How Fast Can My Inverter Drive My Motor?

How Fast Can My Inverter Drive My Motor? This is a frequently asked question and the answer needs careful consideration. There are power and mechanical limits with a nod to the torque characteristics of a squirrel cage motor. The inverter itself is usually capable of providing the increased frequency. ABB inverters typically output 400Hz – 8 times the mains frequency!

We will explore what to consider when assessing whether a speed increase of an existing inverter-motor unit is possible.

Power Limits

This is usually limited by current – both the motor and inverter must be running below their rated currents before considering increasing the speed and when sped up, still stay below these values.

There should be consideration of the type of load the motor is driving. If it is a fan or centrifugal pump, then the power will increase exponentially – a 2% increase in speed will increase the power consumed by around 6%. However, a constant torque load like an extruder would show a 2% power increase for a 2% speed increase.

Mechanical Limits

Speeding up mechanical systems will increase the mechanical stresses. With fans, recommended blade tip speeds may be exceeded or vibration levels may increase too much. Electric motors and gearboxes all have a maximum design speed which should not be exceeded. Check all this out with the machine manufacturer first.

Other Considerations

Above its rated speed, an electric motor enters the ‘constant power zone’ which means the available torque reduces with increasing speed. This reducing torque will still need to be able to drive the load though.

A fan or pump will also create more noise with increasing speed, this may be a hazard.

All mechanical systems have resonant frequencies – these are usually designed out not to be the same as operational speeds. However, increasing the speed of the system may mean that resonance is met with dire consequences.

Really another question needs to be asked first, Why? Why do you need to increase the speed? If you need to increase the speed of your motor and are considering achieving this with the aid of the inverter, talk to us first. When we know the specifics of the situation, we can give relevant and pertinent advice. Our engineers can be contacted by email [email protected] or by calling 0115 944 1036