When the frequency is lowered, the inverter lowers the voltage in the same proportion; since the magnetic flux is maintained, the motor can produce close to its rated torque. Shaft power, however, falls with speed: power (kW) ≈ torque (Nm) × speed (rpm) ÷ 9,550.

The real limit is cooling. HEM motors are of type IC411, cooled over the frame surface by a fan mounted on the shaft. As speed falls, the fan's airflow drops markedly; the motor draws the same current to deliver the same torque but finds it harder to get rid of the heat.

The assessment depends on the load characteristic:

  • Fans and centrifugal pumps: The required torque falls quickly with speed; heating at low speed is often not a problem.
  • Constant-torque loads such as conveyors, mixers and extruders: High torque is needed at low speed too; in long operation the motor may overheat.

Example

The 7.5 kW 132M-4 motor has a rated torque of 49.1 Nm. Delivering the same torque at around 25 Hz (about 730 rpm), its shaft power is 49.1 × 730 ÷ 9,550 ≈ 3.75 kW; but the cooling air is not what it is at 50 Hz. If this operation lasts long, the continuous torque limit or a separately powered fan should be clarified in the order. For torque values by power and speed, the electric motor torque table can be used.