Discussion
A six-pole induction motor is to run at 900 rpm from a variable-frequency drive. The frequency required is nearest to
Answer: D
Correct answer: 45 Hz
Under a variable-frequency drive the machine runs just below the synchronous speed set by the frequency. Taking 900 rpm as the synchronous speed for a six-pole machine,
\[ f = \frac{N_s P}{120} = \frac{900 \times 6}{120} = 45 \text{ Hz} \]The 30 Hz answer follows from using four poles and 90 Hz from inverting the ratio.
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