Power Electronics and Motor Drives
This domain covers controlled conversion between AC and DC and the drives built on it. You should recognise device symbols and characteristics (diode, thyristor, MOSFET, IGBT), compute average DC output of rectifiers, analyse choppers, and explain how a variable-frequency drive controls an induction motor, including harmonics, braking and protection.
Core rules
- Single-phase rectifiers: half-wave average Vm/π; full-wave 2Vm/π; fully controlled full-wave Vdc = (Vm/π)(1 + cosα).
- Three-phase fully controlled bridge: Vdc = 1.35 VLL cosα; half-controlled: Vdc = 1.35 VLL (1 + cosα)/2.
- A freewheel diode clamps the output to zero and improves the power factor with inductive loads; α > 90° in a fully controlled bridge inverts the output and allows regeneration.
- Choppers: buck Vout = D·Vin, boost Vout = Vin/(1 − D), with D the duty cycle.
- Inverters use PWM to synthesise a variable voltage and frequency; below base speed the drive holds V/f constant to keep flux, and above base speed it weakens the field.
- Starting: DOL, star-delta, soft starter (voltage ramp only) or VFD (full speed control). Braking: rheostatic, regenerative, DC injection.
- Harmonics: six-pulse converters produce 5th and 7th, causing motor heating, extra losses and current distortion; drives need derating, reactors or filters and EMC filtering.
Worked example
A 400 V, 50 Hz three-phase fully controlled bridge supplies a separately excited DC motor with armature resistance 0.5 Ω carrying 60 A at α = 45°. Vdc = 1.35 × 400 × cos45° = 1.35 × 400 × 0.707 = 382 V. The motor back EMF is E = Vdc − IaRa = 382 − 60 × 0.5 = 352 V. If α is increased to 60°, Vdc falls to 270 V, so the current decreases until the new steady state, giving speed reduction. At α = 120°, Vdc becomes negative and the converter regenerates, returning energy to the supply provided the load still has a positive back EMF.
Traps that cost marks
- Using the peak value where the 0.9 or 1.35 factors already assume rms input.
- Forgetting the freewheel diode, so the calculated output goes negative with an inductive load.
- Assuming a soft starter gives speed control - it only ramps voltage.
- Belting a standard motor to a VFD at low speed without considering reduced cooling.
- Ignoring thyristor turn-off (commutation) requirements and the meaning of holding/latching current.