GATE EE 2014 Set 2 — Question 47
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Electrical Machines → Three-Phase Induction Machines → Principle & Types
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Question
A three-phase slip-ring induction motor, provided with a commutator winding, is shown in the figure. The motor rotates in clockwise direction when the rotor windings are closed.
If the rotor winding is open circuited and the system is made to run at rotational speed with the help of prime-mover in anti-clockwise direction, then the frequency of voltage across slip rings is and frequency of voltage across commutator brushes is . The values of and respectively are
Correct answer
(A) f + fᵣ and f
Solution
1.Frequency at Slip Rings (): The frequency of the rotor EMF is determined by the relative speed between the stator magnetic field and the rotor conductors. The stator field rotates at synchronous speed in the clockwise direction. The rotor is driven by a prime mover at speed in the anti-clockwise direction. The relative speed is . Therefore, the frequency at the slip rings is .
2.Frequency at Commutator Brushes (): The commutator acts as a frequency changer. The brushes are stationary in space. They pick up the EMF induced by the stator field, which is rotating at synchronous speed relative to the stationary brushes. Thus, the frequency at the brushes is always the supply frequency , regardless of the rotor's mechanical speed.
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