I received a question from a reader some time ago regarding an application in which a 380V/50 Hz motor was to be installed in a plant with only a 60 Hz supply. The reader intended to feed the motor with a variable speed drive and wanted to know (1) what the impact on the motor’s operation would be and (2) what the effect of over-speeding the motor would be.
The response:

Since you are planning to use a VFD anyway, there really is no problem running at 60Hz, just make sure your VFD is programmed with the proper V/Hz output for that motor. You didn’t post what your line voltage is so I can’t give you the specifics, but the following is an example.

The motor is designed for 380V 50Hz, so the V/Hz ratio is 380/50 = 7.6V/Hz. If you have 480V available, a 480V VFD will be set up to deliver 460V for the motor at 60Hz, and 460/60 = 7.67V/Hz! Close enough to not worry about the motor’s basic design. So let’s then look at the ramifications:

  • Speed: your 980RPM motor will now spin at 60/50 = 1.2 x 980 = 1176 RPM.
  • Torque: you will get the same torque out of that motor at the higher speed. Torque is what is based on that V/Hz ratio. T = HP x 5250 / RPM, so your torque now is 5 x 5250 / 980 = 26.79 ft-lbs. That will not change as you increase speed and voltage together.
  • Horsepower: Same torque at higher speed gives the following: HP = T x RPM / 5250, so 26.79 x 1176 / 5250 = 6HP at the shaft. Your 50Hz motor was 5HP, at 60Hz it will be 6HP. Power went up by 20% but voltage went up by ~21% too, so current will remain almost the same, therefore no significant heat problem.

If you decide on running the motor above base speed – again assuming you have 480V available – you will not be able to maintain that 7.67 V/Hz ratio past the 460V limit; once you increase your speed above 60Hz, your 6HP cannot increase, so your torque will begin to drop rapidly. At 90Hz, the motor will be spinning 90/50=1.8 x 980RPM = 1764RPM. Remember from above, Tq = HP x 5250/RPM, so now your torque dropped from 26.79 to 6 x 5250/1764 = 17.86 ft-lbs. So to get that extra speed you sacrificed 33% of your torque at the shaft. Only you can decide if that is a workable trade-off.

If you have concerns or questions about any of the above, please let us know by visiting our Comments section, or contact us at info@joliettech.com or visit us at joliettech.com and joliettech.com/blog. We’ll be glad to assist. And please join me in a couple of weeks for another column.

As always, thanks for reading –

Jay Baima - Author

Regards,
Jay Baima
Joliet Technologies