Electrical Theory and Fundamentals
Electrical Theory and Fundamentals
This domain tests Ohm’s law, power relationships, series and parallel circuits, AC impedance, power factor, three-phase quantities, transformers, and basic machines. Questions often require selecting the right formula and keeping units consistent.
Core rules
- In series circuits, current is common and voltage drops add; in parallel circuits, voltage is common and branch currents add.
- For AC, use impedance, not resistance alone: $Z=\sqrt{R^2+(X_L-X_C)^2}$. Power factor is real power divided by apparent power.
- For balanced three-phase loads, $P=\sqrt{3}V_LI_L\cos\phi$, $S=\sqrt{3}V_LI_L$, and $Q=\sqrt{3}V_LI_L\sin\phi$.
- Transformer voltage ratio equals turns ratio; current ratio is inverse, allowing for losses and excitation current.
Worked example
A balanced 480 V three-phase heater draws 24 A at unity power factor. Apparent power is $S=\sqrt{3}\times480\times24=19,958\text{ VA}$, about 20.0 kVA. Because the load is resistive, real power is also about 20.0 kW. If the same current had a power factor of 0.85, real power would be $20.0\times0.85=17.0\text{ kW}$ while apparent power stayed near 20.0 kVA.
Traps
Using 480 V where phase voltage 277 V belongs; adding AC quantities algebraically instead of as phasors; ignoring power factor; mixing kW and kVA; forgetting that conductor resistance changes with temperature.