Q5Power Electronics
Question
10 marks
Discuss the principle of working of a three-phase bridge inverter with an appropriate circuit diagram. Draw phase and line voltage waveform on the assumption that each thyristor conducts for 180 degrees and the resistive load is star-connected. The sequence of firing of various SCRs should also be indicated in the diagram.
Answer
A three-phase bridge inverter has six SCRs firing every 60°; with 180° conduction and star-connected resistive load, the phase voltages are six-stepped waveforms and line voltages are 12-stepped waveforms.
Failed to load diagram: No description
- Firing sequence: T1(0°), T2(60°), T3(120°), T4(180°), T5(240°), T6(300°)
- Upper group: T1 → phase A high, T3 → phase B high, T5 → phase C high
- Lower group: T4 → phase A low, T6 → phase B low, T2 → phase C low
- Lowest order harmonics are 5th and 7th (triplen harmonics absent in line voltages)