A 55 kW induction motor drives a plant pump. The design needs two currents, not one. The first is the current the motor draws while it runs. The second is the current it draws at the instant of a direct on line start. This page works one machine through to both numbers.
The inputs
- Shaft rating: 55 kW
- Supply: 400 V, three phase
- Power factor: 0.87
- Efficiency: 0.93
- Poles: 4
- Slip: 1.5 %
Power factor and efficiency come from the nameplate. Both raise the current for the same shaft rating. A guessed pair moves the result by several amperes. Read the plate before you size anything.
The clauses
AS/NZS 3000:2018 Cl 2.5.3.1 sets the coordination between the circuit conductor and the protective device. AS/NZS 3000:2018 Cl 3.6.2 sets the voltage drop allowed to any point of the installation. The run current drives both checks. The start current sits on top of them as a separate duty.
The steps
The tool converts the shaft rating into an input rating with the efficiency. It then applies the power factor and the three phase line factor at 400 V to get the current. The pole count sets the synchronous speed at the supply frequency. The slip percent drops the shaft below that speed. Torque follows from the shaft rating and the shaft speed.
The result
- Full load current: 98.12 A
- Starting current, DOL: 588.7 A
- Synchronous speed: 1,500 rpm
- Shaft speed: 1,477.5 rpm
- Full load torque: 355.5 Nm
The motor runs at 98.12 A. A direct on line start draws 588.7 A. The tool marks that start figure as a six times assumption in a note with every result.
What 588.7 A has to pass through
The start current lasts for the acceleration time only. Three parts of the circuit still see it.
The contactor and the overload relay see the full 588.7 A. Select the starter to the machine utilisation category, not to the 98.12 A run figure.
The upstream protective device sees the same surge. A device sized to 98.12 A on its thermal element must ride through 588.7 A for the acceleration time without tripping. The magnetic setting decides that, and this tool does not check it.
The cable sees a short voltage drop. Drop scales with current, so a start pulls roughly six times the running drop for a few seconds. On a long run that dip can stall the acceleration or drop out a contactor coil elsewhere on the board. Run the cable through the voltage drop tool at 588.7 A as well as at 98.12 A.
The starter changes the number
588.7 A is a direct on line figure and nothing else. A star delta starter lowers it in the star step. A soft starter holds the current to the ramp setting. A variable speed drive starts close to the run current, because it builds the frequency up from zero.
The tool reports the direct on line case because it is the worst case and because it needs no starter data. When the machine sheet gives a locked rotor code or a measured starting current, use that figure instead. Feed 98.12 A into the cable size tool as the design current, then check the start condition separately.
This page is a design aid. Verify every value against the current edition of the standard.
See also: Motor full load current: 11 kW 400 V DOL, Submain cable size for 3‑phase 100 A over 60 m and Motor circuits and starting.