Basic Electrical and Power Calculations in the Workshop
Electrical questions in the workshop test Ohm's law, power, energy and motor input current. Always distinguish input power from output shaft power, and kW from kWh.
Core formulas
- Ohm: V = IR; P = VI = I²R = V²/R.
- Single phase: P = VI cos φ.
- Three phase: P = √3 V I cos φ.
- Efficiency η = output ÷ input; input = output ÷ η.
- Energy in kWh = power in kW × time in hours; cost = kWh × tariff.
Worked example
A 2 kW, 230 V single-phase heater runs 3 h/day for 30 days. Current I = P ÷ V = 2000 ÷ 230 = 8.70 A. Resistance R = V ÷ I = 230 ÷ 8.70 = 26.4 Ω. Energy = 2 × 3 × 30 = 180 kWh. At ₹8.50/kWh, cost = ₹1,530. For a three-phase 7.5 kW motor at 415 V, cos φ 0.8 and η 0.88: input = 7.5 ÷ 0.88 = 8.52 kW; I = 8520 ÷ (√3 × 415 × 0.8) = 14.8 A.
Traps
- Using output kW as input when calculating current.
- Omitting cos φ or √3 in three-phase calculations.
- Mixing W and kW in the energy formula.
- Using 230 V for a 415 V three-phase motor.
- Ignoring standby or no-load consumption in energy estimates.