Electrical Theory and Principles
Electrical Theory and Principles
This domain tests Ohm's law, power relationships, series and parallel circuits, AC impedance, power factor, and three-phase calculations. Questions often require selecting the right formula and distinguishing nameplate, measured, and calculated values.
Core rules
- Ohm's law: V = I × R; power: P = V × I for DC and purely resistive AC; P = V × I × PF for AC.
- Series: current is common; voltages add; resistances add. Parallel: voltage is common; currents add; 1/R_T = 1/R1 + 1/R2 + ...
- AC opposition is impedance Z, combining resistance and reactance: Z = √(R² + X²). Power factor is P/S = cos θ.
- Three-phase power: P = √3 × V_L × I_L × PF, where V_L is line-to-line voltage.
- RMS values are used for AC calculations; peak = RMS × √2 for a sine wave.
Worked example
A 480 V, three-phase load draws 25 A at 0.85 PF. True power is P = 1.732 × 480 × 25 × 0.85 = 17,666 W, or about 17.7 kW. Apparent power is S = 1.732 × 480 × 25 = 20,784 VA, or 20.8 kVA. The difference is reactive power, not usable work.
Traps
- Using line-to-neutral voltage in a three-phase formula when line-to-line is required.
- Forgetting power factor and reporting kVA as kW.
- Mixing peak and RMS values.
- Adding parallel resistances directly instead of using reciprocals.