VFD cuasing motor vibration??

[IanFletcher]

We have a problem with a set of Toshiba H7 series model number VT130H7U415KB varaible frequency driving a sereis of 150HP Toshiba motors on an induced draft cooling tower.

The motors run fine through the full 1800 rpm speed except for one area right around 650rpm - 750rpm where the motors begin to vibrate. The vibrations seem to get increasingly worse the longer the drive is left to run at this speed. We have ruled out vibration from the drive shaft and the fan and we beleive that there is some sort of torque ripple seen at the motor that is causing the vibration but the vendor is denying this claiming that the VFD's are "set to factory standards" and have load and line reactors in place to avoid any sort of harmonics. The cooling towers are some distance away from the drives.

Not knowing anything about these drives, is there some programming feature we should be looking at changing different then the factory settings? We have noted that the stated VFD drive speed differs slightly from the actual motor speed (+/-20RPM).

 

[jraef]

It's just a fact of life with mechanical loads. Any time you apply a VFD to a motor, you can run into an issue with naturally occurring mechanical harmonics at specific speeds. For that reason, almost all VFDs have what is called "frequency skipping" settings (or some such terminology). In your case, the H7 Series calls it "Jump Frequencies", look for it in the manual.

These are specific bandwidths of frequency settings that you program into the VFD when you discover them empirically , as you did here. Then you program the VFD as to how you want it to respond: either hold back a commanded frequency until it is above the threshold, or immediately jump past it.

 

[IanFletcher]

Jraef,

Thank you for your response. I should have mentioned before that we have locked out the mechanical vibration (we beleive).

We have identified areas of the mechanical vibration due to the natural frequencies of the components involved and we have done vibration testing that supported what we have seen in the equipment. These motor speeds have already been programed out of the VFD. The vibration we are seeing seems to orginate from the motor end of the assembly, at the motor speed/freq., and, if left to run, increases from 0.2 in/sec (relatively normal) to well over 0.8 in/sec.

If the equipment is allowed up to full speed and then allowed to coast down through the 750-650 range we don't see a spike in the vibration.

 

[edison123]

Looks like critical frequency of the motor frame. While doing run-up and run-down of the motors (open shaft and no VFD) in my repair shop, we always notice, at certain rpm, the whole motor will shudder and then calm down as the speed changes. All bodies have natural frequency of resonance and your motor appears to have it at 650 - 750 RPM. Only way to ignore it, is to run up and run down past that speed.

 

[edison123]

IanFletcher

I see that we posted almost at the same time. May be the motor is coasting down too fast for you to see that vibration bump at 650-750 RPM. Was the load connected during this coastdown ? May be you can try the motor alone after decoupling the load ?

 

[IanFletcher]

edison123,

You raise a good point.

We considered the natural frequency of the motor frame & the support assembly but we have not had a chance to test it out. The coast down was too quick to check with the equipment we had so there could still be the bump you describe.

We have seen resonante frequencies before but never this severe. I don't know enough about VFD's but I thought we might have some motor cogging thing going on or some kind of feed back wave between the motor and the drive.

I think we will run the motor uncoupled (no load) at this speed and see what pops.

 

[Skogsgurra]

Ian, you can safely forget about cogging and "feedback wave". The latter may exist, but it isn't feedback - it is a reflection that may exist in (long) motor cables. It is a very high frequency phenomenon (megahertz) and has no influence on vibrations. But, if you look at motor voltage (careful!) it may look like controller oscillations. But, as said, at megahertz. And that has nothing to do with the mechanical vibrations.

Also, there is usually no feedback in the drive that can cause this kind of problem. Most fans are scalar controlled and have no closed loops that need attention in the drive.

And, finally, if it were a controller oscillation, it would stay the same over a much wider speed range. Not just in a typical resonance band.

I have had calls from certain industry branches where the "unstable controller theory" seems to very popular. It is usually someone fresh from school that has brought that theory up. They call me (independent) because the drive and motor manufacturer "denies the fact that the controller is unstable". In one case, the guys were so convinced that the controller needed adjustment that they accused me for being bought by the manufacturer when I told them there's no controller to adjust. Sigh..

It would be interesting to have some feedback (no pun) when you have solved the problem. We can all learn from that.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[electricpete]

how long to increase from 0.2 to 0.8 ips?



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[MotorVib]

It looks like you have a natural Freq between 10.8 and 12.5 Hz. That is were I would expect the natural freq. be for the pads that it is sitting on.

If you need to run at that low of speed I would look at change the vibration isolation pads.

The production test spec for most motors is for 0.08 ips max.

Chris

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

 

[electricpete]

Everyone seems confident of a resonance. To what do you guys attribute the increase in vibration over time?

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[Skogsgurra]

A very common cause is dust baked on the fan. Or sometimes corrosion that reduces mass of a blade. Or a missing fan blade. They all produce increased vibration.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[electricpete]

This is what I mean by increasing over time. It suggest to me either it is a control problem or that the vibration at that particular speed (perhaps due to resonance) is creating a rub with thermal effects.

The vibrations seem to get increasingly worse the longer the drive is left to run at this speed.....

The vibration we are seeing seems to orginate from the motor end of the assembly, at the motor speed/freq., and, if left to run, increases from 0.2 in/sec (relatively normal) to well over 0.8 in/sec.

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[IanFletcher]

Thanks for all of your comments.

It's a new installation & very clean so concerns about damaged equipment yet.

The vibration increases from about 0.2 in/sec to 0.8 in/sec over a few minutes time.

The fan is 24' in diameter driven by a right angle gear, a 100" drive shaft (composite) and a TEFC 150HP motor mounted horizontal. The entire assembly is on a tall fiberglass structure.

I believe it could be a control problem but I don't know why it would limit itself to one small speed range. What about a power distortion from a harmonic frequency created by the drive; i.e. the power distortion forces the motor to run faster but the feedback to the drive from the motor is telling the motor to brake?

I guess it still could be a nat. freq, of the motor & support but the vibration seems too strong.

 

[Skogsgurra]

If you hit resonace, the vibrations don't stop increasing until you reach some limit - like elasticity limit.

I recently witnessed a vibration treatment where a structure was taken to resonance (to get internal stress "shaken out"). We got it up to 42 mm/s at resonance. So .8 inch/s (about 20 mm/s) is definitely possible.

If the structure has a high Q, it may take quite a long time to reach maximum amplitude. A minute is not unheard of.


Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[electricpete]

Your comment about resonant vibration being limited only by non-linear behavior applies to undamped resonance.

Real world resonances are often limited within the elastic/linear region by damping.

But as you say, it takes time to put energy into the system to reach the steady state level. Longer time for lower damping. That's why I asked about time. Short time of a few seconds would steer me towards resonance. Long time (1 hour) would steer me toward rub.

A few minutes is a grey area for me. I would tend to lean more torwards resonance for that case.

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[Skogsgurra]

Yes, of course. There is always friction in most systems. And with limited input, it usually limits the vibration.

But, the case I was referring to was taken to elasticity limits. That's how we got rid of stresses in this rather big structure (an "undercarriage" for a 12 MW hydroelectric generator). We had put it on elastic feet (plastic "pucks") to minimize damping.

Before this treatment, we had lots of "local" resonances. After the treatment, most of them were gone and we had a few dominating resonances left.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[BobM3]

Can you mount an accelerometer to various points and do some hammer striking to see what the mechanical resonant frequencies are? Do it with no power to the motor so you are looking purely at the mechanical resonances.

 

[MotorVib]

BobM3

that is somthing that should help and something that I would do also.

do you feel safe running it at .8 ips? if you do what is the upper limit for the vibration? You will most likly have a range that it will work in. Changing the vibration isolation pads Should help.

Chris

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

 

[geefer]

The motors are probably of the hollow or flexible shaft design , One solution would be to fit Stiff rotor type motors. These motors are designed to push the natural resonnance frequency of the motor above full speed , This phenomenon is more pronounced in 2 pole machines but does also occur in 4 and 6 poles. This should have been scoped for you prior to installation. I feel the only option other than changing out the motors ( NOT a VSD issue) Is to as suggested set up the Skip frequencies as outlined in the previous posts. This however does not help if your process needs to be in the range where the resonnance occurs.

 

[itsmoked]

Perhaps a more complex control system if you are having issues with skip frequencies. Have certain motors not run while others run at a higher speed, etc.

Keith Cress
kcress -

http://www.flaminsystems.com