In a geared motor, the output speed is the motor speed divided by the gear ratio. When the inverter changes motor speed, the output speed changes in the same proportion; output torque is roughly motor torque × ratio × gearbox efficiency.

Example

With a 1,460 rpm motor and a 20:1 gearbox, the output is about 73 rpm. If the motor is reduced to 25 Hz, the output drops to about 36 rpm. This is an approximate calculation; slip and the actual ratio change the result slightly.

Points to watch:

  • Ratio selection: The gearbox should do the main speed reduction. Choosing a low-ratio gearbox and running the motor permanently at low frequency strains motor cooling.
  • Lubrication: At very low output speed, oil distribution in splash lubrication may weaken; at high speed oil heating increases.
  • Torque limit: The inverter's torque boost and current limit settings must not exceed the output torque and service factor permitted by the gearbox.
  • Ramps: Very short acceleration and deceleration times put shock loads on the gears.
  • Worm gearbox: Its efficiency varies with speed and is generally lower at low speed; output torque should be checked accordingly.

For ratio and service factor selection see the gearbox selection and use questions guide, and for ready solutions the geared electric motor page.