Above rated frequency, the inverter output voltage stays fixed at mains level. As frequency increases without a rise in voltage, the magnetic flux decreases; this is called field weakening. In this range the motor delivers roughly constant power, so usable torque decreases as speed rises. At higher frequencies the breakdown torque also falls and the motor may become unable to carry the load.

Checklist before going to higher speed:

  • Torque margin: How much torque does the machine need at high speed? In fans and pumps the required torque rises with speed, so the motor quickly becomes overloaded.
  • Bearings: As speed rises, bearing temperature increases and the regreasing interval shortens.
  • Balance and vibration: The rotor and parts on the shaft (pulley, coupling, fan) need finer balance at high speed.
  • Shaft and transmission parts: The permitted speed of belts, pulleys and couplings must not be exceeded.
  • Fan noise and losses: Shaft fan noise and friction losses increase with speed.
  • Driven machine: The pump, fan impeller or gearbox must be suitable for the higher speed.

The maximum permitted speed depends on the number of poles and the frame of the motor; it cannot be given as a general rule and should be confirmed by stating the application in the order. Running 2-pole (3,000 rpm) motors, which are already high-speed, above rated speed needs particular care.