Basic Electrical Principles and Calculations
Core Concepts
Understand Ohm’s law (V = IR), power equations (P = VI = I²R = V²/R), Kirchhoff’s laws, series and parallel circuits, and AC fundamentals (frequency, period, peak, RMS, average values). Know that RMS = peak/√2 for sine waves, and form factor = RMS/average = 1.11 for sine. Understand resistance, resistivity (R = ρL/A), temperature coefficient, and power factor (PF = true power/apparent power). For three-phase systems, know star and delta relationships: in star, line voltage = √3 × phase voltage; line current = phase current. In delta, line current = √3 × phase current; line voltage = phase voltage. Total three-phase power = √3 × V_L × I_L × PF.
Traps in Questions
- Mixing peak and RMS: 230 V mains is RMS; peak is about 325 V.
- Series vs parallel: in series, current is common; in parallel, voltage is common.
- Power factor: kW is true power, kVA is apparent; only true power does work.
- Resistivity units: Ω·m, not Ω/m.
- Three-phase: in star, phase voltage is lower than line voltage; in delta, phase current is lower than line current.
Worked Example / Rule of Thumb
Example: A 2 kW, 230 V single-phase heater. Current I = P/V = 2000/230 ≈ 8.7 A. Resistance R = V²/P = 230²/2000 = 26.45 Ω. For a three-phase 10 kW motor at 400 V, PF 0.8: I = P/(√3 × V × PF) = 10000/(1.732×400×0.8) ≈ 18 A. Rule of thumb: For quick estimates, 1 kW at 230 V draws about 4.3 A; at 400 V three-phase, about 1.5 A per kW.