# Basics of 3-phase Induction Motor

## Introduction

This article will deal with those concepts of 3 phase induction motor which 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 of three phase
induction motor.

What is the *in workine of*

**fundamental difference***and*

**induction motor***? That is even though the equivalent circuit of motor and transformer is same rotor of motor rotates where as secondary of transformer do not.*

**transformer**
Induction motor is a generalized transformer. Difference is that
transformer is an 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 of induced EMF in primary and
secondary side is same where as frequency of rotor EMF depends on slip.
During starting when

*the frequency of induced EMF in rotor and stator is same but after loading it is not.***S = 1**
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 per

*the direction of motion is such that it tries to oppose the change which has caused so it starts chasing the field.***Lenz’s Law**## Power flow diagram of induction motor

*= A*

**Stator input electrical power***= B*

**Stator losses***= C*

**Rotor losses***= P*

**Mechanical output**A – ( B + C ) = P

Roughly B= 0.03A, C= 0.04A

A – 0.07A = P

0.93A = P,

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

## Introduction

This article will deal with those concepts of 3 phase induction motor which 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 of three phase
induction motor.

What is the *in workine of*

**fundamental difference***and*

**induction motor***? That is even though the equivalent circuit of motor and transformer is same rotor of motor rotates where as secondary of transformer do not.*

**transformer**
Induction motor is a generalized transformer. Difference is that
transformer is an 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 of induced EMF in primary and
secondary side is same where as frequency of rotor EMF depends on slip.
During starting when

*the frequency of induced EMF in rotor and stator is same but after loading it is not.***S = 1**
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 per

*the direction of motion is such that it tries to oppose the change which has caused so it starts chasing the field.***Lenz’s Law**## Power flow diagram of induction motor

*= A*

**Stator input electrical power***= B*

**Stator losses***= C*

**Rotor losses***= P*

**Mechanical output**A – ( B + C ) = P

Roughly B= 0.03A, C= 0.04A

A – 0.07A = P

0.93A = P,

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