Rotor balancing
Rotor balancing
(OP)
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
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






RE: Rotor balancing
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If we learn from our mistakes I'm getting a great education!
RE: Rotor balancing
RE: Rotor balancing
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
www.edison.co.in
RE: Rotor balancing
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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RE: Rotor balancing
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.
RE: Rotor balancing
rmw
RE: Rotor balancing
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.
RE: Rotor balancing
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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RE: Rotor balancing
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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Eng-tips forums: The best place on the web for engineering discussions.
RE: Rotor balancing
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Rotor balancing
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.
RE: Rotor balancing
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Rotor balancing
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Eng-tips forums: The best place on the web for engineering discussions.
RE: Rotor balancing
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
RE: Rotor balancing
Guardiano
RE: Rotor balancing
And I still don't agree that the rotor has to be balanced every time the motor is dismantled.
Muthu
www.edison.co.in
RE: Rotor balancing
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.
RE: Rotor balancing
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
www.edison.co.in