Every electric motor fails to convert part of the electrical energy it draws into mechanical power, and that difference becomes heat. Heating is a normal consequence of operation; what matters is that the heat can be carried away by the fan and the frame fins.

The main sources of loss are:

  • Winding (copper) losses: heat produced by current in the winding resistance; they grow quickly as load and current rise.
  • Iron losses: magnetic losses in the stator and rotor laminations; they occur even at no load.
  • Rotor losses: heat from the currents in the squirrel-cage rotor.
  • Mechanical losses: bearing friction and the fan moving air.

Worked example (assumption: 95% efficiency, full load): while the shaft delivers 90 kW, the motor draws 90 / 0.95 ≈ 94.7 kW from the supply; the difference of about 4.7 kW must be continuously dissipated as heat from the frame. The actual efficiency of each model is given in its technical data table. Overload, low voltage, phase imbalance or a blocked fan upsets this balance. A higher efficiency class releases less heat at the same power; for more, see the electric motor selection questions page.