Power Electronics and Drives
Power Electronics and Drives covers semiconductor devices, rectifiers, choppers, inverters and industrial drives. Expect device characteristics, converter formulas and drive selection.
Devices and converters
- SCR is a latching thyristor: gate pulse turns it on, and it turns off only when current falls below holding current.
- MOSFET is voltage-controlled and fast; IGBT combines MOS gate with bipolar conduction.
- Controlled rectifiers vary average output voltage by firing angle α; freewheeling diodes improve input power factor and reduce ripple.
- Choppers convert fixed DC to variable DC; buck, boost and buck-boost topologies are common.
- Inverters convert DC to AC; PWM controls output voltage and reduces low-order harmonics.
Drives
- DC drives use armature-voltage or field control; AC drives commonly use constant V/f control to keep flux approximately constant.
- Soft starters limit starting current; variable-frequency drives allow speed control of induction motors.
Worked example
A single-phase full-wave controlled rectifier has peak input voltage Vm = 325 V and firing angle α = 60°. Average output voltage Vdc = (2Vm/π) cosα = (2 × 325/3.1416) × 0.5 ≈ 103.5 V. Increasing α lowers Vdc; at α = 90° the average becomes zero.
Traps that cost marks
Using average and RMS values interchangeably, forgetting that a freewheeling diode changes the output waveform, and assuming an SCR turns off when the gate pulse ends are frequent errors. In V/f drives, check that voltage and frequency are changed together.