Q2Power Electronics
Question
10 marks
Discuss the three-phase 6-pulse AC-DC converter with RL load. Sketch the input voltage, current and output voltage, current waveforms for delay angles of 45 degrees and 90 degrees.
Answer
A three-phase 6-pulse fully controlled bridge converter produces an average output of (3√6/π)Vph·cosα; for α=45° partial inversion begins; for α=90° average output is zero.
- At α = 45°: Vdc = 0.675 VLL, ripple frequency = 6f, source current lags voltage by 45°.
- At α = 90°: Vdc = 0, pf = 0 (purely reactive), still continuous current for highly inductive load.
- For α > 90°: Vdc becomes negative (inverter operation, energy flows back to source).
- Input current is a quasi-square wave with fundamental displaced by α from the corresponding phase voltage.