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GATE 2014 EE (EE2) – Question 47

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 slip-ring induction motor with commutator winding is shown. With rotor closed it rotates clockwise. Open its rotor and drive it anticlockwise at rotational speed f_r using a prime mover. Find slip-ring frequency f₁ and commutator-brush frequency f₂ respectively.

Diagram for GATE 2014 EE (EE2) question 47
  1. f + f_r and f
  2. f − f_r and f
  3. f − f_r and f + f_r
  4. f + f_r and f − f_r

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Correct answer: (A) f + f_r and f

Explanation

Reverse mechanical rotation increases the relative speed between rotor conductors and the stator rotating field,so the slip-ring frequency is f+fr in electrical-frequency units. A commutator transforms the rotor winding connection as it rotates;at stationary brushes,the induced field variation remains at supply frequency f. Hence **f1=f+fr,f2=f,A**. If fr denotes literal mechanical revolutions per second for a multipole machine,the slip expression requires(p/2)fr.