Basics of 3-phase Induction Motor

Basics of 3-phase Induction 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. 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


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".


    Mar 27, 2014

    Please add me in your website as I am good fan of you

  2. Nitish
    Dec 11, 2013

    When IM starts we have high inrush current, but how this current comes to normal value when motor works at or around rated speed?
    And is their any difference between full load current and name plate rated current?

  3. [...] induction motor (Source ABB) The capacitor-start (CS), or more precisely, capacitor-start, induction-run motor, is a modified split-phase induction motor used for hard-to-start loads. CS motors are efficient [...]

  4. [...] All types of industries are invariably required to install different types of electric motors as prime mover for driving process equipment participating in their respective production line up. [...]

  5. ahmedelwakel
    Mar 24, 2013

    hii ; great post
    could you tell me, magnetizing current is higher in transformer or in induction motor ?

    • TopDrive
      Jun 16, 2013

      Hi, greetings.
      magnetizing current is lower in a transformer, and its a lot lower. In transformers, magnetizing currents its about 4% of the nominal current, but… In induction motors its about 30%.
      This difference is because the high reluxtance value, just like the autor said in the paper.

      Hope to be usefull.

    • Sashidhar
      Jan 30, 2014

      TO Ahmedelwake: I think magnetizing current is less in case of transformers because they do not have an air-gap unlike induction motors!

    • Baljinder Bansal
      Feb 15, 2014

      Magnetizing current is higher in Induction motor as compared to transformer. The reason is that the flux in motor faces much more reluctance due to air gap present between stator and rotor as compared to transformer flux which has less reluctance due to continuous magnetic core. So to overcome this reluctance and magnetize the rotor, magnetizing current is much higher in Induction motor

  6. [...] utilityMotorsOn-site generatorsThere are two types of motors that contribute short circuit current:Induction motorsSynchronous motorsBetween these sources of short circuit current and the point of the short [...]

  7. [...] Circuit Current (part 1) ReactanceSub transient reactance Xd” is the apparent reactance of the stator winding at the instant short circuit occurs, and it determines the current flow during the first few cycles [...]

  8. umarz
    Feb 24, 2013

    Why induction motor draws 6-7 times of full load current during starting using DOL?

    • Asif Eqbal
      Feb 24, 2013

      In three phase induction motor the magnitude of induced EMF in rotor depends upon on the slip of induction motor.This induced EMF effectively decides the magnitude of rotor current. Rotor current in running condition is given by:

      I2r = sE2/√R22 + (sX2)2
      During starting speed of motor is zero and hence slip is unity. So magnitude of rotor induced EMF is very large at start. As rotor conductors are short-circuited, the large induced EMF circulates very high current through rotor at start.The condition is exactly similar to transformer secondary short-circuited and excited by rated voltage. The current through shorted secondary is very large so primary also draws a very high current. Similarly in induction motor when there is high current in rotor stator also draws high current from supply.

  9. ainsliem
    Feb 16, 2013

    A good post, but you should define LT and HT in the 2nd half of the article. I assume you mean Low Torque and High Torque, but it is unclear.

    • Asif Eqbal
      Feb 24, 2013

      LT and HT meant here low tension and high tension or low voltage and medium voltage. Intended to highlight the difference in winding configuration of motor connected by low voltage supply (less than 400V) and motors connected to medium voltage supply (more than equal to 3.3kV)

  10. [...] it and it behaves like a bar magnet, as shown in Figure 1.As the current starts flowing, the magnetic flux lines of the coil will interact with the flux lines of the permanent magnet.This will cause a movement of the coil [...]

Leave a Comment

Tell us what you're thinking... we care about your opinion!
and oh, not to forget - if you want a picture to show with your comment, go get a free Gravatar!

+ 1 = nine

Subscribe to Weekly Download Updates: (free electrical software, spreadsheets and EE guides)

EEP's Android Application
Electrical Engineering Daily Dose Daily dose of knowledge and news from
Electrical Engineering World