We received the following question from a customer:

Q: I have 3-phase incoming voltage measured at 505VAC on a nominal 480VAC distribution system. Will this voltage level affect my drives and motors rated for 460VAC nominal?

A: Drives are almost all rated at +10/-10% voltage input (some are +15/-10; others are +10/-15…). A typical North American drive for controlling a 460V motor (motor nameplate rating) is built to accommodate an input of 380-480V at +10/-15%, giving an allowable range of 323 – 528V input. In theory, any voltage input within this range will be taken in by the drive, converted and inverted to a set output of 460V (or 480V – you set it in drive parameters to match the motor nameplate value).

But “in theory” doesn’t much impact the plant floor. In practice, you still need to be certain of the drive’s allowable input range. Some drives rated for non- North American voltages are rated for 400V nominal; once you get up to 480V you start having problems. Such drives can sometimes be found driving equipment manufactured outside the US. What we are seeing is occasional difficulty with proper motor deceleration at higher than nominal input voltages. That is most likely to happen with the Euro/Asian versions optimized for the 400V/50hz range which are not equipped with brake choppers and braking resistors. What happens is the DC bus voltage gets higher than the desired ~500VDC. When attempting to decelerate the motor, it acts for a time as a generator and feeds power back into the drive. At high enough levels, this can overload an already highly loaded DC bus, and cause it to overheat or fault. If you have a properly rated 380/480 volt drive for the North American market, you should be fine at 505VAC in. But pay attention to drive conditions, especially if the drive sees over-voltage and DC bus faults. If you get them, and you are sure the problems are not in the field, one method of addressing the issue is installing an isolation transformer with taps ahead of the drive to reduce the incoming voltage. If you do, you’ll also get the added benefit of eliminating any upstream harmonics/noise problems.

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 www.joliettech.com and www.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

Jay Baima
Joliet Technologies