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Electric Motor Calculator
Free engineering tool for electric motors: kW ⇄ HP conversion, synchronous speed (RPM), full-load torque and full-load current. Based on standard three-phase asynchronous (AC) motor formulas.
kW ⇄ HP (metric) Conversion
Synchronous Speed (RPM)
ns = — rpm
ns = 120 · f / p
Full-Load Torque
T = — Nm
T = 9550 · P(kW) / n(rpm)
Full-Load Current (3-phase)
I = — A
I = P / (√3 · U · cosφ · η)
Slip
s = — %
s = (ns − n) / ns × 100
Efficiency & Loss
η = — % · loss — kW
η = Pçıkış / Pgiriş
Power Triangle (kW–kVAr–kVA)
Q = — kVAr · S = — kVA
S = P / cosφ · Q = P · tanφ
VFD: Target Speed → Frequency
f = — Hz
f = n · p / 120
About the formulas
These calculations apply to standard three-phase asynchronous (induction) electric motors. Actual full-load speed is slightly below synchronous speed due to slip (e.g. a 4-pole motor runs ~1450–1480 rpm). Current and torque depend on the motor's rated power factor and IE3/IE4/IE5 efficiency class.
Need a motor for a specific power and speed? Browse our power & speed range or the full electric motors catalogue. DRG Motor supplies IE3/IE4/IE5 motors from 0.55 to 355 kW.
Synchronous speed reference (50 Hz)
| Poles | Synchronous (rpm) | ≈ Full-load (rpm) |
|---|---|---|
| 2 | 3000 | ~2900 |
| 4 | 1500 | ~1450 |
| 6 | 1000 | ~960 |
| 8 | 750 | ~720 |
For 60 Hz multiply synchronous values by 1.2 (e.g. 4-pole = 1800 rpm).
Frequently asked questions
How many HP is 1 kW?
1 kW equals about 1.36 metric HP (BG). For example a 7.5 kW motor ≈ 10.2 HP. Use the converter above for any value.
How is synchronous speed calculated?
ns = 120 · f / p, where f is frequency (Hz) and p the number of poles. At 50 Hz: 2 poles = 3000, 4 poles = 1500, 6 poles = 1000 rpm.
How do I find motor torque?
T (Nm) = 9550 · P(kW) / n(rpm). A 7.5 kW motor at 1450 rpm produces ≈ 49.4 Nm.
How is three-phase full-load current calculated?
I = P / (√3 · U · cos φ · η). Use the rated voltage, power factor and efficiency from the motor nameplate.
Why is the real speed lower than synchronous speed?
Asynchronous (induction) motors run with slip, so the loaded speed is slightly below synchronous — e.g. a 4-pole motor runs ~1450–1480 rpm instead of 1500.