Discussion
A 3-phase induction motor having 08 poles runs at 727.5 rpm. If the supply frequency is 50 Hz, the e.m.f. in the rotor will have frequency of __________.
Answer: C
The rotor emf frequency is 1.50 Hz.
Find the synchronous speed, then the slip; the rotor frequency is the slip times the supply frequency.
\[ \begin{aligned} N_{s} &= \frac{120f}{P} = \frac{120 \times 50}{8} = 750\ \text{rpm} \\ s &= \frac{750 - 727.5}{750} = 0.03 \\ f_{r} &= s f = 0.03 \times 50 = 1.5\ \text{Hz} \end{aligned} \]
1.54 Hz comes from dividing the speed difference by the rotor speed, 727.5, instead of the synchronous speed. 15 Hz is a decimal slip, and 50 Hz is the rotor frequency only at standstill.
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