Chasing Down Motor Whine

[Smitty771]

This motor has a terrible, high-pitched whine when energized, and I'm trying to figure out the cause and eliminate it, if possible.

Manufactured by Diehl
Rewound for only 220v previous to my acquisition
Nameplate info (220v): 2 HP, SF 1.15, RPM 860, FLA 7.8, 3 phase

It will be driven by a VFD. First I assumed the whine was because of the VFD's switching frequency (SF), but I tried two different VFDs at every available SF up to 16 kHz with no change to the whine. I then pull-started it and ran it on single phase 220v, which confirmed the whine is not VFD-related. I turned the motor passively without energizing it using a different motor and a v-belt; It reached 200 RPM over its rated RPM and was silent at all speeds between 0 and the max, proving that it only whines when energized. It has new bearings. I opened the motor again (after replacing the bearings) to more closely-inspect, paying special attention to the thin (ferrous) metal bell flare airfoils at each end. I thought those would be most susceptible to magnetic resonance. Upon inspection, I found no loose welds, and when impacting the thin metal in many places with a screwdriver handle, I got solid tones of the same pitch, indicating no looseness. There is a small gap between the outer perimeter of the airfoils and the end bell castings. I'm not sure it that's enough to allow them to be vulnerable to magnetic resonance.

Someone I know has suggested energizing the stator without the rotor and end bells just for a second or two to help determine if the whine coming from the stator. I've read several posts here about energizing a motor stator only for testing, and I get an incomplete picture. More than one post warned to never energize a stator [only] with full voltage. The idea of energizing this stator to learn if that's where the whine is intrigues and tempts me, but after what I've read so far, it seems riskier than I'm comfortable with. Also, if I applied a much lower voltage to the stator (as suggested in various posts here), I'm not sure that very low voltage would generate the whine that I hear when the motor is at full voltage and fully assembled.

I would welcome any insights on this noisy motor.

 

[waross]

Well 220 Volts is not a standard 60 Hz voltage.
860 is not a standard 50 Hz speed.
Your supply my be 208 Volts or 240 volts.
If you still have a VFD available, try setting the voltage to 200 Volts and see if it whines.
Is the whine frequency a multiple of 60 Hz?
Does the whine frequency change with the VFD frequency?
You may have a reveresed coil.
To check, with the leads labelled 1,2,3;
Connect 2 and 3 and put 6 Volts DC across 1 and 2-3.
Then (With rotor removed) check the polarity pattern around inside the stator with a compass.
Then connect lead 3 and 1 and apply 6 Volts dc across 2 and 3-1.
Check with a compass.
Connect 1 and 2 and apply 6 volts across 3 and 1-2.
You will see a repeating pattern of North and South.
A revered coil will break the pattern.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Smitty771]

Thanks for your input, waross. I suspect the 220 volt label has to do with the manufacture date of this motor and the changes to electrical standards and nomenclature over time. I realize the RPM is unusual, which is what makes this motor so useful to me and makes it difficult to affordably replace. Just to reiterate... this motor has been rewound at some time in the past before I got it. I took it to a local motor shop to make some inquiries about it. They told me that when a shop rewinds a motor, they stick to the original specs, though whoever rewound this one didn't bother with the convention of stamping through the voltage on the nameplate that is no longer available--in this case, 440v. So, I suppose when it was rewound, it may have been done in a way that favors a voltage slightly different than 220v.

I have the VFD in perpetuity. Your suggestion of programming the VFD with a slightly different voltage is an idea that had not occurred to me; I'll try that.

I measured the frequency of the whine while the motor was running at 60 Hz. The screenshot of that measurement is below. It showed three predominant frequencies: ~930 Hz, ~2800 Hz, and ~5600 Hz, but the frequency that sounded most dominant to my ears was the 930 Hz. None of those frequencies are exact multiples of 60 Hz. The whine frequency does go lower as I lower the motor frequency using a VFD, but it doesn't get very tolerable to human ears until a power frequency of 45 Hz or lower is used. Listen the audio recording of the motor running at 60 Hz on a VFD.

I performed the phase sequence test you described with the compass and a 9 volt battery. Thank you for that suggestion. The results indicated that these windings are correctly phased.

[UPDATE] I just got the motor back together. I tried changing the VFD voltage setting to 208 instead of 220. No change. I set the VFD to 230 volts. No change. I believe that I've pretty much ruled out everything except a wiggly wire in the stator or a loose lamination in the rotor, though I could find no physical evidence of those problems.

 

[waross]

You frequency is related to speed.
Your no load speed is close to 900 RPM.
I wonder if you have a bad squirrel cage casting in the rotor?
A discontinuity in the squirrel cage may be exciting each slot as it passes.


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Smitty771]

You frequency is related to speed.
Your no load speed is close to 900 RPM

I measured actual speed and it was right at 900, so even closer than the nameplate states.

I wonder if you have a bad squirrel cage casting in the rotor?
A discontinuity in the squirrel cage may be exciting each slot as it passes.

A friend suggested either a loose, wiggly wire in the stator or a loose lamination in the rotor, then pointed me to Glyptal products. Another suggested looking into having the stator vacuum dipped. I'm reluctant to invest significantly more money into this motor, so it seems I will just have to live with it and maybe wear ear plugs. Can't win them all, I guess.

Thank you for your responses.

 

[waross]

See if you can get the rotor tested at a reasonable cost.
Stator issues often hum at 120 Hz.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Smitty771]

See if you can get the rotor tested at a reasonable cost.

I'll ask the local motor repair shop about that. Can't hurt to get a price.

 

[waross]

Ask if they think that a growler test will show anything.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[BrianE22]

Is there a mix-up between 929 Hz and 900 RPM (15 Hz)?

 

[waross]

Is there a mix-up between 929 Hz and 900 RPM (15 Hz)?

The number of slots may factor in.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Smitty771]

Ask if they think that a growler test will show anything.

$100 for a growler test. That's not so low that I definitely want to do it, but it's not so high that it's absolutely out of the question. He said it would be definitive; if it passed the growler test, it's a good rotor, otherwise something's wrong. I forgot to ask the guy, "What then?" If it passes, I'm out $100, I know the rotor is good, and my motor still whines. If it detects something wrong with the rotor, I'm out $100, I know something is wrong with the rotor, but does that mean anything can be done about it?

There's one test I haven't done yet, and it's easy enough to do that I will likely do it just to satisfy my curiosity. When I ran it on single phase power by pull-starting it, it had the same whine, but I didn't try different wiring configurations. If I try the other two wiring configurations on single phase, skipping one different phase with each wiring configuration, it may sound different one of those times. If so, that would suggest, I think, something wrong in the windings (wiggly wire). If not, it would further support the idea of the problem being in the rotor. [UPDATE] I just finished doing this test, and the whine remained unchanged in every wiring configuration with single phase. I don't know that this is an authoritative exoneration of the windings because there are complicated magnetic and electrical dynamics still surging very near to the phase that gets skipped each time, but this is at least some additional information that seems to agree with other indicators that the problem may be in the rotor.

I do find it intriguing that the dominant frequency of the whine is almost exactly the same as the [actual] RPM, but I don't know if that's correlative or just a coincidence.

 

[waross]

Do you know how many slots the motor has?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

You may try connecting a 12 Volt battery to two leads and turning the rotor rather slowly with a string.
A faulty rotor may show cogging.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Smitty771]

Do you know how many slots the motor has?

36 slots. Does that give anyone any ideas?

You may try connecting a 12 Volt battery to two leads and turning the rotor rather slowly with a string.
A faulty rotor may show cogging.

The motor is back together now, and I think I'm going to proceed with the rest of the project (making an interface for this motor to drive a metal lathe) for now. When I get everything ready to use, I'm planning to wear ear plugs. We'll see if that makes the noise tolerable. If not, I will have to revisit the problem. If I pursue the investigation and find there's something wrong with the rotor, it only puts me in worse position of having to find an entirely different motor (or keep using this one with the noise). At the moment, it seems that the motor will function just fine and simply be a great bother to the human using it.

I really appreciate the responses and suggestions to help me with this problem. Thanks.

 

[zeusfaber]

Have you measured the noise intensity? Are you in damaging territory, or just mildly irritating?

Beginning to wish I'd worried about that sort of thing a bit more when I was younger.

A