Power Electronics and Drives: Converters, Choppers, Inverters and Motor Control
Power Electronics and Drives tests semiconductor devices and the converters that interface them to machines - rectifiers, choppers, inverters and adjustable-speed drive schemes. Expect average-value and harmonic questions.
Devices and commutation
A thyristor latches when anode current exceeds the latching current and turns off only when current falls below the holding current; natural (line) commutation uses supply voltage reversal, forced commutation needs an external circuit. Turn-off time limits the maximum switching frequency.
Converter formulas
- Single-phase full converter (continuous conduction): V_o = (2V_m/π)cos α.
- Three-phase full converter: V_o = (3V_ml/π)cos α = 1.35 V_L cos α.
- Buck chopper: V_o = D·V_s; boost: V_o = V_s/(1 − D); buck-boost inverts polarity.
- A six-pulse converter injects harmonics of order 6k ± 1 (5th, 7th, 11th, 13th); a 12-pulse arrangement cancels the 5th and 7th.
Drives
Constant V/f control maintains flux and maximum torque capability below base speed; above base speed, flux weakening reduces torque. DC drives use armature voltage control below base speed and field control above it.
Worked example
A single-phase full converter is fed from 230 V, 50 Hz with firing angle 60° into an R load. V_m = 230√2 ≈ 325 V. V_o = (2 × 325/π) × cos 60° = 206.9 × 0.5 ≈ 103.5 V.
Traps
Applying continuous-conduction formulas when the load current is discontinuous, mixing RMS and average values, and forgetting that α > 90° puts the converter into inverter mode only if an active load is present.