Power Electronics and Drives
Core concepts
Know the semiconductor switches (diode, SCR, MOSFET, IGBT) and their safe operating areas, then the converter families: uncontrolled and controlled rectifiers, choppers, inverters, and AC voltage controllers. Understand firing angle effects on average output voltage, ripple, and power factor. For drives, study DC motor speed control (armature and field control), induction motor V/f control, slip-power recovery, and PWM techniques. Harmonic content, THD, and filter design matter.
Traps examiners set
- For a controlled rectifier, average voltage falls as firing angle increases; it is not constant.
- Freewheeling diodes improve input power factor and reduce ripple but do not change the ideal average voltage formula.
- Inverters need commutation; line-commutated vs forced-commutated distinction is a favourite.
- V/f control keeps flux constant only below base speed.
- SCR turns off only when current falls below holding current, not by gate signal.
Worked rule of thumb
Single-phase full-controlled rectifier with resistive load: Vdc = (2*Vm/pi)cos(alpha). Example: Vm = 325 V (about 230 V RMS), alpha = 60 degrees, so cos = 0.5 and Vdc = (2325/3.1416)*0.5 is about 103 V. Check whether the load is resistive or inductive before applying the formula.
Practice strategy
Draw the waveform for one cycle; it exposes whether the formula assumes continuous or discontinuous conduction.