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
Step 1: Current per conductorFor a lap winding the number of parallel paths equals the number of poles, :Step 2: WaveshapeThe armature terminal current is dc, but each conductor's current reverses whenever its coil is commutated, i.e. each time the conductor moves from under one pole to under the next pole of opposite polarity. Between commutations the conductor carries a constant or ; during the short commutation interval the current changes roughly linearly from to . So the conductor current is an alternating trapezoidal wave (nearly rectangular), not a dc level and not a sinusoid.Step 3: FrequencyOne full cycle of conductor current corresponds to passing one pair of poles, soEach half-cycle therefore lasts .Option (C) shows an alternating trapezoidal wave of with half-cycles of , which matches. Option (D) has the right amplitude and timing but a sinusoidal shape.Answer: (C)
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