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Electrical Engineering QUESTION #2177
Question 1
A three-phase induction motor runs at $1440\,\text{rpm}$ on a $50\,\text{Hz}$ supply. The number of poles and percent slip are respectively:
  • 4 poles, $4\%$✔️
  • 6 poles, $4\%$
  • 4 poles, $6\%$
  • 8 poles, $4\%$
Correct Answer Explanation
Synchronous speed for $P = 4$: $N_s = \frac{120\times50}{4} = 1500\,\text{rpm}$. Slip $s = \frac{1500-1440}{1500}\times100 = 4\%$. For $P=6$: $N_s=1000\,\text{rpm}$, which would give negative slip — physically impossible.