## 15/12/2019

### Rotor emf When the three phase supply is given to the three phase induction motor, the rotating magnetic field is produced which rotates at synchronous speed. Starting condition The speed of the rotor during starting condition is zero therefore the relative speed between stator rotating magnetic field and rotor speed is         Ns – N = Ns When the rotor conductors cut the rotating magnetic field, an emf will be induced in it. Let us consider that the induced emf in the rotor is E2 Running condition Let us assume that the induced emf in the rotor is E2’.  The relative speed between stator rotating magnetic field and rotor speed is Ns – N.  The induced emf in the rotor condition is                     Relative speed                     Rotor emf                          Ns                                             E2                          Ns – N                                      E2’                          E2’ = [ Ns – N / Ns  ] E2                          E2’ = s E2 The slip at starting is unity therefore the rotor induced emf is same as that of starting condition.  However the rotor induced emf decreases as the speed increases or slip decreases. Effect of slip on rotor parameters The effect of slip on rotor parameter is summarized as below.  As the slip decreases during running condition, all the parameter decreases during running condition. Rotor parameter Starting condition Running condition Frequency of rotor emf f f’ = sf Rotor induced emf E2 E2’= s E2 Rotor resistance R2 R2 Rotor reactance X2 = 2πfL X2’ = s X2 =  s( 2πfL ) Rotor impedance Z2 = √ R22 + X22 Z2’ = √ R22 + X2’2 = √ R22 + ( sX2 )2 Rotor current I2 = E2 / Z2 I2 = E2 / √ R22 + X22 I2’ = E2’/Z2 I2’= s E2 / √ R22 + ( sX2 )2 Rotor power factor Cos F2 = R2 / Z2 = R2 / √ R22 + X22 Cos F2’ = R2 / Z2’ = R2 / √ R22 + ( sX2 )2

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