Power Electronics and Drives
Power Electronics and Drives tests controlled rectifiers, choppers, inverters, AC voltage controllers, DC-DC converters and motor drives. ESE numericals focus on average voltage, RMS current, duty ratio and drive torque-speed control.
Core rules
- Single-phase full converter with continuous current: Vdc=(2Vm/pi)cos(alpha); a semi-converter has (Vm/pi)(1+cos(alpha)).
- Buck converter: Vo=DVs in continuous conduction; boost: Vo=Vs/(1-D); buck-boost: Vo=-DVs/(1-D).
- SCR turns on by gate pulse and latches above latching current; it turns off when current falls below holding current and reverse voltage is applied.
- Inverter output is rich in harmonics; PWM shifts harmonics to higher frequency and controls fundamental magnitude.
- DC motor armature voltage control gives constant-torque operation below base speed; field weakening gives constant-power operation above base speed.
Worked example
A buck converter has Vs=100 V and duty ratio D=0.4. In continuous conduction, Vo=DVs=40 V. The reasoning assumes ideal switches and continuous inductor current; if the inductor current reaches zero, the DVs relation no longer holds.
Traps
Using the buck formula in discontinuous conduction; confusing latching with holding current; forgetting freewheeling-diode conduction; and assuming a line-commutated inverter can operate without an AC source for commutation. In drives, check whether the load is constant torque or constant power.