Motor With VFD Slows Down When Frequency Above 150

T

The Mighty Ginsu

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Aug 11, 2016
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I am trying to make my new/used 3HP motor work with my VFD, which is rated for 3HP. I am replacing a 2HP motor which worked fine. I set the top frequency at 167, which is about 5000 FPM.

The new motor runs great, but if you go above 150 Hz, it starts to slow down again. I know it can't be sucking too much power; it has no load on it, and the VFD is supposed to be big enough to run the motor.

Any ideas?

The motor is 230/460, about 7.5 amps, and 1745 RPM.

I do not have total confidence in the wire connections, as I threw it together in a hurry just so I could provide Ebay feedback and let the seller off the hook.
 
normally speaking, a motor does just fine at 2X freq, (120 Hz), but almost 3X at 160 hz is a tad more than it should be. While you might get one motor to run at 160 hz, another motor might not.
 
Ken H> said:
normally speaking, a motor does just fine at 2X freq, (120 Hz), but almost 3X at 160 hz is a tad more than it should be. While you might get one motor to run at 160 hz, another motor might not.
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Yep. Just went through this at work with an air handler retrofit for our paint booth.
 
No need to ever go that fast, so does it really matter why ?


I suspect the bearings are getting hot and tighten up.
 
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The bearings should not be a problem, because they should be good for something like 14K, minimum. I was worried about the armature whacking the inside of the stator, but I did the math, and it looks like it should only be experiencing force something like 1.7 times what you would expect in a 3600-RPM motor.

I asked the same question at Practical Machinist, and they sort of flipped out. They said the wiring can act as a low-pass filter when you go too fast, and they had some concerns about stuff like explosions.

Makes me wonder how so many people here get up over 5000 FPM with 1800-RPM motors.
 
5.5" drive wheel, 1800 rpm motor at 120hz 5100 sfm.
 
Sooner or later you would expect to lose grunt with a big diameter wheel. Maybe I should have gone with 3600 RPM in the first place.
 
Yeah, but...I'm running a 3600 RPM motor, it's only 1hp, with a 3.5" drive wheel. If I use a pusher and a 36 grit belt on a flat platen, I can hear it slow down a bit. I can't even hear it if I'm using a 10" wheel. So with a 3hp motor, I really doubt you're going to tax it with a 5.5" wheel if max SFM is your goal.
 
The Mighty Ginsu said:
(...) I was worried about the armature whacking the inside of the stator, but I did the math, and it looks like it should only be experiencing force something like 1.7 times what you would expect in a 3600-RPM motor.(...)
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Can you share your math? 3x the speed gives you 9x the centrifugal force... are you referring to something else?

In any case, my observation matches Ken's: my motor would run happily at 120hz, but not much more. There're at least 2 control problems:
- High speed makes timing more critical. The controller may not have the time resolution to accurately generate higher frequencies.
- High speed means less time for the inactive coil's electrical noise to settle before the controller detects the zero-cross.
 
gammarad said:
Can you share your math? 3x the speed gives you 9x the centrifugal force... are you referring to something else?
Click to expand...

I just saw this, three years later.

It's not going three times as fast. It's going about 1.3 times as fast...as a 3600-RPM motor. I was working on the assumption (based on some iffy research I did) that the guts of a top-quality 1800-RPM motor were not likely to be weaker than those of a 3600-RPM motor made in the same factory, so I used 3600 for comparison purposes.

I generally run it at around 90 Hz, so I don't really worry about what happens at 167 these days. I'm not afraid to turn it up, but it cuts like crazy as it is.
 
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