A motor is protected by current, not by power. The Motor Calculator (Rated Current, Fusing, Supply Cable) works from the nameplate to the complete design: rated current, starting current, motor protection switch, back-up fuse and supply cable.
The formula
In = P ÷ (η × √3 × U × cos φ) – the mechanical rated power P is divided by efficiency η, the three-phase factor √3, mains voltage U (400 V) and power factor cos φ. All values are on the nameplate.
Step by step
- Enter nameplate data: rated power in kW, efficiency, cos φ and voltage
- Select starting method: direct-on-line (5–8 times rated current), star-delta (about ⅓ of that) or soft starter/VFD
- Read the results: rated current, starting current, setting for the motor protection switch and recommended back-up fuse
Worked example
4 kW motor, η = 0.88, cos φ = 0.82, 400 V: In = 4000 ÷ (0.88 × 1.732 × 400 × 0.82) = 7.55 A. With direct-on-line starting, up to 53 A flow briefly – which is why the back-up fuse needs a slow characteristic, and the motor protection switch is set to the rated current, not the starting current.
Practical tips
The motor protection switch is set exactly to In – set too high it does not protect, set too low it trips at start-up. Star-delta starting reduces the starting current to about a third, but also the starting torque: unsuitable for loads with high breakaway torque (compressors). Variable frequency drives limit the starting current to 1 to 1.5 times and are usually the first choice today.
