Power Electronics and Drives
Power Electronics and Drives tests switching devices, converter waveforms, and motor control. Many questions ask for average voltage, ripple, or conduction mode rather than device physics.
Core rules
- A thyristor latches on gate current and turns off only when anode current falls below holding current and commutation removes forward voltage.
- Single-phase full-wave controlled rectifier average voltage is V_dc = (2V_m/pi)cos(alpha) for continuous conduction.
- Buck converter V_o = D V_s; boost converter V_o = V_s/(1 - D); buck-boost V_o = -V_s D/(1 - D), for continuous conduction.
- Inductor current ripple in a buck converter is Delta I_L = V_s D(1 - D)/(L f_s).
- PWM inverter fundamental amplitude is controlled by modulation index; V/f control keeps air-gap flux approximately constant below base speed.
- Drives are classified by quadrant; regenerative braking requires a path for energy return.
Worked example
A buck converter has V_s = 48 V, D = 0.4, L = 100 microhenry, and f_s = 50 kHz. Output V_o = 0.4 × 48 = 19.2 V. Ripple Delta I_L = 48 × 0.4 × 0.6/(100×10^-6 × 50×10^3) = 11.52/5 = 2.304 A. The reasoning uses duty ratio for average voltage and volt-second balance on the inductor for ripple.
Traps
Using the ideal converter gain outside continuous conduction; mixing average and RMS values; forgetting freewheeling diode conduction; and assuming a thyristor turns off when the gate signal is removed.