Discussion

A four-pole induction motor on a 50 Hz supply runs at 1,440 rpm. Its slip is

Answer: D

Correct answer: 4 per cent

The synchronous speed is

\[ N_s = \frac{120 \times 50}{4} = 1500 \text{ rpm} \]

so the slip is

\[ s = \frac{1500 - 1440}{1500} = \frac{60}{1500} = 0.04 = 4\% \]

The 96 per cent figure is \( N/N_s \), the per-unit speed, and 6 per cent comes from dividing 60 by 1000 rather than 1500.

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