Slip in Induction Motor

Induction motor slip The slip in an induction motor or in an induction machine is an important parameter that describes the relative speed between the synchronous speed and the actual speed of the induction motor expressed as the percentage of synchronous speed shown in equation (1).

The induction motor is an asynchronous motor and it never runs on synchronous speed so there is a slight difference between synchronous speed and actual motor speed. And this speed difference between synchronous speed and the actual motor speed is known as slip speed. Or in other words we can say that the speed of the stator RMF with respect to the rotor is known as slip speed.

By Definition s.Ns = Ns - N N = Ns - s.NS

………………….

(i)

Ns = 120F/P

Where, Ns = Synchronous Speed N = Actual Speed of Motor S = slip sNs = Slip speed F = Supply Frequency P = Number of Poles on Stator

Effect of slip on the Induction Motor Parameters Effect on the Frequency Let the frequency of the voltage applied to the stator winding is F and the Frequency of voltage and current induced in the rotor F’.

F’ = s.F

Effect on the Rotor Reactance Let Xr be the rotor reactance Xr = 2πFL at starting But under running condition F’ = s.F Xr’ = 2π s.F.L So, Xr’ = s.Xr

Effect on the Voltage At the starting of the induction motor induced EMF in the rotor is Er. Er = 4.44 kw F’.N.ɸ At starting slip is equal to 1 F’ = F So, Er = 4.44 kw F.N.ɸ But under the slip condition F’ = sF So, Er’ = 4.44 kw s.F.N.ɸ Er’ = s.Er Similarly Ir’ = Er’ / Zr’

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