Method
Motor input, apparent power, full-load current and optional shaft torque.
It covers two cases: three phase and single phase. Equations that apply to only one of them are tagged.
Equations
k = 1single phase
k = √3three phase
Pin = Poutη
S = PinPF
IFL = 1000 · Poutk · U · η · PF
T = 9550 · Poutnwhen the speed is given
Symbols and units
| Symbol | Quantity | Unit |
|---|---|---|
| Inputs | ||
| Pout | Rated output (shaft) power | kW |
| U | Supply voltage | V |
| η | Motor efficiency | 0–1 |
| PF | Power factor | 0–1 |
| n | Speed (optional) | rpm |
| Intermediate values | ||
| k | Phase factor | – |
| Results | ||
| IFL | Full-load current | A |
| Pin | Electrical input | kW |
| S | Apparent power | kVA |
| T | Shaft torque | N·m |
Assumptions and limits
- Starting current, breakers, contactors and protection are not estimated.
In practice
- Use the line-to-line voltage for three-phase motors. The constant 9550 ≈ 60 000 / 2π turns kW and rpm into N·m.
- Starting current, cable size and protection are separate checks.
Worked example
The input values are the calculator's own verification test, run through the calculator. They show the working; they are not design values.
Method: Three phase.
Inputs
| Pout | Rated output (shaft) power | 15 kW |
| U | Supply voltage | 400 V |
| η | Motor efficiency | 0.9 0–1 |
| PF | Power factor | 0.85 0–1 |
| n | Speed (optional) | 1,450 rpm |
Results
| IFL | Full-load current | 28.3015 A |
| Pin | Electrical input | 16.6667 kW |
| S | Apparent power | 19.6078 kVA |
| T | Shaft torque | 98.7931 N·m |
A preliminary check for engineer review, not an engineering approval. The project calculation to the applicable codes, authority requirements and specification governs.