Source of Large Motor Vibration

[boscorio]

My organization is working on a problem with a 15,000 HP motor which exhibits two to three times the level of vibration when voltage is applied than when not. The rotor and rotor bearings have been test throughly and they are balanced and true. The vibration levels are very low when the rotor is spinning without voltage applied to the stator. This phenomena occurs in the field and in tests at the factory. There are several of these motors in service and all exhibit the same phenomena but one has vibration levels that are exceptionally high when power is applied. The stator is the last component of the machine not yet subjected to intensive scrutiny. That is begining now. Suggestions for the direction of the investigation are greatly appreciated.

 

[electricpete]

good point ukpete. He also called it 1x so I have assumed he meant 515cpm.

 

[jbartos]

Suggestion: The vibration QA/QC test report may also be available from the manufacturer.

 

[boscorio]

UKpete,
Sorry for my confusion. The primary frequency of vibration at low speed (515RPM) is 8.58 Hz and at high speed (600RPM)is 10.0Hz.

 

[edison123]

boscario,

did you get the no. of stator & rotor slots ?

What does the motor(s) manufacturer have to say for these vibrations ?

 

[boscorio]

edison123,
I'll get the stator/rotor slots .... I'll need to lookup the drawings once I'm back in my office.

The manufacturer doesn't really have a clue thus far but they claim that the electrical design has been thoroughly reviewed an should be fine. They are reluctant to share the details of their review with us.

Early on in the issue we discovered that the rotor was designed with insufficient material in the "spider" arms that hold the rotor lamination core to the shaft. The rotor was re-designed and new rotors were produced. We had assumed at that time that the silver bullet had been found and indeed it helped the problem (as we assume it stopped some of the dynamic air gap variation while the machine was running) but the vibration issue did not entirely go away. The manufacturer has tried to balance the "electrical" vibration out with a comprimise balance (comprimise between the mechanical balance and reducing the electrical vibration) but with marginal success.

They are now looking at performing performing a surge test on the coils and if nothing is found there then they are thinking of discecting the thing. Ultimately they are looking at rewinding the stator.

Thanks for your interest!

 

[Shortstub]

Boscorio,
I was referring to what you called the "spider arms!" Were there three or four? We had a such a machine vibrate intermittently with the 3-arm spider.

The root cause was unexpected "weakness" of rotor's 3-armed spider (wye-configuration). The initiating phenomenon was shaft-rub where the shaft exited thru a wooden (hard-to-believe) bushing in the surrounding enclosure that contained the forced-air cooling system. The rotor was also replaced with one having a 4-armed spider (cross-configuation)

 

[electricpete]

Since they are focusing on the stator: other obvious checks are winding resistance, current balance while unloaded, and watts loss while unloaded. Surge comparison as you say can identify a miswinding. Air gap if not already checked and stator bore roundness. And before you tear it down why not stator core loop test while monitoring for hot spots.

I did not hear any indication that you had positively ruled out open rotor bars. It's unusual in a new motor but you never know.

 

[edison123]

boscorio,

Another thing that comes to my mind about these vibrations - stator coils being loose in the slots.

 

[electricpete]

Normally for looses stator coils one would expect 2*LF.

BUT, seems possible we might also see vibration at syncronous speed. Boscorio - have you done a high-resolution spectrum to confirm whether this is in fact the vibration is at 1*running speed or syncronous speed?

 

[boscorio]

electricpete,

The 1x vibration frequency is at running speed, not synchronous speed. The peak of the spectrum occurs at that frequency and not at the synchronous frequency.

thanks for your interest!

 

[jbartos]

Suggestion: The motor might be disassembled and the rotor manufacturing accuracy checked.

 

[edison123]

boscario,

another thought. This being a PAM motor, the speed change is obtained by manipulating the winding connections. If the winding connections do not ensure pole-phase group symmetry, then uneven magnetic pull across airgap will result leading to 1 X RPM electrical vibrations.

Could you mail me the stator winding connection diagrams for both the speeds to eewemf@myrealbox.com ?

 

[plc123]

Hi,
it looks like syncronous motor,
So stator supplied with 3 phase AC voltage
(hopefully symmetric - better to check).
If vibration appears during load - reason
in one coil pair on the stator provides
different level of magnetic flux.
Assuming coils itself are OK,
I would check connection pole to core - this is real resistance to flux or
check voltage between artificial star center and real star center in winding - may be difference.

Thanks,
Vladimir.

 

[boscorio]

edison123,

I have tried to get my hands on those drawings but the manufacturer won't give them up. They're claiming the drawings are proprietary and they don't want to give away their designs. Hmmmm.....

I understand that PAM motors have an inherient imbalance in their flux patterns due to the nature of the method that generates the poles for a given speed. Despite the best of designs, it appears that such an imbalance may be something one must live with.

If I do come up with any drawings then I will certainly keep your offer in mind. Thank you.

 

[steamboat444]

Buscorio;

You mention in your original post that there are a number of motors exhibiting the same behavior, but one is worse than the others. You also mentioned that these motors have been rewound because of an original problem. (I have to admit that my head is spinning with the amount of quality information you are getting from this site)

Especially if the "other" motors display unacceptable performance, this sounds an awful lot like a motor design flaw that is exacerbated in one particular motor by an exceptional electrical or mechanical defect. My quess is one of the mentioned possibilities upsetting the electrical "balance" of the motor.

Given the size of these motors and the fact that there is more than one motor involved, it sounds like there could be some sizeable monetary consequences. This could degenerate into what we politely call a "Urinating Contest" here in the states. If you don't find an obvious answer/solution shortly, I would get a highly reputable motor test lab on my side. Have them wring that motor out both electrically and mechanically. If they can't find it, you can at least claim to have done your due diligence on this problem.

Best of Luck