VFD cuasing motor vibration??
VFD cuasing motor vibration??
(OP)
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).
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).





RE: VFD cuasing motor vibration??
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.
RE: VFD cuasing motor vibration??
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.
RE: VFD cuasing motor vibration??
RE: VFD cuasing motor vibration??
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 ?
RE: VFD cuasing motor vibration??
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.
RE: VFD cuasing motor vibration??
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
www.gke.org
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RE: VFD cuasing motor vibration??
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RE: VFD cuasing motor vibration??
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
RE: VFD cuasing motor vibration??
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RE: VFD cuasing motor vibration??
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: VFD cuasing motor vibration??
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RE: VFD cuasing motor vibration??
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.
RE: VFD cuasing motor vibration??
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
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: VFD cuasing motor vibration??
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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RE: VFD cuasing motor 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
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: VFD cuasing motor vibration??
RE: VFD cuasing motor vibration??
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
RE: VFD cuasing motor vibration??
RE: VFD cuasing motor vibration??
Keith Cress
kcress - http://www.flaminsystems.com