Power Electronics and Drives
Power Electronics and Drives tests switching devices, rectifiers, choppers, inverters, AC voltage controllers, and motor drives.
Core rules
- SCR is a latching thyristor: it conducts when forward-biased and gate-pulsed, and turns off when current falls below holding current. MOSFET is voltage-controlled; IGBT combines a MOS gate with low on-state drop.
- Single-phase full-wave controlled rectifier: Vdc=(2Vm/π)cosα for a resistive load with continuous conduction.
- Buck chopper: Vo=DVs; boost: Vo=Vs/(1−D); buck-boost: Vo=−VsD/(1−D).
- Inverter converts DC to AC; PWM controls fundamental voltage and reduces low-order harmonics. V/f control keeps induction motor flux approximately constant.
- Freewheeling diode provides an inductive-load current path and improves input power factor.
Worked example
A single-phase full-wave controlled rectifier has peak input voltage Vm=325 V and firing angle α=60°. Vdc=(2×325/π)cos60°=(650/3.1416)×0.5≈103.5 V. The average output is positive because α<90°.
Traps
An SCR does not turn off by removing the gate pulse; natural or forced commutation is needed. Do not use the half-wave average Vm/π for a full-wave bridge. In a boost converter, output is higher than input, not lower.