A 3-Phase 440 V, 50 Hz induction motor has 5% slip. The frequency of induced rotor emf will be

Question

A 3-Phase 440 V, 50 Hz induction motor has 5% slip. The frequency of induced rotor emf will be

1. A. 50 Hz
2. B. 2.5 Hz (Correct answer)
3. C. 5 Hz
4. D. 2 Hz

Correct Answer

Option B — 2.5 Hz

Detailed Solution & Explanation

The correct answer is 2.5 Hz.

Key Points

• The frequency of the induced rotor EMF is the slip frequency: f_r = s x f.
• Here the slip s = 5% = 0.05 and the supply frequency f = 50 Hz.
• Therefore f_r = 0.05 x 50 = 2.5 Hz.

Additional Information

• At standstill the slip is 1, so the rotor frequency equals the full supply frequency of 50 Hz — the rotor behaves like the short-circuited secondary of a transformer.
• As the motor accelerates the slip falls, and with it the rotor frequency; at synchronous speed the slip would be zero and no EMF would be induced at all.
• The supply voltage of 440 V is not needed for this calculation and is included as a distractor.