Q3Power Electronics
Question
4 marks
Discuss the symmetrical and asymmetrical AC-DC converters.
Answer
Symmetrical converters use fully controlled (all SCR) bridges, while asymmetrical converters use half-controlled (mixed SCR and diode) bridges; they differ in output voltage range and freewheeling behaviour.
- Both half-cycles are controlled by firing angle α.
- Average output voltage: Vdc = (2Vm/π)cosα for single-phase.
- Output voltage range: -2Vm/π ≤ Vdc ≤ +2Vm/π.
- Can operate as inverter (α > 90°) for regenerative braking.
- No inherent freewheeling path; requires separate freewheeling diode for inductive loads to prevent discontinuous current.
- Only positive output voltages possible (0 to 2Vm/π); cannot invert.
- Built-in freewheeling path through diodes — inherently better for inductive loads.
- Average output voltage: Vdc = (Vm/π)(1 + cosα).
- Better input power factor than fully controlled converter at the same Vdc.
- Simpler gate drive circuit (only two SCRs to fire).
- Cannot regenerate power back to source.