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A 6-pole, 3-phase Induction motor operates from a supply whose frequency is 60 Hz. Calculate the speed of the rotor, when the slip is 0.03.
1. 1164 rpm
2. 976 rpm
3. 816 rpm
4. 900 rpm

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Correct Answer - Option 1 : 1164 rpm
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) = 60 Hz

\({N_s} = \frac{{120 \times 60}}{6} = 1200\;rpm\)

Slip (s) = 3 % = 0.03

Now rotor speed is

Nr = Ns (1 – s) = 1200 (1 – 0.03) 

N= 1164 rpm

Rotor current frequency (fr) = 0.03 x 60

f= 1.8 Hz

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