Slip of Induction Motor

Slip of Induction Motor

We have seen that rotor rotates in the same direction as that of R.M.F. but in steady state attains a speed less than the synchronous speed. The difference between the two speeds i.e. synchronous speed of R.M.F. (N_s) and rotor speed (N) is called slip speed. This slip speed is generally expressed as the percentage of the synchronous speed.

So slip of the induction motor is defined as the difference between the synchronous speed (Ns) and actual speed of rotor i.e. motor (N) expressed as a fraction of the synchronous speed (Ns). This is also called absolute slip or fractional slip and is denoted as 's'.

Thus s = N_s – N / N_s  …(Absolute slip)

The percentage slip is expressed as,

% s = N_s - N / N_s ×100

In terms of slip, the actual speed of motor (N) can be expressed as,

N = Ns (1 - s)       ... (From the expression of slip)

At start, motor is at rest and hence its speed N is zero,

s = 1 at start

This is maximum value of slip s possible for induction motor which occurs at start. While s = 0 gives us N = N_s which is not possible for an induction motor. So slip of induction motor cannot be zero under any circumstances.

Practically motor operates in the slip range of 0.01 to 0.05 i.e. 1 % to 5 %. The slip corresponding to full load speed of the motor is called full load slip.

Example 5.6.1 A 3-phose , 50 Hz, induction motor has 2 poles. If the slip is 2 percent at a certain load, determine the speed of the motor.

Solution :

Example 5.6.2 A 12 pole, 3 phase alternator is coupled to an engine running at 500 r.p.m. It supplies an induction motor which has a full – load speed of 1440 r.p.m. Find the slip of the motor.

Solution :

Example 5.6.3 A 3-phase, 4 pole, 50 Hz induction motor is running at 1440 r.p.m. Determine the slip speed and slip.

Solution :

Examples for Practice

Example 5.6.4 A 4 pole, 3 phase induction motor is supplied from 50 Hz supply. Determine its synchronous speed. On full load, its speed is observed to be 1410 r.p.m. Calculate its full load slip.  

[Ans.: 6 %]

Example 5.6.5 A 4 pole, 3 phase, 50 Hz, star connected induction motor has a full load slip of 4 %. Calculate full load speed of the motor.  

[Ans.: 1440 r.p.m.]

Review Question

1. Define the term slip of an induction motor. Explain its significance.