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GATE 2025 EE – Question 46

Power Electronics · Single-phase and three-phase voltage and current source inverters, sinusoidal pulse width modulation · 2 marks · Multiple choice

The 3-phase modulating waveforms ($v_a(t)$, $v_b(t)$ and $v_c(t)$), used in sinusoidal PWM in a Voltage Source Inverter (VSI) are

$v_a(t)=0.8\sin(\omega t)$ V

$v_b(t)=0.8\sin\left(\omega t-\dfrac{2\pi}3\right)$ V

$v_c(t)=0.8\sin\left(\omega t+\dfrac{2\pi}3\right)$ V

where $\omega=2\pi\times40$ rad/s is the fundamental frequency. The modulating waveforms are compared with a 10 kHz triangular carrier whose magnitude varies between +1 and −1. The VSI has a DC link voltage of 600 V and feeds a star connected motor. The per phase fundamental RMS motor voltage, in volts, is closest to

  1. 169.71
  2. 300.00
  3. 424.26
  4. 212.13

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Correct answer: (A) 169.71

Explanation

In the linear region the peak fundamental pole voltage is $m\dfrac{V_{dc}}2=0.8\times300=240$ V (modulation index $m=0.8$ against a carrier of amplitude 1). For a star-connected motor with balanced three-phase modulation this is the phase-voltage peak, so the RMS per-phase fundamental is $240/\sqrt2=169.71$ V.