Blew a motor...

Yes, we did. We overlooked 2 things. We put a 25HP 480VAC motor on a VFD. We didn't realize it was a 2 pole motor (3600RPM). The VFD was setup for 1800RPM (4-pole). The drive ran at 60Hz, but at the incorrect base speed for the motor. Normally shouldn't be an issue, but the motor wasn't an inverter rated motor. Cooked the winding insulation, took about a week.

My question is: Did the motor fail because the incorrect RPM was set on the VFD OR was it due to the motor wasn't an inverter rated motor OR was it because the motor was running at something other than 3475 RPM?

TIA,

Greg

P.S. The drive is in basic Volts/Hz mode....

it's always bad to run a non-inverter duty motor at 50% or less speed for extended lengths of time....the cooling fan can't pull air through the motor to cool it properly.

the motor failed because the set-up was wrong, or..

because the incorrect RPM was set on the VFD AND the motor wasn't an inverter rated motor AND the motor was running at something other than 3475 RPM

Interesting....I didn't think inverter duty had anything to do with cooling. It's a standard TEFC motor...If we had an application where we were at low speed and high torque, we would use TEBC. This application was low torque at zero speed.

Greg

I don't think inverter duty has anything to do with cooling, it has to do with the rating of the insulation.

IMO go TENV for these types of applications, there expensive and bigger but sure saves a lot of hassle.

I can't see why screwing up the poles in the drive has anything to do with your problem

Greg Dake said:

This application was low torque at zero speed.

Greg

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3600 RPM motor for a low torque zero speed app? That seems odd to begin with.

But I'm guessing that the VFD had to come up with some extra current to get the motor to behave like a 4-pole motor. Either that, or you ended up with some wild harmonics because of the setup error.

Was there a reactor between the VFD and the motor?

And no blower? If I have a motor hooked up to a PLC, I'll always spec out a blower to go with it.

I've heard the argument that the motor will be setup so that it never runs below such-and-such speed, and therefore doesn't need a blower. But that only works as long as the motor speed never gets changed. Besides, the motor fan output drops like a rock when the motor isn't run at rated speed.

It's just cheap insurance.

We have several hundreds of motors on VFD's running PID loops tec.. with no blower....and no issues.

I was also under the impresson that inverter duty has to do with the winding insulation.

Greg

No reactor....the motor is less than 100 feet away from the VFD.



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I took out a spindle motor one time. I can't remember exactly what the problem was but it seems the drive only changed the freq and not the volts. We got a new spindle in and set the drive to lower the volts with the htz and it worked fine. Best I remember it went quick. less than 5 min all the smoke was out.

I don't think inverter duty has anything to do with cooling, it has to do with the rating of the insulation.

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I think just having more insulation helps cooling. It takes more heat to short windings.

vdf set up

after u do a parameter set up, then u start the motor, u have to chech the current reading and compare it with the motor rate and the vdf max. current set up , then confirm it with the motor temp.that will alwayas save ur motor.

First, Greg, a V/Hz drive really knows nothing about the actual motor speed. It is concerned only with regulating Hz, voltage, and monitoring amps. The resulting motor shaft speed is due to the frequency.

Second, having the wrong motor nameplate data on a V/Hz drive puts the motor at risk only with respect to the Full Load Amps and the drive's ability to protect the motor thermally. Of course, if your load at no point overloads the motor, the drive protection is not needed and, even if it is wrong due to incorrect nameplate data, no damage occurs.

Third, insulation class determines the motor's ability to cope with heat. This is an entirely separate issue from insulation that is rated for inverter output pulses rather than nice clean sine waves. A TEFC motor is also affected by reduced speed because its shaft fan slows down. At 25hp, pretty much any commodity motor will tolerate a 4/1 full-load speed turndown and much more at less than full load.

In view of all that, and since you didn't tell us how the motor insulation failed, I would have to speculate that the motor was probably Insulation Class B which is never adequate for 460V PWM inverter duty. The failure mode is generally insulation blowthru in the endturns.

Doing everything the same but using an Insulation Class F motor with an MG1 Part31 endorsement would likely result in normal motor life.

Having said all that, I want to be clear that entering motor nameplate data correctly is rock-bottom essential for reliable motor-drive operation regardless of system sophistication. Make a practice of always doing it completely and correctly. There is no way the drive can protect the motor correctly without it. It would be the same as putting the wrong heaters in an overload block.

Thanks for the responses. The motor was an insulation class F, but the supplier said it wasn't inverter rated. I'm assuming that the MG1 part 31 defines inverter rated. I looked on several motors that I know are inverter rated in our plant and I don't see MG1 Part 31 on the nameplate anywhere. How would I know a motor has the NEMA MG1 part 31 rating by looking at the motor?

Greg

Greg, you have identified a real problem. Motor manufacturers use terms like "inverter ready", "inverter rated", and "inverter duty" with reckless abandon and no definition of the terms so, at bottom, such terms are useless.

NEMA, however, does have a specific spec for testing motor insulation to 1600V on pulse power plus high temp bearing grease. It is MG1 P31. If a motor is tested to these standards, regardless of whether it is TEFC, TEBC, TENV, or whatever, you can trust that it is going to tolerate VFD output pulses.

A motor that meets MG1 P31 will almost always mention that on the nameplate. The price premium for this endorsement is very small and a real bargain when doing variable speed.

One caution tho, just because a motor is rated MG1 P31, does NOT mean it is suitable for the application you have in mind. For example, a rated TEFC motor is still not going to cool properly at 100rpm and full torque. MG1 P31 is about the insulation.

Greg Dake said:

No reactor....the motor is less than 100 feet away from the VFD.

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I've seen them burn up at less than 100 feet when we had four motors connected to the same VFD. We added a reactor to the VFD's output which smoothed out the VFD pulses and fixed the problem.

These motors were not inverter-rated, but with the addition of the reactor we never had any other burnt motors again. They don't cost much, and they really do help.


As for the blower, it's just cheap insurance. We had a number of lifts here that started their motors (a dual-speed Demag motor with a creep speed motor piggybacked onto it) 144 times an hour.

Added to this was that the lift was seriously overloaded, and not particularly maintained well. When we opened up the disconnect you could smell the burnt insulation.

We were burning up motors at an unbelievable rate until we added a blower to the back of the motor.

BTW, we first contacted Demag for an "official" Demag blower. It not only didn't fit, but they wanted $1700 for what was essentually a "****-fan" (bathroom fan).

So we got some $75 muffin fans (about 12" across) and machined the backplate of the motor to accept them. We never burned up a motor on these lifts again.

So for $75 plus less than 8 hours of plant maintenance work per motor, we saved tens of thousands of dollars in just motor rewinds alone. This doesn't even begin to include our production downtime costs when we burned up a motor (which we rate at $5000 a minute).

So for next to nothing, we covered up serious design and maintenance flaws permanently.


And here's a link to AutomationDirect's VFD/Motor FAQ that has some pretty good info:

http://automationnotebook.com/2005_Issue_4/fyi_Issue4_2005.html

One thing further, Greg. Assuming a 460V service and a drive carrier frequency less than or equal to 8khz, there is still a limit to motor lead length before additional filtering is necessary.

The guidelines I use are: given the above and an MG1 P31 motor, at or above 100hp, you are good for 250 feet of motor lead length. At 10hp and smaller, you are only good for 60 feet. You should extrapolate linearly between these two points.

Addition of a 5% motor lead reactor doubles the above numbers. A dv/dt filter will triple the numbers. Beyond that, you need a sine filter (very expensive).

what is the rating of your VFD?