Electrical Engineering - Power Electronics
Power Electronics tests semiconductor switches, controlled rectifiers, choppers, inverters, AC voltage controllers and drives. The exam emphasizes average and RMS calculations, commutation, harmonic content and motor-control applications.
Converters
Single-phase full converter with continuous current: V_dc = (2V_m/π)cos α. A half-controlled converter has a freewheeling diode, so V_dc cannot go negative. Three-phase full converter: V_dc = (3V_mL/π)cos α, where V_mL is peak line-to-line voltage.
Choppers and inverters
Buck chopper: V_o = DV_s; boost chopper: V_o = V_s/(1 − D). In a current-source inverter, commutation is load-dependent. PWM trades switching loss for reduced low-order harmonics; the sinusoidal PWM modulation index controls the fundamental output.
Worked example
A single-phase full converter is fed from 230 V rms, 50 Hz, with α = 60° and continuous conduction. V_m = 230√2 = 325.3 V. V_dc = (2 × 325.3/π)cos 60° = 207.1 × 0.5 = 103.5 V. For a highly inductive load, the rms supply current is approximately the DC load current, so the input displacement factor is cos α = 0.5; real power is V_dc I_dc.
Traps
Using V_m instead of V_rms for the supply; applying the continuous-conduction formula when load current is discontinuous; forgetting that α > 90° can invert only with an active load or back-EMF; and equating displacement factor with power factor when harmonics are present.