage


9

Construction

1. Stator – the part i.e., three phase windings, which is stationary.

2. Rotor – the part which rotates and is connected to mechanical

load via shaft.

Stator stamping


10

Stator core with 3-phase winding


117/10/2013 PRB/EEE/SCE

12

Stator Assembly


13

Stator terminal connections


14

Squirrel cage rotor

/rotor winding

Short circuits all

rotor bars.

Skewed slots


15

Squirrel cage rotorEnd ring

Copper barsEnd ring


16

Stator


17

3-PH, HV, SCIM, 4155HP/3.1MW, 11kV, 2974r.p.m., 195A 50 Hz BROKEN ROTOR BAR – LIFTED OUT OF THE SLOT


18

3-PH, HV, SCIM, 4155HP/3.1MW, 11kV, 2974r.p.m., 195A BROKEN ROTOR BAR – LIFTED IN THE SLOT SECONDARY DAMAGE TO HV STATOR


19

Symbolic representation of squirrel cage induction motor

Star connected stator


20

Delta connected stator


21

Slip ring OR phase wound rotor

Slip ringsShaft3-phase

windingSkewed slots


22

Symbolic representation of slip ring induction motor


23

Skewed slots

The slots on the rotor are always not parallel to the rotor shaft but

are usually skewed. (slanting angle of rotor slot with respect to

shaft axis)

Purpose of skewing

1.It provides uniform torque.

2. It reduces the magnetic locking of the stator and rotor.

3. It reduces the magnetic humming noise while running.


24

Comparison

Wound or slip ring rotor Squirrel cage rotor

Rotor consists of three-

phase winding similar to

the stator winding.

Rotor consists of copper

bars and is short circuited

by two end rings.

Construction is

complicated

Construction is simple

Slip rings and brushes are

present to add external

resistance

Slip rings and brushes are

absent, external

resistance cannot be

added

Frequent maintenance is

necessary

Maintenance free


25

The rotors very costly The rotors are cheap

High staring torque can be

obtained and controlled

Moderate starting torque

which cannot be

controlled

Rotor resistance starter

can be used

Rotor resistance starter

cannot be used

Speed control by rotor

resistance is possible

Not possible

Rotor copper losses are

high ,hence efficiency is

less

Vice versa

Used for high starting

toque loads ,such as

lifts,hoists,cranes,elevators

, compressors etc.,

Used for low starting

toque loads ,such as

lathes, drilling machines,

fans blowers, water

pumps, grinders etc.,7/10/2013 PRB/EEE/SCE

Induction Motor

Documents

Transcript of Induction Motor