Power Electronics and Drives
Power electronics converts and controls electrical energy using switching devices - diodes, thyristors (SCRs), MOSFETs, and IGBTs. Know the basic converter families: uncontrolled rectifiers (diodes), controlled rectifiers (thyristors, phase-angle control), DC-DC choppers (buck, boost, buck-boost), and inverters (DC to AC, using PWM for voltage and frequency control).
For drives, understand that variable-speed control of AC motors uses a variable-voltage variable-frequency (VVVF) inverter, keeping the V/f ratio roughly constant to maintain flux and torque capability. Below base speed, constant torque is available; above base speed, field weakening reduces torque. For DC drives, armature voltage controls speed (below base) and field control extends speed above base.
Know the role of freewheeling diodes, snubbers, and the difference between line-commutated (natural) and forced commutation. Understand harmonics: a six-pulse converter produces characteristic harmonics of order 6k ± 1 (5th, 7th, 11th, 13th), and twelve-pulse configurations reduce them.
Worked example: A buck chopper with input 100 V and duty cycle D = 0.4 gives output Vout = D × Vin = 40 V. For a boost converter, Vout = Vin/(1 − D); with D = 0.5 and Vin = 100 V, Vout = 200 V. Remember duty cycle D is the fraction of the switching period the switch is on.
Traps: confusing buck and boost formulas; assuming a thyristor turns off when gate signal stops (it needs current to fall below holding current); and forgetting that PWM reduces low-order harmonics but introduces switching-frequency components.