basics of 3-phase induction motor (part 1) _ eep

7/30/2019 Basics of 3-Phase Induction Motor (Part 1) _ EEP

1/4

/5/12 Basics of 3-phase Induction Motor (part 1) | EEP

1/4lectrical-engineering-portal.com/basics-of-3-phase-induction-motor-1

electrical-engineering-portal.com

Basics of 3-phase Induction Motor

http://electrical-engineering-portal.com/basics-of-3-

phase-induction-motor-1

Basics of 3-phase Induction Motor (part 1)

Introduction

This article will deal with those concepts of3 phase induction motorwhich are essential

prerequisite for proper selection, procurement, installation and maintenance of the same.

Before any actual discussion on motor is started It will better to have a comparison of

starting behavior of induction motor and transformer because as per the equivalent

circuit representation a 3 phase induction motor is generalized transformer.

It is assumed that readers are already familiar with the elementary concept of principle of

operation and construction ofthree phase induction motor.

What is the fundamental difference in working principle ofinduction motorand

transformer? That is even though the equivalent circuit of motor and transformer is

same rotor of motor rotates where as secondary of transformer do not.

Induction motoris a generalized transformer. Difference is that transformer is an


2/4



alternating flux machine while induction motor is rotating flux machine. Rotating flux is

only possible when 3 phase voltage (or poly phase) which is 120 degree apart in time is

applied to a three phase winding (or poly phase winding) 120 degree apart in space

then a three phase rotating magnetic flux is produced whose magnitude is constant but

direction keeps changing. In transformer the flux produced is time alternating and not

rotating.

There is no air gap between primary and secondary of transformer where as there is a

distinct air gap between stator and rotor of motor which gives mechanical movability to

motor. Because of higher reluctance ( or low permeability) of air gap the magnetizing

current required in motor is 25-40% of rated current of motor where as in transformer it is

only 2 -5 % of rated primary current.

In an alternating flux machine frequency ofinduced EMF in primary and secondary side

is same where as frequency of rotor EMF depends on slip. During starting when S = 1

the frequency of induced EMF in rotor and stator is same but after loading it is not.

Other difference is that the secondary winding and core is mounted on a shaft set in

bearings free to rotate and hence the name rotor.

If at all secondary of a transformer is mounted on shaft set at bearings the rate of cutting

of mutual magnetic flux with secondary circuit would be different from primary and their

frequency would be different. The induced EMF would not be in proportion to number of

turns ratio but product of turn ratio and frequency. The ratio of primary frequency to the

secondary frequency is called slip.

Any current carrying conductor if placed in magnetic field experience a force so rotor

conductor experience a torque and as perLenzs Lawthe direction of motion is such

that it tries to oppose the change which has caused so it starts chasing the field.

Power flow diagram of induction motor

Stator input electrical power= A

Stator losses = B

Rotor losses = C

Mechanical output= P

A ( B + C ) = P

Roughly B= 0.03A, C= 0.04AA 0.07A = P

0.93A = P, Hence efficiency = (P/A) x 100 = 93%


3/4



Power flow diagram of induction motor

Why LT motors are delta connected and HT motors arestar connected?

Reason is techno commercial.

1. In star, phase current is same as line current. But phase voltage is 1/1.732 times

line voltage. So insulation required in case of HT motor is less.

2. The starting current for motors is 6 to 7 times full load current. So start-up power

will be large if HT motors are delta connected. It may cause instability (voltage dip)in case small Power system. In starred HT motor starting current will be less

compared to delta connected motor. So starting power is reduced. Starting torque

will also be reduced. (It will not be a problem as motors are of high capacity.)

3. Also as current is less copper (Cu) required for winding will be less.

4. LT motors are delta connected.

1. Insulation will not be problem as voltage level is less.

2. Starting current will not be problem as starting power in all will be less. So noproblem of voltage dips.

3. Starting torque should be large, as motors are of small capacity.


4/4



Comparison of star and delta motor starting

LT motors have winding delta connected.

1. In case it is having stardelta starterthan they are started as Star connected motor.

2. After it attains 80% of synch speed the changeover takes place from star to original

configuration delta.

3. In star the voltages across the windings are lesser that is 1/1.732 times that available

in delta so current is limited.

4. When it goes to delta again voltage is full line voltage so current increase even though

it is lesser than the line current it remains higher than the line current drawn in star

connection at reduced voltage. So cables for motor are sized for this current that is what

it draws in delta connection.

References:

1. NEMA MG-1.

2. Industrial Power Engineering and Application Hand Book by K C Agarwaal.

3. Industrial Power System Hand Book by Shoaib Khan.

4. Theory and Calculation of Alternating Current Phenomena by Charles Proteus

Steinmetz

5. Motor protection relay (MM30) manual from L&T

basics of 3-phase induction motor (part 1) _ eep

Documents