Rotor balancing 1

[Guardiano]

Hello forum members,

I am on a site where is running a 3500 kVA 400 V steam driven turbo alternator. The alternator was recently rewounded, both rotor and stator. Should the rotor be balanced before putting into operation ? There is a little viration on the alternator and I presume that this come from the irregular transmission of the weight of the rotor when the rotor is turning. I have asked for a complete vibration monitoring exercise on the set but there is very little commitment to do it.

Guardiano

 

[ScottyUK]

Yes, it should definitely have been balanced. Ideally it should have had an overspeed run too. You should check that the rotor is correctly aligned and centred in the airgap too.



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If we learn from our mistakes I'm getting a great education!

 

[jan63]

the rotor needs to be balanced everytime it is removed and then installed

 

[edison123]

"the rotor needs to be balanced everytime it is removed and then installed".

No, only when any mechanical modification was done on the rotor (like rewinding) or if the rotor was removed due to unbalance problem in the first place.

In the OP's case, I'm surprised the winder did not do a balancing after rotor rewinding. Compared to the cost of rotor winding, balancing is only a small fraction. What is the speed of the generator ?

Muthu

http://www.edison.co.in

 

[electricpete]

One question is whether the rotor should have been balanced in the shop. The answer to that depends on what was included in the customer specification.

Also the type of balance to be performed in the shop should be identified in the spec. If it is a rigid rotor, low speed balancing is adequate. If the machine operating speed is above approx 75% of flexible rotor critical, then slow speed balance has limited effectiveness. Operating speed balance is more effective, but may be cost prohibitive at the repair shop.

Another question I gather you may be asking is whether there should be field balancing done upon installation. It depends in part on the previous two items. If it is a flexible rotor not balanced at operating speed in the shop, then needs field balancing. There may be other reasons to want to do a field balance if there is a suspicion the vibration will be high such as thermal changes.

Once faced with high vibration, as you say an analysis is appropriate because unbalance is not the only possible cause. Field balancing certainly might be a way to correct it.

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[byrdj]

I'm not that familiar with 3MW size. following a rotor rewind, most times only a low speed balance is performed. A low speed only ensures the unit can be place in service for final balancing. If rewind is performend in shop that has capibilities, a running speed balance would be an option.

if the unit is running, a balance program can now performed with equipment that gathers phase angle wrt a known referance on the rotor as well as magnitude.

If vibration dynamic responses are known, either from OE or past trails will be benificial in determine the placement of the correction weight. If not known, vibration verse speed decay will be need. knowing how many critiacal speeds and speed differance away from each determines the effect of the correction on the measured phase angle.

 

[rmw]

Is this a 2 pole rotor or a 4 pole rotor?

rmw

 

[rhatcher]

I would have to agree with jan63 on this one, the rotor should be balanced every time it is removed and replaced. In the shop where I work, we balance all rotors regardless of customer specification. This is considered by us to be a good standard practice.

However, even with a good low speed balance, we sometimes have to 'trim balance' the rotor once it is installed in the motor and running at speed. I've always wondered why that was true.

Thanks to electricpete (star) for explaining why that is possible. I never considered that a rotor may be flexible but, I can see how that may be a problem.

In our case, vertical motors are particularly problematic, especially two and four poles. I do not mean to hijack the thread but, can anyone explain why this is the case.

 

[electricpete]

However, even with a good low speed balance, we sometimes have to 'trim balance' the rotor once it is installed in the motor and running at speed. I've always wondered why that was true...

In our case, vertical motors are particularly problematic, especially two and four poles. I do not mean to hijack the thread but, can anyone explain why this is the case.

Our repair shop guy had mentioned that during final shop solo run, he needs to perform trim balance on vertical motors much more than horizontal, even though both have their rotors balanced before assembly.

I think the reason is: the pre-assembly balancing is done on shaft journals without bearings, and so it can correct for imbalance in rotor but it cannot correct for eccentricity in the bearing mounting. The upper bearing mounting for vertical motor is more complex, with thrust collar between shaft and bearing – provides more opportunity for bearing eccentricity.

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[electricpete]

Other problems with verticals: the upper thrust bearing may require minimum downthrust. When no load is provided, even if no skidding occurs, the radial stiffness of the bearing can decrease in absence of thrust (picture a single angle bearing without downthrust...easy to move it sideways).

And there is the matter of the lower guide bearing - misapplied per bearing manfuacturer's which tells us every beairng needs a minimum load. Obviously radial load is missing from the vertical motor guide beairng and sometimes there is no preload mechanism either.

After installed in the plant and coupled vertical machines seem to have more vvariability in the vibration. Maurice Adams makes this observation in his book on Rotating Machinery Vibration and it matches our experience. He attributes it partially to low loading on the radial beairngs... along with variability of the load that is present.

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[electricpete]

Last comment about verticals - if you have similar size rotorsfor a vertical and horizontal and balance them both to the same level and have same unbalance force, the vertical motor tends to have higher vibration simply because it is more tall/thin rather than short/stout. If you are pushing on a footlocker, it is easier to rock it if it's standing on its end (like vertical), then if it's flat on the floor (like a horizontal)

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[rhatcher]

Thank you very much electricpete. One of the balancing technicans where I work recently asked me why there was a difference. I could not provide an answer. I did suggest that a vertical motor was less stable on it's base than a horizontal one but, I failed to recognize the difference that the bearings make.

If I understand you correctly, 'trim balancing' should be expected on vertical motors and it should be planned for as a matter of routine. In addition, the need to 'trim balance' should not be considered as an indication that the original low speed balance was incorrect. It is a characteristic of the rotor and bearings for this design. Is this correct?

BTW, I will print this thread for the technicians in my shop. They will be glad to know this. Stars to you again.

 

[electricpete]

'trim balancing' should be expected on vertical motors and it should be planned for as a matter of routine. In addition, the need to 'trim balance' should not be considered as an indication that the original low speed balance was incorrect. It is a characteristic of the rotor and bearings for this design. Is this correct?

That's the way I understand it. Let's say we have a rotor that is perfectly balanced on the journals and then we add just 0.001" eccentricity due to the thrust runner. At 3600 rpm that creates an unbalance exceeding ISO G6.3 (from chart on page 4 of 11 here

http://www.irdbalancing.com/downloads/TechPaper1BalQualityReqmts.pdf

). You can also see from the chart the same eccentricity has not as much effect for slower machines.


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[electricpete]

I think maybe the terminology "thrust runner" applies only to Kingsbury style bearings. But a similar part might be named "retainer" or "collar" for rolling element bearings. The part would mount to the shaft at one elevation and provide a seat for the bearing inner ring at a lower elevation. Such a part is always needed in an oil lubricated vertical bearing in order to provide room for a shaft standpipe which must go higher than the inner ring rollers, but not as high as where the collar/retainer mounts to the shaft.

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[galiano]

Hello,

Same rotors with the same amount of unbalance will produce equal centrifugal forces from unbalance mass. Vertical machines have lower stiffness in radial direction, that's why the vibration amplitude will be greater then on horizontal machines. And I had allways thinking that bearing misalignment only produce greater bearing static preload (DC component), but not periodic force ?

Thanks

 

[Guardiano]

Thanks all for your comments; The machine is a 4 pole one.

Guardiano

 

[edison123]

At that speed (it's not 2 pole and hence a low speed balancing would suffice) and MW (less than 5 tons mass ?), the rotor should have been balanced by the rewinder after it was rewound.

And I still don't agree that the rotor has to be balanced every time the motor is dismantled.

Muthu

http://www.edison.co.in

 

[rhatcher]

edison123, we balance every rotor, whether it is a recondition or a rewind, whether it is wound or squirrel cage. We also perform vibration checks during the every test run.

This is mostly because we cannot assume that the original balance is good. Also, we cannot assume that after being in our shop that the original balance, if unchanged, is still good. The end user expects a vibration free motor to be returned to them and the only way we can be sure of this is to perform the rotor balance and the vibration checks.

In the case of the OP, had the service shop performed rotor balancing and vibration checks, they would not be questioned now about the vibration.

 

[edison123]

rhatcher

I too balance most rotors that comes to my shop, especailly after rewinds or other mechanical changes like change of shaft, fans, sliprings etc. But I just don't subscribe to the rule that the rotor has to be balanced every time the motor is dismantled.

For example, I get motors for routine overhaul (bearing change, winding clean-up etc.). In such cases, I run the motor on arrival and check the vibrations. If they are well within the limits, I don't balance the rotor just because I opened them up.

Let sleeping dogs lie is my motto.


Muthu

http://www.edison.co.in