Motor operation at more than rated speed

[eletest]

In one of our newly commissioned plants we have a motor Motor ( rating 415 V,2.2 KW,4.3 Amps, 1455 rpm).This motor is powered through VFD ( V/f control ). The normal operating range for the motor was 265 to 1465 rpm . However for meeting the process requirement , the motor is required to be run at 2300 rpm .While achieving the same through VFD , we were able to achieve 2300 rpm at around 455V . The load current measured was around 1.4 to 1.6A which is within the rated current for the motor. Also no heating is observed in the motor while running at 2300 rpm.My query is that whether it is possible to keep operating this motor continuously at 2300 rpm . What are the problems that we may face ?

 

[Skogsgurra]

Not many problems expected. You have lowered the V/hz ratio and that's fine as long as the motor doesn't have to deliver full torque. The motor is in so-called "constant power" range and that means that the available torque is lower than at rated speed.
Bearings could be a problem, but I don't think that will be an issue at that low speed.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[jraef]

Why I agree with Gunnar, I always tell people who ask this question of me the following:
1) Just because you CAN does not mean you SHOULD
2) My opinions are all null and void the moment the motor parts start flying around the room...

The point is, this is a question that can really ONLY be answered with any authority by the manufacturer of the motor. There are 4 issue of contention: Cooling, bearings, balance and torque, and the torque issue really relates back to causing an overload. So if you are not causing an overload, which your stated evidence suggests, then the 3 issues are specific to that particular motor. As a GROSS generality, most motor mfrs only use one set of bearings that are used in the 2 pole designs, so are going to be rated for that speed (ie 3000RPM in your case). But is that 100% universal? Absolutely NOT, so only your motor mfr knows for sure. The same can be said for rotor balancing. Cooling" no idea.

Call your motor rep and ask them.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[electricpete]

Can you clarify what you meant by cooling. I only think of cooling problems at low speed.

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(2B)+(2B)' ?

 

[jraef]

Some motor cooling fans (if used) have a band width of effectiveness. So at 2X rated speed for example, they might begin to move LESS air, which may be right at the time you need MORE (depending on the torque situation) or at the very least, the added windage losses further deplete the useable power from the motor. In the old NEMA MG1-12.48 design spec, motors were only expected to be operated at 20-50% over speed (depending on size) and only for a minute, so that's what the cooling fans were designed for. Some fans are fine with running over speed, but only the motor mfr can know for sure, which again is my point.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[Skogsgurra]

You are probably right, Jeff. You have a tendency to have just that.

But, a 2.2 kW motor that doesn't draw more than half rated current at the moderately higher speed doesn't deserve all this anxiety. The bearings will be fine. The windings will be fine. The motor will be fine. I guarantee that. And, if that motor fails, I have several motors in that size available over here in Granbergsdal. Just come and get one!



Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.