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GATE 2014 EE (EE2) – Question 35

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

Constant V/f control maintains fundamental terminal voltage/frequency ratio from zero to base frequency. Which statement describes low-frequency operation?

  1. Stator flux increases above rated value.
  2. Stator flux decreases below rated value.
  3. Motor saturates.
  4. Stator flux stays at rated value.

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Show answer and explanation

Correct answer: (B) Stator flux decreases below rated value.

Explanation

The air-gap emf is approximately V−IsRs. At low frequency,the stator resistance drop is substantial relative to the reduced supply voltage. Consequently keeping terminal V/f constant does not keep internal E/f constant:flux falls below rated value, **B**. A low-speed voltage boost is used to compensate this drop.