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Backward-Field Slip

A 220 V, 6-pole, 50 Hz, single-phase induction motor runs at 950 rpm. Its slip with respect to backward rotating field will be

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Answer & Explanation

Correct AnswerC. 1.95

Quick Explanation

For 6 poles and 50 Hz, synchronous speed is 1000 rpm. At N=950 rpm, forward slip sf=(1000−950)/1000=0.05. Backward-field slip is sb=2−sf=1.95. Therefore option C.

Formula / Key Relation

Ns=120f/P=1000rpm; sf=0.05; sb=2−sf=1.95

Detailed Explanation

Correct Answer

C — 1.95

Concept and Reasoning

For 6 poles and 50 Hz, synchronous speed is 1000 rpm. At N=950 rpm, forward slip sf=(1000−950)/1000=0.05. Backward-field slip is sb=2−sf=1.95. Therefore option C.

Calculation / Key Relation

Ns=120f/P=1000rpm

sf=0.05

sb=2−sf=1.95

Exam Takeaway

Single-phase motor backward slip = 2 − forward slip.

Common Mistake

Do not calculate backward slip using the ordinary forward-slip expression with the same sign of speed.

Quick Trick

Single-phase motor backward slip = 2 − forward slip.

Common Mistake

Do not calculate backward slip using the ordinary forward-slip expression with the same sign of speed.

Exam Tip

Single-phase motor backward slip = 2 − forward slip.

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QP-112-202425.pdf2024Q. 165

Backward-Field Slip

Quick Explanation

Formula / Key Relation

Detailed Explanation

Correct Answer

Concept and Reasoning

Calculation / Key Relation

Exam Takeaway

Common Mistake

Quick Trick

Common Mistake

Exam Tip

Previous Exam Appearances

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