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
Let be the number of poles. The supply frequency gives a stator field rotating clockwise at with . The prime-mover speed is expressed as a frequency, (the speed in electrical-frequency units).
1.Frequency at the slip rings (): Slip rings are connected to the rotor winding, so they see the EMF induced by the relative speed between the stator field and the rotor conductors. The field rotates clockwise at while the rotor is driven anti-clockwise at , so the relative speed is . Therefore
2.Frequency at the commutator brushes (): The commutator acts as a frequency changer: the voltage collected by stationary brushes has the frequency of the field's motion relative to the brushes, not relative to the rotor. The stator field rotates at relative to the stationary brushes, so
independent of the rotor's mechanical speed.Answer: (A) and Continue learning with Success Tracker
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