painfully loud motor 1

[foretmark]

Is there any electrical mechanism that I'm just not able to figure out that would cause a motor running under typical conditions to make an exceptionally loud noise under load? I'm fairly certain it's mechanical, but I'm being told it could be electrical and I just want to rule out anything wierd. It is a pump motor and the pump and motor have both been replaced seperately and together and the noise continues. I'm told it's coming from the motor/pump assy.

 

[dpc]

Is it running on an adjustable freq drive? If it is not, the noise is probably mechanical in nature, although it could certainly be coming from the motor (bearings, etc).

Motors on adjustable freq drive can produce some interesting noises.

"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller

 

[foretmark]

no, no VFD or softstart or anything, direct through an overload and contactor.

 

[starkopete]

Hi,
When you cut the power does the noise cease immediately or does it continue as the motor coasts down? Generally, if the noise is electrical it will quit as soon as the power is cut. Thanks

 

[TurbineGen]

fortemark, there could be a number of problems from bad bearings, to a cavitating pump, to poor alignment coupling, to broken rotor bars, .... and list goes on. A few questions: Have you tried running the motor without the load attached? Can you/have you done a vibration test? I have experienced this several times in the last few months. Each time it was related to the pump cavitating or being restricted. I'm not saying that must be your problem, just suggesting it be checked. Perhaps a realignment between the motor and pump wouldn't hurt either.

------------------------------------------------------------------------
If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[foretmark]

the time between the motor power being cut and actually stopping is too short to determine. Probably 1s or less. I don't actually have it in front of me, but I know the system.

 

[itsmoked]

Pumping what?

What does the noise sound like?

Got a recording?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[1000V]

Hi here are my suggestions!

Run motor light (split coupling) to determine motor or pump. Maybe rotating unit bearings are U/S? Is the wear plate rubbing on the impellor?
Turn pump and motor by hand if you can to get a feel for the bearings.
Check non return valves/autovalves if any.

Does it have a spider coupling? Check spider. This can be done on the run with a strobe. Check tyre if it's a tyre type.

Sorry if I'm over simplifying. I am just a lowly shift electrical tech in a paper mill.




Has vibro analysis picked up that it is an electrical problem? Quite often I am handed jobs from the VA department saying that there is a problem with a motor electricaly only to find there isn't!

I would bet it is a process or mechanical problem. If your work is like mine, that makes it your problem.

 

[waross]

If the noise is present after both the motor and the pump have been changed, look for a hydraulic problem. Try throttling the flow. You may have a critical flow situation where the pressure pulses from the impeller blades are reflected from a bend in the discharge piping and arrive back at the pump just as a following pulse is starting. That is why VFDs for pump duty have programmable "Forbidden frequencies".
You may have a "forbidden frequency" at of near line frequency.
I remember a 4 inch diameter hole blown in the side of a pump casing by a VFD that had not been programmed for the forbidden frequency properly. The sad part was that it happened twice before it was corrected.
A long straight pipe run downstream of the pump is conducive to this problem.
There are the basic solutions.
Change the frequency.
Change the number of impeller blades.
Change (usually shorten)the length of straight discharge piping.
Is this a new or a changed installation? Did the noise start following some change?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[foretmark]

The pump is pumping hyd fluid into a pressure vessel....what I'm told they've done is remove everything between the pump and vessel (valves, etc) and ran a hose directly to it. Everything is fine until it gets to some (not sure what) pressure....then the noise begins. Running the fluid with no back pressure does not create the noise.

There is no vibration measuring equipment available to my knowledge.

I've heard several descriptions of what it sounds like, and I heard it over the phone....kind of like a boeing 747 coming to a stop on a runway - upside down. Can't really remember the other descriptions, all similar though

The coupling is a lovejoy. I think that was inspected, but not sure if they replaced it.

I'll see if the discharge pipe has a different configuration than usual, however, I'm fairly certain is a configuration we use regularly as far as plumbing the pump in. The longest run of pipe (tubing in this case) would be no more than a couple feet if that...

We ran the thing here and did not experience the noise...apparently it began on site. Another thing we've considered is a damaged diaphragm in the pressure vessel. Pretty much all other parts have been swapped.

I should know in a few days what they find and I'll post it...

 

[electricpete]

Electromagnetic noise is certainly a possibility. It usually does vary with load. In my experience fractional-slot large motors (which are sometimes encountered in slow speed applications) are particularly vulnerable. We have two families of large fractional slot motors (one 324rpm and one 900 rpm). Both of them purr at no-load and SCREAM under load. In our case it is not indicative of a problem, simply a consequence of the design (rotor slots, stator slots etc)

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[jraef]

The pump is pumping hyd fluid into a pressure vessel....what I'm told they've done is remove everything between the pump and vessel (valves, etc) and ran a hose directly to it. Everything is fine until it gets to some (not sure what) pressure....then the noise begins. Running the fluid with no back pressure does not create the noise.

Sounds to me as though it's load related from that statement. The only possibility I can think of is a broken rotor bar in the motor. As the load increases, the problem becomes more evident. A good motor rewind shop can find that if they know to look for it.


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[electricpete]

If you can get a spectrum of the noise that may help to confirm

Typically the frequency of electromagnetic noise would be a pattern of 2*LF sidebands around/including rotor bar pass frequency.

For example if LF = 60hz, speed= 30hz and you have 40 bars, the frequencies would likely be:
30 * 40 = 1200hz
30 * 40 -120 = 1080hz
30 * 40 +120 = 1320hz

You might also have pattern of 2*LF sidebands near 2*RBPF, 3*RBPF etc.

Even if the noise didn't turn out electromagnetic, the spectrum may provide some clues as to the source.

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[electricpete]

Rotor bar problem would typically be an oscillating noise (the frequency of oscillation is pole pass frequency usually around 2hz at most for large motors (slow enough that you can hear the pulsating). Exact pole pass frequencycan be determined from the speed (such as by strobotach) and compared to pulsating frequency to help validate or disprove rotor problem. There are multitudinous other ways to check... vibration analysis is a start, current signature analysis most conclusive. However rom your description and lack of mention of pulsation I still think simple electormagnetic noise much more likely than rotor bar problem.

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[itsmoked]

If he's replaced the motor and the pump why would the problem be "a rotor bar"?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[oldfieldguy]

Hydraulic pump? And it doesn't get noisy unless there's pressure?

Has anybody checked the bypass valve. Hydraulic pumps are usually positive displacement. If the pressure gets too high on the outlet, a bypass valve opens and runs the overpressure back around to the suction side. On some pumps this is very noisy.

These are usually spring-loaded relief valves and a broken or weak spring will let it start 'bypassing' at a lower pressure.

old field guy

 

[waross]

Good call, oldfieldguy. LPS

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[1000V]

Sounds like a fitters problem to me

 

[rbulsara]

what I'm told they've done is remove everything between the pump and vessel (valves, etc) and ran a hose directly to it.

Why? Was that before the noise started or after? It may be that they may have removed a check valve that may have kept the fluid coming back to the pump from the vessel??

 

[electricpete]

If your motor is 8-pole or slower?

If so, I will pretty much guarantee that electromagnetic noise is exactly what you're seeing.

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