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GATE 2019 EE – Question 48

Power Electronics · Single and three-phase configuration of uncontrolled rectifiers · 2 marks · Multiple choice

The line currents of a three-phase four wire system are square waves with amplitude of 100 A. These three currents are phase shifted by $120^\circ$ with respect to each other. The rms value of neutral current is

  1. 0 A
  2. $\dfrac{100}{\sqrt3}$ A
  3. 100 A
  4. 300 A

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Show answer and explanation

Correct answer: (C) 100 A

Explanation

The neutral current is the sum of the three square waves, $i_N=i_a+i_b+i_c$. At any instant two currents have one sign and the third the opposite, so $i_N=\pm100$ A, which is itself a square wave of amplitude 100 A. Its rms value is 100 A.