Engine Driven Generator Vibration Amplitudes
Engine Driven Generator Vibration Amplitudes
(OP)
I have been testing a V6 (4.2L) natural gas fuel, spark ignition, emergency generator running at 3600 RPM. The 1X RPM vibration amplitudes at all load conditions approach 1 inch per second on the generator bearing (spectrum peak). There is only 1 generator bearing on the outboard end of the generator.
The crest factor approaches 5 on this unit. This indicates that the energy in the waveform is not very close to a sine wave. This is common for internal combustion reciprocating engines. The amplitude observed in the waveform spikes is 5 g's (peak) which at 60 Hz is equivalent to 5 inches per second (peak) or about 250 mils (pk-pk)!!
These are prototype units and there has been some rotor/stator rubbing. The generator air gap is only 1 millimeter.
My data indicates that there may be some resonant amplification at the generator bearing. Amplitudes in the vertical direction are about 5X higher than in the horizontal direction, and there are some other indicators, as well.
(1)Are these extreme levels common on engine driven generators?
(2)Can the rubbing be expected with vibration levels like this and such a small air gap?
(3)Won't these high vibration levels significantly shorten the life of the generator bearing?
(4)Is the spiked nature of the waveform energy more or less damaging to the generator bearing and windings than a more sinusoidal waveform?
Skip Hartman
The crest factor approaches 5 on this unit. This indicates that the energy in the waveform is not very close to a sine wave. This is common for internal combustion reciprocating engines. The amplitude observed in the waveform spikes is 5 g's (peak) which at 60 Hz is equivalent to 5 inches per second (peak) or about 250 mils (pk-pk)!!
These are prototype units and there has been some rotor/stator rubbing. The generator air gap is only 1 millimeter.
My data indicates that there may be some resonant amplification at the generator bearing. Amplitudes in the vertical direction are about 5X higher than in the horizontal direction, and there are some other indicators, as well.
(1)Are these extreme levels common on engine driven generators?
(2)Can the rubbing be expected with vibration levels like this and such a small air gap?
(3)Won't these high vibration levels significantly shorten the life of the generator bearing?
(4)Is the spiked nature of the waveform energy more or less damaging to the generator bearing and windings than a more sinusoidal waveform?
Skip Hartman





RE: Engine Driven Generator Vibration Amplitudes
RE: Engine Driven Generator Vibration Amplitudes
I think you were a factor of 10 off on your calculation.
At 3600cpm, 5g's => 5ips => ~ 25 thousandths of an inch (peak to peak).
Also, I'll point out that conversion applies for a sinusoidal waveform (not a spikey-impact waveform). The better way to determine the actual peak-peak discplacement would be a double integration of your velocity waveform. Since the time waveform spikes at 2600cpm are much narrower than a half-cycle of 3600 sinusoidal waveform, you will find the double-integration should give you a much smaller peak-to-peak result.
Also the relationship between rotor motion and bearing vibration deserves some consideration.
You're probably aware of all that, but I just wanted to throw in those comments.
RE: Engine Driven Generator Vibration Amplitudes
RE: Engine Driven Generator Vibration Amplitudes
Just one thought, since you mentioned you've had evidence of rubbing, when was the last time you balanced the rotor? Is this a single phase generator?
EP....double integrate velocity to get displacement? Am I having a brain fart this early in the morning?
RE: Engine Driven Generator Vibration Amplitudes
Pete - Thanks for pointing out my error on the displacement. I agree that the conversions are only accurate for a sine wave. Do you think the FFT more closely represents the amplitudes? Or is it somewhere in between what is displayed in the FFT (which seems artificially low in these cases) and what you get using the inaccurate (for this wave shape) waveform relationship conversion formula? I was using accelerometers so I have an acceleration waveform and I will try the Excel time domain integration we have talked about in the past and see what I come up with. I agree this type of waveform is common on recips. I just wonder if the amplitudes and impulsive nature of the energy are enough to damage bearings and reduce winding life significantly. The manufacturer also needs to meet various vibration specs that are out there, which it is currently exceeding.
Fred - This is a 3 phase, 80 kW generator prototype. The component manufacturers (engine and generator) each do their own balancing before the units are coupled. I am a big believer, in the fact that once these rotor components are joined you have a new rotor and the fact that independent component balancing was done may mean very little. No dynamic balancing has been done after assembly and I am recommending that the first step be balancing a couple of these units to determine what improvements can be made that way. On most rotating machinery you can get a pretty good idea from the phase relationships how successful balancing will be. I think that determination is harder to make in advance with the reciprocating engine. It is certainly worth a try before they start trying to redesign things.
Skip Hartman
http://www.machinerywatch.com
RE: Engine Driven Generator Vibration Amplitudes
Skip - Intuitively it seems like hi-crest factor waveforms must be tougher on almost any equipment than a corresponding sinusoidal waveform with the same overall velocity. One thing that comes to mind is fatigue... doesn't that depend roughly on the number of cycles times the peak amplitude? In that case you have the same number of cycles but much higher peak amplitude for the hi-crest-factor waveform. Likewise certaily plastic deformation will be higher with a higher peak stress.
So, it seems reasonable to me that the true peak would be a better indicator of the damaging potential of nonsinusoidal vibration than an rms value.
But that of course brings us to another philosophical argument... do we monitor vibration because the vibration itself is destructive of because it is a symptom of someting else destructive going on? Probably a little of both as you know.
As far as practical experience one way or another I don't have any. I do know that compressors have a more frequent overhaul schedule (3-10yrs) than pumps (10yrs to never), but not necessarily because the bearings are degrading.
RE: Engine Driven Generator Vibration Amplitudes
Skip Hartman
http://www.machinerywatch.com
RE: Engine Driven Generator Vibration Amplitudes
(We already said the impacting waveform creates more plastic deformation and fatigue.)
Another aspect is that the impacting waveform has a spectrum which occupries many more harmonics and therefore has a much higher probability of exciting a resonance somewhere within the machine.
RE: Engine Driven Generator Vibration Amplitudes
On an SI I4 engine you can see up to 20g at second order, due to the inertial problems with L/R ratio, so if you have an automotive generator it will probably be OK for a hundred hours, but I'm guessing that you have a shaft driven generator which may not be as tolerant.
Cheers
Greg Locock
RE: Engine Driven Generator Vibration Amplitudes
Anyway you look at it, the vibration levels that you measured on the generator bearing are too high. A few years ago, I did extensive vibration testing of several diesel engine generator sets. The single bearing generators had high vibrations for several reasons:
1. The overhung exciter was resonant at 4x running speed. Excitation from the engine that was coincident with this natural frequency caused high vibration. After 500 to 1000 hours of operation, the exciter would fail. You can easily check this with an impact test of the generator shaft with the unit down. We also used a strain gage telemetry system and proximity probes to take measurements during operation.
2. Depending on the mounting configuration, the end of the skid could be resonant causing high vibration at the generator bearing. We looked at rigid mounting and spring isolators. The best arrangement was rigid mounting to the skid and spring isolators between the skid and floor. The extra beam length was removed from the end of the skid to lower the vibration amplitudes.
3. High 1x vibration could be an indication of mechanical unbalance or an electrical problem. If the vibration occurs when the generator is loaded, and then immediately reduces in amplitude when then generator is de-energized, the problem is electrical. We found some loose connections in the generator. On the other hand, mechanical unbalanced will cause 1x vibration that varies with the speed squared.
To download other papers on vibration go to
http://www.engdyn.com/papers_main.htm
RE: Engine Driven Generator Vibration Amplitudes
Greg-Thanks for your comments on typical engine vibration levels. You are right, this is a shaft driven generator with a single bearing on the outboard end. The highest vibration levels were measured on the generator bearing, not the engine. This is where my concern lies.
Skip Hartman
http://www.machinerywatch.com
RE: Engine Driven Generator Vibration Amplitudes
The engine/generator sets I was telling you about ran at 1800 RPM.
To confirm if your vibration is strictly due to unbalance or if a lateral critical speed may be involved, I would suggest performing an unloaded speed sweep. The amplitude of the 1x vibration should change with the speed squared.
If you find a peak or an increasing slope with a phase shift near the operating speed, you may have a critical speed problem. An impact test of the rotor with the unit down would help confirm this.
Regards,
Troy Feese
RE: Engine Driven Generator Vibration Amplitudes
Skip Hartman
http://www.machinerywatch.com