Discussion
The frequency of the rotor currents in an induction motor is
Answer: C
Correct answer: the slip times the supply frequency
The rotor conductors cut the rotating field at the relative speed \( N_s - N \), and the frequency of the emf induced in them is proportional to that relative speed:
\[ f_r = s f \]At standstill \( s = 1 \) and the rotor frequency equals the supply frequency, which is why a wound rotor at rest behaves as a transformer secondary. At normal running slips of a few per cent the rotor frequency is only a hertz or two, and that low frequency is why the rotor reactance is small in normal operation.
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