Electrical Engineering core subjects
Electrical Engineering core covers circuits, electrical machines, power systems, control systems, measurements, and power electronics. Expect steady-state circuit calculations, machine speed and torque relations, per-unit power-system reasoning, and basic converter waveforms.
Core rules and concepts
- DC circuits: series resistances add; parallel conductances add. Thevenin equivalent: open-circuit voltage and equivalent resistance with sources killed.
- AC circuits: rms = peak/√2 for a sinusoid; impedance Z = R + jX; power factor = cos φ; real power P = VI cos φ.
- Machines: synchronous speed N_s = 120f/P, where f is frequency in Hz and P is poles. Slip s = (N_s − N_r)/N_s. Transformer emf per turn is approximately equal on both sides.
- Power systems: per-unit values simplify voltage levels; fault current depends on source impedance; protection coordination uses time-current grading.
- Control: open-loop transfer function, closed-loop T = G/(1 + GH); stability by Routh-Hurwitz or Nyquist.
- Measurements: ammeter in series, voltmeter in parallel; CT and PT ratios scale readings.
- Power electronics: diode conducts when forward-biased; SCR needs a gate pulse and turns off when current falls below holding current.
Worked example
A 4-pole induction motor runs at 1440 rpm from a 50 Hz supply. N_s = 120 × 50/4 = 1500 rpm. Slip = (1500 − 1440)/1500 = 0.04, or 4%. The reasoning shows the order: frequency and poles give synchronous speed first, then slip.
Traps
Using peak voltage where rms is required, or forgetting √2 in power calculations. In three-phase systems, confusion between line and phase values in star and delta. In transformers, rated kVA is not the same as kW output. In measurements, a voltmeter with low resistance loads the circuit and changes the reading.