The GATE Grind

GATE 2014 EE (EE1) – Question 48

Electrical Machines · Three-phase induction machines: principle of operation, types, performance, torque-speed characteristics, no-load and blocked-rotor tests, equivalent circuit, starting and speed control · 2 marks · Multiple choice

A three-phase, 50 Hz, six-pole induction motor has rotor resistance 0.1 Ω and reactance 0.92 Ω. Neglect stator voltage drop and assume constant rotor resistance. Full-load slip is 3%. Find maximum/full-load torque ratio.

  1. 1.567
  2. 1.712
  3. 1.948
  4. 2.134

Practise this question in The GATE Grind →

Show answer and explanation

Correct answer: (C) 1.948

Explanation

With stator drops neglected, maximum torque occurs at sm=R2/X2=0.1/0.92=0.108696. The torque ratio is Tmax/T(s)=(1/2)(sm/s+s/sm). At s=0.03this is approximately1.9496, closest to **1.948,C**. It is a ratio of electromagnetic torques, not powers.