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GATE 2016 EE – Question 34

Electrical Machines · DC machines: separately excited, series and shunt, motoring and generating mode of operation and their characteristics, speed control of dc motors · 1 mark · Multiple choice

A 4-pole, lap-connected, separately excited dc motor is drawing a steady current of 40 A while running at 600 rpm. A good approximation for the waveshape of the current in an armature conductor of the motor is given by

Four candidate waveforms. (A) A constant 10 A. (B) A constant 40 A. (C) A trapezoidal wave alternating between +10 A and -10 A, with each half-period 25 ms. (D) A sinusoid of peak 10 A with half-period 25 ms.
  1. (A)
  2. (B)
  3. (C)
  4. (D)

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Correct answer: (C) (C)

Explanation

A lap winding on 4 poles has 4 parallel paths, so each conductor carries $\frac{40}{4} = 10$ A. The current reverses each time the conductor moves from under a north pole to under a south pole. At 600 rpm the rotor makes 10 revolutions per second, and a pair of poles passes every $\frac{1}{20}$ s, so each half-period is 25 ms. The current changes direction quickly at commutation, giving a trapezoidal wave between +10 A and -10 A.