Power Electronics and Electrical Machines
Power Electronics and Electrical Machines tests converter topologies, switching loss, magnetic devices, and machine torque-speed behavior. Expect duty-cycle calculations, continuous/discontinuous conduction, and induction or DC machine relations.
Core rules
- Ideal buck: Vout=D Vin; boost: Vout=Vin/(1-D); buck-boost inverts.
- Inductor volt-second balance gives steady-state duty relations; capacitor charge balance gives output ripple.
- Switching loss rises with frequency and current/voltage overlap; conduction loss depends on RMS current and RDS(on).
- Induction motor synchronous speed ns=120f/P; slip s=(ns-nr)/ns.
- DC machine back emf Ea=Kφω; torque Te=KφIa.
Worked example
A buck converter has Vin=24 V, D=0.4, and ideal components. Vout=D Vin=0.4×24=9.6 V. If the load is 10 Ω, Iout=0.96 A. In continuous conduction, the inductor average current equals Iout. The switch conducts for DT, so its average current is about D Iout≈0.384 A; the diode carries the rest. If the diode has a 0.7 V forward drop, the output is lower and the duty must be adjusted, so ideal calculations are only a first pass.
Traps
Using boost and buck duty formulas interchangeably; ignoring the CCM/DCM boundary; forgetting diode drop and switch resistance; using line frequency instead of electrical frequency for slip; and treating motor slip as zero at full load.