Power Electronics and Electric Drives
Power Electronics and Electric Drives
This domain tests controlled rectifiers, choppers, inverters, AC voltage controllers and drive control. Expect average-voltage, duty-cycle and V/f numericals.
Core rules
- An SCR latches when anode current exceeds the latching current; it turns off only when anode current falls below the holding current. A gate pulse cannot turn off a conducting SCR.
- A single-phase full-controlled rectifier with continuous current has average output V = (2Vm/π) cos α.
- Buck chopper: Vo = DVs. Boost chopper: Vo = Vs/(1 − D). D is the on-time fraction.
- Inverters produce variable-frequency variable-voltage output; PWM controls both voltage and harmonic content. A freewheeling diode provides a path for inductive current and improves input power factor.
- For induction motor drives, keep V/f constant below base speed to maintain flux. Above base speed, field weakening reduces torque capability.
- A four-quadrant drive can reverse speed and torque, allowing regenerative braking.
Worked example
A buck chopper has Vs = 200 V, duty D = 0.35 and R = 10 Ω. Vo = 0.35 × 200 = 70 V. Io = 70/10 = 7 A. Load power = 70 × 7 = 490 W. At 1 kHz, the period is 1 ms and the on-time is 0.35 ms.
Traps
Do not confuse average and RMS output. Check whether α is measured from the start of the positive half-cycle. Do not assume a gate signal turns off an SCR. Do not apply V/f control above base speed.