What is the synchronous speed of a 3-phase, 6-pole motor at 25 Hz?

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Multiple Choice

What is the synchronous speed of a 3-phase, 6-pole motor at 25 Hz?

Explanation:
The speed at which the stator’s magnetic field rotates is the synchronous speed, and for a 3-phase machine it’s found with Ns = 120 f / P, where f is the supply frequency in hertz and P is the number of poles. Here, f is 25 Hz and the motor has 6 poles, so Ns = (120 × 25) / 6 = 3000 / 6 = 500 RPM. This is the speed of the rotating magnetic field; the rotor runs slightly slower under load due to slip, but the question asks for the synchronous speed. The other speeds would require different pole counts (for example, 125 RPM would need 24 poles, 250 RPM would need 12 poles, and 600 RPM would imply an odd number of poles, which isn’t typical for a 3-phase machine). So the synchronous speed is 500 RPM.

The speed at which the stator’s magnetic field rotates is the synchronous speed, and for a 3-phase machine it’s found with Ns = 120 f / P, where f is the supply frequency in hertz and P is the number of poles. Here, f is 25 Hz and the motor has 6 poles, so Ns = (120 × 25) / 6 = 3000 / 6 = 500 RPM. This is the speed of the rotating magnetic field; the rotor runs slightly slower under load due to slip, but the question asks for the synchronous speed. The other speeds would require different pole counts (for example, 125 RPM would need 24 poles, 250 RPM would need 12 poles, and 600 RPM would imply an odd number of poles, which isn’t typical for a 3-phase machine). So the synchronous speed is 500 RPM.

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