Discussion
The synchronous speed of an induction motor in revolutions per minute is
Answer: A
Correct answer: \( 120f/P \)
The stator winding produces a field that rotates at the synchronous speed. A pair of poles is traversed in one cycle of the supply, so with \( P \) poles the field completes \( 2f/P \) revolutions per second, and in revolutions per minute
\[ N_s = \frac{120f}{P} \]With \( f = 50 \) Hz a four-pole machine gives \( 120 \times 50 / 4 = 1500 \) rpm and a six-pole machine 1000 rpm. The other expressions either use 60 in place of 120, which is the per-pole rather than per-pole-pair figure, or invert the roles of \( f \) and \( P \) and so make the speed fall as the frequency rises.
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