Question

Calculate the percentage slip of an induction motor having 6 poles fed with 50 cycles supply rotating with an actual speed of 980 rpm.
1. 3%
2. 4%
3. 1%
4. 2%

Answer 1

Correct Answer - Option 4 : 2%
Concept:

The synchronous speed is given by

\({N_s} = \frac{{120f}}{P}\)

Where,

f is the supply frequency in Hz or C/s

P is the number of poles

The induction motor rotates at a speed (Nr) close but less than the synchronous speed.

Slip of an induction motor is given by,

\(s = \frac{{{N_s} - {N_r}}}{{{N_s}}}\)

Where,

Ns is the synchronous speed

and Nr is the rotor speed

Rotor current frequency (fr) = (s)(f)

The slip is negative when the rotor speed is more than the synchronous speed of the rotor field and is in the same direction.

Calculation:

Given - 

Number of poles (P) = 6

Frequency (f) = 50 Hz

N_r = 980 rpm

\({N_s} = \frac{{120 \times 50}}{6} = 1000\;rpm\)

\(s = \frac{{{N_s} - {N_r}}}{{{N_s}}}= \frac{{{1000} - {980}}}{{{1000}}} = \frac{20}{1000}\)

s% = 2 %