Correct Answer - Option 2 : 70 Hz
Concept:
Synchronous speed:
\({N_s} = \frac{{120f}}{P} \)
Where
f = frequency
P = Number of poles
Relating speed with respect to rotor = Nactual - Nr
The speed with respect to stator = Nactual
Speed of rotor = Nr
Calculation:
Given
f = 50 Hz
P = 4
Nr = 600 rpm (in opposite direction)
\({N_s} = \frac{{120f}}{P} \)
\({N_s} = \frac{{120f}}{P} = \frac{{120 \times 50}}{4} = 1500\;rpm\)
Relating speed with respect to rotor = 1500
1500 = Nactual - (-600)
Nactual = 2100 rpm
\({f_s} = 2100 \times \frac{4}{{120}} = 70Hz\)
fs = 70 Hz
