A 3-phase, 4-pole, 400 V, 50 Hz squirrel-cage induction motor has slip 0.02. The rotor-flux speed, in mechanical rad/s as seen by a stationary observer, is closest to
The rotor magnetic field rotates relative to the rotor at slip speed. Adding that to the rotor's mechanical speed gives synchronous speed with respect to the stator.
For 4 poles and 50 Hz, $n_s=120f/P=1500$ rpm. In mechanical radians per second this is $1500(2\pi/60)=157.08$.
Thus **157 rad/s, C**. The rotor body itself turns at 1470 rpm, but its flux does not rotate at that speed relative to the stationary observer.
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