At start, the following information is correct for induction motor.

Question

At start, the following information is correct for induction motor.

I. Speed = 0

II. Slip = 0.5

III. Slip = 1

IV. Slip = 0

Choose the correct answer.

1. A. I, IV
2. B. IV
3. C. I and III (Correct answer)
4. D. I, II

Correct Answer

Option C — I and III

Detailed Solution & Explanation

The correct answer is I and III.

Key Points

• At the instant of starting, the rotor has not yet begun to turn, so Speed = 0 (statement I is correct).
• Slip is defined as s = (Ns - N) / Ns. With N = 0 this gives s = Ns / Ns = 1 (statement III is correct).
• Slip = 0 (statement IV) would mean the rotor is running at synchronous speed, which never happens in an induction motor and certainly not at start.
• Slip = 0.5 (statement II) corresponds to half synchronous speed, not standstill.

Additional Information

• At start the slip of 1 makes the rotor frequency equal to the supply frequency, and the rotor draws a very heavy current — typically 5 to 7 times the full-load value.
• This is precisely why starters such as the star-delta, autotransformer or soft starter are used on larger machines.
• Under normal running conditions the slip settles at a small value, usually between 2% and 5%.