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Home / Technical Articles / Basics of 3-phase Induction Motor (part 1)

Introduction to 3-ph motor

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.

Basics of 3-phase Induction Motor (part 1)
Basics of 3-phase Induction Motor (part 1)

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 fundamental difference in working principle of induction motor and 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 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 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 per Lenz’s Law the 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= 0.04A
A – 0.07A = P
0.93A = P, Hence efficiency = (P/A) x 100 = 93%

Power flow diagram of induction motor
Power flow diagram of induction motor

Why LT motors are delta connected and HT motors are star 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 no problem of voltage dips.
    3. Starting torque should be large, as motors are of small capacity.
Comparison of star and delta motor starting
Comparison of star and delta motor starting

LT motors have winding delta connected.

1. In case it is having star delta starter than 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.


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

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More Information

Asif Eqbal

Bachelor of Engineering in Electrical & Electronics engineering, from Manipal University, (Karnataka), India in 2006. Presently involved in the design of EHV outdoor substation and coal fired thermal power plants for more than seven years. Motto of joining EEP as a contributor is to share my little engineering experience and help the budding engineers in bridging the conspicuous gap between academics and Industrial practice. “If you have knowledge, let others light their candles with it, so that people who are genuinely interested in helping one another develop new capacities for action; it is about creating timeless learning processes".


  1. Stanley Ogidi
    May 04, 2019

    Have an idea to invent an electric power generating machine but I need you assistance, thanks ,please you can whatsap me on this line for discussion +2348034681128

  2. rajaram maity
    Jan 18, 2018

    please explin how much i set the derived earth fault both idmt and instantaneous .600 kw 3.3kv induction motor .there are a relay siemens argus 7sr17

  3. Yadav Rishikesh
    Nov 10, 2017

    What happens if we use silicon steel core in induction motor for path of flux instead of air gap?

  4. Mehdi
    Dec 31, 2016

    I want to know how we can defined the turn ration between the primary and secondary in induction motors.

      Jan 14, 2017

      please guide me that how much an LT 3 phase induction motor will consume current in star configuration at start

      for example we have a 37W/50 hp motor so what will be starting current consumption for this motor and motor is connected in star delta configuration. i m confused it will be six times or 3 times of the rated current.

    • Yadav Rishikesh Balasaheb
      Nov 10, 2017

      What happens if we use silicon steel core in induction motor for path of flux instead of air gap?

  5. pankaj
    Aug 15, 2016


    Can anyone tell me, how to find the winding data of fresh stator. if slots & HP is known

  6. John Stewart
    Nov 07, 2015

    Hi Asif,
    There’s a slight misunderstanding in your explanation on the reason for Star and Delta connections.
    In fact the reason we use Star connection on higher voltage machines is to drop the Volts per pole.
    The motor performance doesn’t need to change at all because the turns per coil and conductor sizes are changed to suit the changed phase volts.
    Star connection reduces the volts per phase, so in design, the turns are reduced proportionately and the conductor size increased proportionately.
    That means the flux density in the iron remains the same, so shaft torque and starting curve is the same.
    There is no real commercial advantage either. There is the same total amount of copper in the slot.

  7. Michael
    Oct 02, 2015

    Could you please explain/ draw how to connect a three pole two way with off rotary switch to a three phase motor in order to obtain two speeds with off between.

  8. virender singh
    Sep 11, 2015

    give me ac motor winding data calculator and ac motor winding data list 0.5 hp to350hp

  9. Saroj
    Sep 05, 2015

    Can you please explain Steinmetz equivalent circuit of induction motor, mainly the relationship between frequency of supply and equivalent resistance across the rotor side??

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