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GATE 2014 EE (EE2) – Question 23

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 · 1 mark · Multiple choice

A three-phase, four-pole self-excited induction generator supplies a load at f₁. Partially removing load changes frequency to f₂. Speed is maintained at 1500 rpm in both cases. Which is true?

  1. f₁,f₂ > 50 Hz and f₁ > f₂
  2. f₁ < 50 Hz and f₂ > 50 Hz
  3. f₁,f₂ < 50 Hz and f₂ > f₁
  4. f₁ > 50 Hz and f₂ < 50 Hz

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Show answer and explanation

Correct answer: (C) f₁,f₂ < 50 Hz and f₂ > f₁

Explanation

For generating operation,the rotor runs faster than the rotating field. At1500rpm and4poles,the electrical frequency must therefore be below50Hz. Removing load reduces the magnitude of negative slip,bringing frequency closer to50Hz from below. Thus **f1,f2<50Hz and f2>f1,C** under the intended stable generating operation.