Power Electronics and Motor Control
Power Electronics and Motor Control
This section tests semiconductor power devices, rectifier and chopper circuits, inverters and variable-frequency drives used for industrial motor control.
Core rules
- Diode: uncontrolled; conducts when forward biased.
- SCR (thyristor): conducts when the anode is positive and a gate pulse is applied; it turns off only when current falls below holding current. Firing angle α controls output.
- TRIAC: bidirectional, used for AC phase control such as lamp dimmers and fan regulators.
- IGBT and MOSFET: voltage-controlled gate, fast switching, used in inverters and VFDs.
- Single-phase half-wave controlled rectifier: Vdc = (Vm/2π)(1 + cos α). Full-wave controlled: Vdc = (2Vm/π) cos α.
- Chopper: DC to DC; pulse-width modulation controls average output.
- Inverter: DC to AC. A VFD follows the V/f constant rule to keep flux and torque capability constant below base speed.
Worked example
A single-phase full-wave controlled rectifier is fed from 230 V RMS and fires at α = 60°. Find the DC output. Vm = 230 × √2 = 325 V. Vdc = (2 × 325/π) × cos 60° = 206.9 × 0.5 = 103.5 V. As α increases beyond 60° the output falls; at 90° it would be zero.
Traps that cost marks
- Using RMS instead of peak voltage in the rectifier formula.
- Forgetting cos α and treating a controlled rectifier as a diode bridge.
- Assuming an SCR turns off when the gate pulse stops; it turns off at current zero.
- Running a VFD at low frequency without reducing voltage, causing magnetic saturation and high current.
- Omitting a freewheeling diode across an inductive load, causing voltage spikes.