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How this is calculated#

The tool finds the running and starting current of a motor, with its speed and torque. It works from the shaft rating, the voltage, the power factor, the efficiency and the number of poles. It reads no table, so it cites no clause.

1. Full load current. The tool divides the shaft power by the efficiency. That gives the input power. It divides the input power by the power factor. It then divides by the voltage, and on three phases by the square root of three.
2. Synchronous speed. It multiplies the supply frequency by 120 and divides by the number of poles.
3. Shaft speed. It reduces the synchronous speed by the slip you enter, as a percentage.
4. Full load torque. It divides the shaft power by the shaft speed, with the usual constant for rpm.
5. Starting current. It multiplies the full load current by 6. This assumes a direct-on-line start.

Worked example#

• 15 kW four‑pole motor at 400 V. The power factor is 0.85, the efficiency is 0.9 and the slip is 3 %. The full load current is 28.3 A. The synchronous speed is 1,500 rpm. The shaft speed is 1,455 rpm. The full load torque is 98.45 N·m. The direct-on-line starting current is 169.81 A.

Limits#

The factor of 6 is a general assumption. A real motor can draw more or less. Read the nameplate or the data from the maker when you have it. A soft starter or a drive gives a lower starting current, and the tool does not model them.

The tool does not size the cable, the breaker or the overload relay. It does not check the voltage drop at start. The slip is an input. It is not calculated from the load.

See also: Motor full load current, 11 kW at 400 V, Motor starting current, DOL, 55 kW and Motor circuits and starting.