Power Electronics and Drives
What the exam tests
Power Electronics and Drives tests controlled rectifiers, choppers, inverters, AC voltage controllers, and motor drives. Expect average/rms output calculations, device commutation, and speed-control principles.
Core rules
- A thyristor turns on by gate pulse when forward-biased and turns off when current falls below holding current; natural commutation uses AC supply, forced commutation uses an external circuit.
- Single-phase full-controlled bridge with resistive load: Vavg = (2Vm/π) cos α. With inductive load and continuous current, the same cosine form applies but output can reverse for α > 90°.
- Chopper: Vout = DVs for step-down; boost gives Vout = Vs/(1−D); buck-boost gives −DVs/(1−D) in continuous conduction.
- PWM inverter controls fundamental voltage and frequency; V/f control maintains flux in induction motor drives.
- Freewheeling diode improves input power factor and provides a path for inductive current.
Worked example
A single-phase full-controlled bridge is supplied at 230 V rms, 50 Hz, with resistive load and firing angle 60°. Vm = 230√2 = 325.3 V. Vavg = (2×325.3/π) cos 60° = 207.1×0.5 = 103.5 V. The rms output would be higher than this average, so do not use the average value for heating calculations.
Traps
Do not mix average and rms values. Do not assume a fully controlled converter can invert without an active load or back EMF. Do not ignore overlap and device drops in practical calculations. In drives, constant V/f is invalid above base speed; field weakening reduces torque capability.