Industrial Electronics and Control
Scope
The paper tests power semiconductor devices, rectifiers, controlled drives and basic PLC/relay control used in industry.
Core rules
- Diode: uncontrolled rectifier. Half-wave average V_dc = 0.318 V_m; full-wave V_dc = 0.636 V_m; bridge ripple frequency is twice the supply frequency.
- SCR: turns on with a gate pulse and latches until current falls below holding current; used in controlled rectifiers, choppers and soft starters. TRIAC: bidirectional, for AC power control; DIAC: trigger device for TRIAC.
- IGBT and MOSFET: fast switching for inverters and VFDs. A VFD controls speed by keeping the voltage-to-frequency ratio (V/f) nearly constant.
- PLC: reads inputs, executes the ladder program, drives outputs. Standard sensors: limit switch, proximity sensor, photoelectric sensor, pressure switch, 4–20 mA transmitter.
- Contactors and overload relays provide motor switching and overload protection; interlocks prevent conflicting operations.
Worked example
A half-wave rectifier is fed from 230 V AC RMS. Peak voltage V_m = 230 × 1.414 = 325 V. Average DC output = 0.318 × 325 = 103.4 V. The diode must have a peak inverse voltage rating above 325 V, so a 400 V or 600 V device is chosen. If a full-wave bridge is used, V_dc = 0.636 × 325 = 206.7 V and the ripple frequency is 100 Hz for a 50 Hz supply, allowing a smaller filter capacitor for the same ripple.
Traps that cost marks
Using RMS instead of peak in rectifier formulas, assuming an SCR turns off when the gate signal is removed, and fitting a VFD without screening the motor cable. In PLC work, confusing normally-open field contacts with normally-closed program instructions reverses the logic.