Motor power & current calculator
Three-phase current I = P / (√3 × U × cos φ × η). A 4 kW four-pole motor on 400 V at cos φ 0.81 and 89.1 % efficiency draws 8.00 A, which is the full-load current WEG prints for it, and turns 26.3 N·m at 1450 rpm.
Enter the shaft power from the nameplate (or the torque and speed), the supply, the power factor and the efficiency. The power triangle is drawn to the power factor, beside the motor.
- Current I
- Torque T
- Input power P1
- Apparent power S
How it’s calculated
Three-phase I = P / (√3 × U × cos φ × η), single-phase I = P / (U × cos φ × η), with P the shaft power and U the line voltage. Input P1 = P / η; apparent S = P1 / cos φ. Torque T [N·m] = 9550 × P [kW] / n [rpm].
Four-pole IE3 motors at 400 V 50 Hz, full load
| Output, kW | Frame | Speed, rpm | η, % | cos φ | I, A (WEG) | I, A (ABB) | I, A (Siemens) |
|---|---|---|---|---|---|---|---|
| 0.75 | 80 | 1420 | 82.5 | 0.82 | 1.6 | 1.68 | 1.75 |
| 1.5 | 90L | 1450 | 86.0 | 0.80 | 3.15 | 3.2 | 3.15 |
| 2.2 | 100L | 1435 | 87.0 | 0.80 | 4.56 | 4.3 | 4.4 |
| 4 | 112M | 1450 | 89.1 | 0.81 | 8 | 8.6 | 7.9 |
| 7.5 | 132M | 1465 | 91.5 | 0.85 | 13.9 | 14.7 | 14.3 |
| 11 | 160M | 1470 | 92.2 | 0.83 | 20.7 | 21.1 | 20.5 |
| 15 | 160L | 1470 | 93.0 | 0.82 | 28.4 | 28.5 | 28.5 |
| 22 | 180L | 1470 | 93.6 | 0.83 | 40.9 | 41.5 | 41 |
| 37 | 225S/M | 1480 | 94.6 | 0.86 | 65.6 | 69.8 | 66 |
| 55 | 250S/M | 1480 | 95.3 | 0.86 | 96.9 | 97.8 | 96 |
| 75 | 280S/M | 1485 | 95.6 | 0.87 | 130 | 133 | 133 |
Why does a motor draw more than its kW?
The nameplate kW is what the shaft gives out. The supply also pays for the losses (the efficiency takes those) and carries a magnetising current that flows back and forth without doing work (the power factor takes that). So the volt-amps are the kW divided by both, and the current is the volt-amps over √3 × the line voltage on a three-phase supply.
How do you get torque from kW and rpm?
T = 9550 × P / n, with T in N·m, P in kW and n in rpm: 9550 is 60 000 / 2π, rounded. A four-pole motor on 50 Hz runs a little under 1500 rpm at full load, so its torque in N·m is a little over 6.4 times its kW.