Correct Answer - Option 3 : 45 rpm
Concept:
The speed difference between Synchronous Speed (Ns) and Rotor speed (Nr) is called slip speed. It is given by:
Slip speed = Ns - Nr
Synchronous speed is given by:
\({N_s} = \frac{{120 \times f}}{p} \)
where f is frequency and p is number of pole
Calculation:
Given:
p = 4 and f = 50 Hz
\({N_s} = \frac{{120 \times f}}{p} = \frac{{120 \times 50}}{4} = 1500\;rpm\)
Slip speed = Ns - Nr = 1500 - 1455
Slip speed = 45 rpm