Power Electronics and Drives
What this domain tests
This domain tests controlled rectifiers, choppers, inverters, AC and DC drives, harmonics, and commutation.
Core rules
- A thyristor conducts after a gate pulse when forward-biased and turns off when current falls below holding level. Firing angle alpha controls average output voltage.
- Single-phase full-controlled bridge in continuous conduction: V_dc = (2 V_m/pi) cos alpha. Increasing alpha reduces the average voltage; above 90 degrees it can invert.
- Buck chopper: V_out = D V_in. Boost chopper: V_out = V_in/(1-D). Duty ratio D is on-time divided by switching period.
- Inverters use PWM to control fundamental voltage and reduce low-order harmonics. SPWM pushes dominant harmonics to higher orders around the carrier frequency.
- V/f control keeps air-gap flux approximately constant below base speed; above base speed, field weakening reduces available torque.
Worked example
A single-phase full-controlled bridge is supplied at 230 V RMS, 50 Hz, with alpha = 60 degrees and continuous conduction. Find the average DC voltage. V_m = 230 x sqrt(2) = 325.3 V. V_dc = (2 x 325.3/pi) cos 60 = 207.1 x 0.5 = 103.5 V.
Traps
- Using RMS instead of peak voltage in rectifier formulas.
- Assuming continuous conduction when the load current actually becomes discontinuous.
- Forgetting device voltage drops in low-voltage converters.
- Applying V/f control above base speed without accounting for field weakening.