Control Engineering

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Any problems to watch for when overdriving a motor?
May 19, 2008

The reader who sent in this question said he normally sizes motors to run at between 75% and 100% of rated speed, but asks about overdriving by 150% by setting the variable frequency drive (VFD) at 90 Hz, rather than 60 Hz.

My knee-jerk reaction was that most AC motors can take it for short bursts provided the load is light, but not for continuous duty. The drive can certainly take it as long as current and voltage stay within specifications. The motor can take it as long as it doesn’t get too hot.

Not having had the experience myself, however, I asked my friend John Mazurkiewicz at Baldor Electric Company what he thought.

“There is no issue for the Variable Frequency Drive (VFD) in an over-speeding application,” he said. “The VFD can supply the required frequency above 60 Hz continuously; however, the mechanical characteristics of the motor are the determining factor.

“In terms of a conventional standard efficient three phase AC induction motor, the motor insulation and type of bearings should be reviewed prior to over-speeding the motor.

“The heat inside the motor will increase in an over-speed condition and the motor must have an insulation rating that can withstand the increase in heat. A Class F or Class H motor insulation is preferred when over-speeding an induction motor.

“In an over-speed condition, the bearing grease is subjected to higher temperatures and therefore becomes thinner. Bearings can lock-up due to excessive heat buildup in the bearing journal. Additionally, once the grease becomes thin it can leak from the bearing journal into the stator assembly of the motor. Bearings that are not properly lubricated become hot and will eventually fail.”

So, the bottom line is that you can do it, but watch out! Review the motor specifications carefully before attempting it in continuous duty. A call to the manufacturer’s tech support line probably would be in order as well. Overspeeding on a continuous basis would probably void any motor warranty and likely would shorten the motor’s service life.

A better choice would be to look into getting a motor rated for the higher speed, or adding a gear drive to provide the higher shaft speed. I know it seems like a waste of money, but buying a single motor that lasts through its normal service life is a lot less expensive in the long run than replacing the lower-rated motor every, say, 2 years.

The closest I’ve come to overspeeding a motor for a long time is running continuously a fan motor intended for intermittent duty. It failed after the equivalent of about 2.5 years continuous service.

Posted by Charlie Masi on May 19, 2008 | Comments (1)