Slip in Induction Motor

Can Rotor Speed of a Induction Motor N =  Ns ?

     As the rotor rotating, the magnitude of rotor induced emf ( proportional to relative speed between stator rotating field and rotor speed ) is reduced.

     Let us assume that the rotor rotates at synchronous speed, so the emf induced in the rotor conductors would become zero and consequently there is no torque on the rotor causing the rotor to fails back in speed. Once the rotor falls back in speed, the relative speed increases and again rotor picks up the speed. But due to inertia of rotor, this does not happen in practice and rotor continuous to rotate with a speed slightly less than the synchronous speed of the rotating field. 

    The induction motor never rotates at synchronous speed. The speed at which it rotates is hence called as sub synchronous speed and motor called as asynchronous motor.

Slip Speed :

     The slip speed is defined as the difference between the synchronous speed ( Ns ) and the actual speed of the motor ( N ).

Slip :

     The slip is defined as the difference between the synchronous speed ( Ns ) and the actual speed of the motor ( N ), expressed of fraction of synchronous speed.

Thus slip,

S  =  Ns – N / Ns    . . .  ( 1 )

Percentage slip,

% S  =   ( Ns – N / Ns ) × 100      . . .  ( 2 )

The actual speed of the motor can be expressed, from the above equation as

sNs  =  N-  N

N  =  N-  sNs

N  =  Ns ( 1  -  s )

      From equation ( 1 ) and ( 2 ), it may be noticed that the relative speed or slip speed ( Ns  -  N ) of rotor and rotating magnetic field is directly proportional to the slip.

At start ( or ) stand still. N  =  0


        s  =  1  this is the maximum value of slip.

    slip can not be zero because when s  =  0, N  =  Nand the torque is zero. 

     At no-load slip is about 1 and at full load it is about 3-5. The variation in slip and therefore speed, from no-load to full load is very small. Thus a 3-phase induction motor is known as a constant speed motor.            

Slip in Induction Motor

    Above figure shows the rotor speed and slip variation, when the torque is varied from zero to full load value.

  Also check out  Effect of Slip on Induction Motor Parameters

i.e., on

            Rotor frequency

            Rotor induced emf

            Rotor power factor 

            Rotor current

            Rotor resistance

            Rotor reactance

            Rotor impedance

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