GATE EE 2016 Set 1 — Question 34
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Electrical Machines → DC Machines → Armature Reaction & Commutation
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Question
A 4-pole, lap-connected, separately excited dc motor is drawing a steady current of while running at . A good approximation for the waveshape of the current in an armature conductor of the motor is given by
Correct answer
(C) [figure]
Solution
1.Current per conductor: For a lap-connected machine, the number of parallel paths is equal to the number of poles . Here, . The current in each armature conductor is:
2.Waveshape: In a DC motor, the current in an armature conductor is alternating because the conductor passes under alternating North and South poles. The commutator acts as a mechanical inverter to maintain unidirectional torque, but the conductor current itself is trapezoidal/rectangular.
3.Frequency: The frequency of the armature current is . The time period is .
Option (C) shows an alternating trapezoidal waveform with a peak of and a half-cycle duration of (total period ), which matches the calculated values.Continue learning with Success Tracker
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