How to use it
- Choose the supply: DC, single phase or three phase.
- Enter the rated output power and pick kW or hp.
- Enter the voltage (line-to-line for three phase), the efficiency in % and, for AC, the power factor.
- Read the full-load current, the electrical input power and, for AC, the apparent power in kVA.
Formulas
With rated output P (W), efficiency η and power factor cos φ:
- Input power: P_in = P / η.
- DC: I = P_in / V.
- Single phase: I = P_in / (V × cos φ).
- Three phase: I = P_in / (√3 × V × cos φ).
- Apparent power (AC): S = P_in / cos φ.
1 hp = 745.7 W (mechanical horsepower).
Worked examples
7.5 kW, 400 V three phase, η 90 %, cos φ 0.85: P_in = 7,500 ÷ 0.9 = 8,333 W; I = 8,333 ÷ (1.7321 × 400 × 0.85) = 14.15 A; S = 8.333 ÷ 0.85 = 9.804 kVA. The motor is 10.06 hp.
1 hp, 230 V single phase, η 75 %, cos φ 0.8: P_in = 745.7 ÷ 0.75 = 994.3 W; I = 994.3 ÷ (230 × 0.8) = 5.4 A.
1 kW DC motor at 24 V, η 85 %: P_in = 1,176 W; I = 1,176.5 ÷ 24 = 49.02 A.
Tips
- Part load changes everything. Below full load both efficiency and power factor fall, so current does not drop in proportion to the load.
- Star or delta? Use the line voltage the motor is connected to: a 230/400 V motor in delta on a 230 V line draws √3 times the current it draws in star on 400 V.
- Variable-speed drives draw input current set by the drive’s own power factor, not the motor’s.