Limitations of increasing motor speed

The normal frequency in Australia is 50 Hz and a client of mine runs his compressor motors up to 60 Hz which gives him greater output. But he only gets away with it because the machines are from the United States and are rated to operate at 60 Hz. Be careful because there could be a significant increase in power at the higher speeds in 60Hz.

Before you attempt to do this verify with the compressor motor manufacturer if you motor is VFD rated, and how much over speed it can be taken without damage to the motor. Once you have that you have to check to make sure the distance from the MCC / VFD to the motor is not long enough to create harmonic distortions. Next verify the temperature increases in the motor due to VFD adjustments and make sure overheating does not damage the motor. Finally verify with the VFD how much heat is generated inside the VFD due to the variations in voltage and amperage, what is removed from the circuit will end up as heat, so you could end up with cooling requirements.


Increase in frequency at rated voltage has the following limitations.

1. As motor speed increases beyond the synchronous speed it run in constant power motor. At increased speed the torque goes on reducing as speed increase.
2. The normal induction motors have cooling fan driven from same output shaft. As motor speed increase fan speed increase which consumes more torque available on shaft, results in reduction of net torque available for load driving.
3. Normally in such case fan should be driven from separate inline motor.
4. If you go for frequency higher than 150 Hz. the motor bearings to be checked for their RPM ratings. Beyond 8500 rpm ceramic bearings are preferred for running.