Vibration problems with an electrical generator 1

[AlbertFM]

Dear all,

I am dealing with a 35 kW and 90.000 rpm electrical generator to work coupled to a micro gas turbine.

I face some problems when I try to ramp-up the generator. When it pass through 550 seems to stay in a critical speed and later on at 750 Hz an other one. We can pass the first one but the second one destroys the system.

Trying to find out what is the source of the problem I have simulated some parts and I've used the hammer test to find the natural frequences of the system and all the components. What annoys me is that all the natural frequencies are at least 3 times bigger and I am not able to find the source of the problem.

Does anybody have any usefull comment that can helps me to solve the problem?

I will be really pleased with your help.

Thanks in advance

 

[GregLocock]

You'd struggle to excite torsionals with a hammer test.

Guessing the machine is about 0.3m long it will be intensely modal by 750 Hz - that is there will be many modes, and the shapes will be complex.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[AlbertFM]

Greg,

Rotor is .25 m long and we measure it without the coupling of the turbine. With the turbine I know that we shoud expect some torsional vibration.

Do you think that the magnetism can be the responsible of torsional vibrations?

Is it true that if we make the rotor shorter we would have less vibration problems at that frequency?

Thanks for your help

 

[SomptingGuy]

Is the generator bought-in or home-made? How is it coupled to the turbine?

- Steve

 

[AlbertFM]

SomptingGuy,

The generator is home-made. The coupling is made by a steel small bar that goes inside both shafts. However, we do not still have problems with it because we are not running it coupled. First we need to run both systems separately till 90.000 rpm.

 

[GregLocock]

I don't see why torsionals should not be excited by the magnetic forces, but I have no direct experience. Can you upload a waterfall or Campbell's plot?

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GregLocock]

Actually, just guessing, are you talking about first order?




Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[AlbertFM]

I will prepare some information.

When the first order or 1xRPM component reach 500 and 750 Hz the vibration seems to increase a lot. I don't know what happens at higher speeds because the vibration is too severe to try it. It seems to destroy the system.

This morning I've seen something interesting.

The natural frequency of the shaft is around 2250 Hz at one end, 3000 Hz at the other end and 7000 Hz at the middle (thicker) part with the magnetics. Besides, we do have big harmonics (specially in the axial direction). During all the ramp-up we can see the first order and its harmonics.

Do you think that the harmonics can ressonate with the shaft natural frequencies?

 

[MotorVib]

What typ of bearings are you using? Also how is the rotor supported when the natural frequency was taken.

Chris



"In this house, we obey the laws of thermodynamics." Homer Simpson

 

[AlbertFM]

We're using ball bearings. The bearings are in contact with the stator.

Here is attached some data from our measurements. Unfortunately I cannot send you a campbell diagram because we use videos of the spectrums to see the data.

 

[MotorVib]

From what you have send and told us I don't see any simple way of finding it, but I do know a way that you should be able to figer it out.

You will need to do Modal Analysis of the rotor. It was the best way that I have found to find problems on rotors. Once you have the Modal Analysis you can make a calibrated model of the rotor. With the calibrated model as a starting point any change that you would like to change on the rotor can be check back to the FEA.

Chris

"In this house, we obey the laws of thermodynamics." Homer Simpson

 

[AlbertFM]

MotorVib,

To find the fundamental vibration mode shapes and corresponding frequencies I've done FEA, calculations and also hammer testing. In all this calculations I find the natural frequencies 2 or 3 times higher than the problem frequency.

I've studied also rotor+stator, stator alone and so on

Do you know what else I can do?

Thanks for your help

 

[TPL]

1X at 500Hz corresponds to 30,000rpm which is pretty fast.

How was the rotor assembly balanced? Even a small amount of residual unbalance will generate significant forces at this speed.

Things will likely get worse if you move towards the 2 mode shape at higher speeds.

I would suggest that you get hold of some proximity probes to see if these shed any light on what the shaft is doing. (I know you would not normally use these transducers with rolling element bearings, but it seems you have significant vibration and likely large shaft defelections)

 

[AlbertFM]

TPL,

We balance at low speed but with 2mg of residual imbalance.

The norm says that with this tolerance we won't have problems at nominal speed ( 1500 Hz- 90.000 rpm) due to imbalance.

Actually, I'm not sure if the problem is due to a deflection because the speed range where I see what is supposed to be the ressonance is too wide. (from 650 Hz to something more than 900 Hz).

If the problem is a deflection due to the centrifugal force, it is going to affect at the imbalance right? What I don't understand is why we find so many harmonics at the axial spectrum. May be unbalance?

We have prepared three tests:

We will restrict the axial movement of the shaft to check what happens with the harmonics and the possible ressonances.

The second one will be using shaft with a stiffer material to reduce deflection.

The third will be reduce the distance of the bearings also to try to control the deflection.

Unfortunately we cannot use proximity proves because we don't have space for them.

Thanks a lot

 

[TPL]

Tell Norm he could be wrong...

A low speed balance might get you through the 1st critical speed but, there is no reason to assume that a low speed balance will be valid for 2nd and higher modes

How does the system destroy itself at 750Hz? If its a rub then yes unbalance (M*r*w^2) might well be causing shaft deflection that uses up all available clearances.

Getting vibration data in bode and polar formats would assist in figuring out what is happening


Quote:When it pass through 550 seems to stay in a critical speed

When a rub develops, the effective stiffness of a shaft increases and perpetuates the contact.

Have you considered using a microphone to detect the onset of any problems?

 

[BobM3]

Are you getting modulation of the 2 (or more) natural frequencies? 3000 hz - 2250 hz = your 750 hz.

 

[AlbertFM]

TPL,

When I said Norm I referred to a handbook but it is true that the Norm I use does not take in account the possible modes of the shaft.

Today I have tried to restrict the axial movement of the shaft and I've seen something interesting; First of all the harmonics in both axial and radial directions were smaller and furthermore the 1XRPM component (radial) was much more smaller than other times. Secondly when the shaft has passed through 550 Hz vibration has not increased as it used to happen. Unfortunately the vibration at 750 remains too high.It seems ressonance but is very very wide (at least from 650 to 900). At this moment I cannot check the bode plot because I don't have the equipment.

I was thinking on increasing the speed of the balanced but I don't uderstand how is possible that the shaft has two modes at this "low" speeds (550 Hz and 750 Hz) if simulations, calculus and hammer says that critical speeds are above 2000 Hz. I think that I am missing something related critical speeds, mode shapes...

BobM3,

I thought that modulation occures when two shaft speeds are quite near. In my case there is only one speed (one shafts) because we don't measure with the turbine coupled. Besides the speed of the turbine and the generator will be the same.

Tomorrow I will check what happens with a stiffer shaft. I hope that if it is a problem of deflection we won't experience such a big vibration above 750 Hz.

 

[MotorVib]

When you did your calculations did you factor in the bearing being a weak spring.

If you think that it could be a balance take a look at ISO 1945 it is on balance and it can give you a good idea if you are in the range for gas turbine.

Chris

"In this house, we obey the laws of thermodynamics." Homer Simpson

 

[BobM3]

Modulation could occur if there was indeed a 2250 hz and a 3000 hertz vibration occuring at the same time. You mentioned earlier that you measured (or calculated) that those two frequencies were present on each end of the shaft.

 

[AlbertFM]

MotorVib,

My hand calculations were just an approximation. In FEA simulations, Ansys added weak springs to solve the case (at least it was said in an information message).

To think about a reasonable residual imbalance I've used ISO 1940. This standard does not takes into account if the rotor pass through critical speeds. It just says which is the maximum residual imbalance to work at your nominal speed. I have doubts in this area because I am not sure to face a critical speed. I haven't found any natural frequency of any element of the system (hammer test) that can cause it.


BobM3,

I will investigate modulation because I don't know enough about it. I still don't understand this phenomenon. How can two high natural frequencies be substracted if they're not beeing excited? It could happen if the harmonics were the responsibles but the vibration of the harmonics that pass through that frequencies is not really high. In any case I will look for some information about this problem...


Thanks,