The rated current of a three-phase motor is I = P2 / (√3 × U × η × cos φ). Even with the same power, a different efficiency, cos φ or voltage gives a different current.

7.5 kW IE3, 400 Vcos φEfficiencyRated current
2-pole (2,930 rpm)0.8890.1%13.7 A
4-pole (1,460 rpm)0.8490.4%14.3 A
6-pole (970 rpm)0.7989.1%15.4 A

The values come from the IE3 tables in the HEM Motor catalogue. As the pole number rises, the share of magnetising current grows, cos φ falls and the current increases.

Other reasons for the difference:

  • Voltage: the same motor draws slightly more current at 380 V than at 400 V; 15.0 A versus 14.3 A in the 4-pole example.
  • Load: current measured on site depends on the load at that moment; the nameplate current applies only at rated load.
  • Design and efficiency class: from another manufacturer or IE class, the current for the same kW can differ by a few percent.
  • Phase imbalance: voltage imbalance also unbalances the currents.

For this reason, overload protection must be set to the nameplate current of the motor actually installed, not that of another motor.