Slip in Induction Motor or in Induction Machine
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.
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
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’
Importance of Slip in Induction Motor
The slip is an important parameter to be considered while studying the induction machine. It greatly affects the performance of the induction motor. Torque and efficiency of the induction motor significantly depends on the slip. If there is no slip or slip is equal to zero then the actual motor speed is equal to the synchronous speed and there is no relative motion between the rotor and the stator rotating magnetic field. If there is no relative motion between rotor and stator rotating magnetic field then there is no induced EMF and current in the rotor. Hence no torque is developed in the induction motor.
Torque in an induction motor
Under the running condition s is very low so (sX2)2 is negligible and can be neglected
So, for low slip value
So, from the above equation it is clearly seen that for low slip value Torque is directly proportional to slip.
