Motor Whining

[HYDL]

I am trying to learn what I can about Motor Whining. We have a cooling tower motor at our building that has a significant Whine. It is served by a VFD and I'm thinking that this may be the reason. Anyone have any input in this regard?

 

[jraef]

Most likely that is the reason. VFDs work by simulating AC power with a complex series of DC pulses. The switching on-and-off of these pulses will make your motor windings vibrate on an almost molecular level, hence the "whine". You can reduce it, but not without sacrifice. Most VFDs offer the ability to modify what is called their "carrier frequency", the base rate at which the pulses are fired. By increasing the carrier frequency you "decrease" the whine. I put that in quotes because you don't really decrease it, you just move it into the range beyond human hearing. The cost of doing that is to increase what are called switching losses in the drive, so your overall efficiency goes down a little and the VFD and motor put out a little more heat.

Not all VFDs are capable of this, especially as you get into larger horsepower (you didn't say what it was), but it is worth checking out if the noise really bothers you. If not, leave it alone. It is not hurting the motor, in fact lower carrier frequencies are better for the motor than higher ones.

JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[Skogsgurra]

To add to jraef's recommendations: Some inverters have a parameter that you can set to get a randomly variable switching frequency. It is the same technique that is used to reduce EMI in some cases.

It sometimes, not always, results in a more acceptable sound from the motor. Sometimes it also gets more annoying - sounds like the bearings are full of sand or very near their end of life. If the option is there, just try it. It doesn't cost anything and it doesn't hurt motor or inverter.

Sometimes, you can even go down in frequency to avoid resonances that you may have in motor, fan or supporting structure. If, for instance, your carrier frequency is 2.5 kHz, it may be a good idea to try something like 2 or even 1.6 kHz.

There is also (the rather expensive) solution to add a sine filter between VFD and motor. That will give you the best solution. Quiet motor and reduced bearing problems.

Gunnar Englund

http://www.gke.org

 

[ScottyUK]

...reduce EMI...

Or just spread it over a broader spectrum to defeat the EMI tests...?

----------------------------------

Sometimes I only open my mouth to swap feet...

 

[itsmoked]

Now, now, Scotty not defeat... p a s s the EMI tests. Say after me "paaasss the test".

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[powerjunx]

"skogsgurra, ..It sometimes, not always, results in a more acceptable sound from the motor. Sometimes it also gets more annoying - sounds like the bearings are full of sand or very near their end of life. "

skogsgurra, It's true. Such audible emission really annoying!

We have a problematic machine. To the extent a thorough check up of the unit was resorted, ended up to found nothing except the VFD itself is the culprit until tuning up was carried out.

 

[Skogsgurra]

Yes Scotty (and Smoked). It is to pass the test. Smear out the peaks so you get under the dB lines.

Gunnar Englund

http://www.gke.org

 

[ozmosis]

Just to add a couple of points to what jraef and gunnar advise:
increasing the carrier frequency on certain drives and certain makes will automatically de-rate the drive (reduce the output current capability), so increasing the carrier frequency, whilst it will reduce the audible noise, may create other problems with the drive system (e.g. the VFD going into current limit). So be careful and take any steps gradually (i.e. if your CF is 2kHz, don't go straight to 16kHz) and observe the effect of any changes.
Also, fitting a sine filter between the VFD and motor will reduce the effects of motor noise but it sometimes just puts it into the sine filter. We had an issue in a theatre once where a motor was too noisy. Fitted a sinefilter and cured the motor noise but moved the noise to the filter. We had to make sure the filter was sited well away from the stage.

Another point: cooling tower motors are usually the cheapest of the cheap motors. Could it be the bearings?

 

[jraef]

Last month I worked on a Ski resort chair lift drive system where the motor noise was so bad that skiers had been complaining all though this past season (the motors were outside next to the boarding area). The resort had been sold a "solution" by a vendor of putting reactors on the drive(s), which they just got around to installing. The reactors then made so much noise that the operators who would be working in that building threatened to get OSHA (US workplace safety organization) involved because of exposure to harmful noise; the pitch apparently was so penetrating that headphones were not effective (although I suspect they were angling for the really expensive noise cancelling headphones myself).

They had 2 x 250HP closed loop PDL vector drives, but it turned out the motors were only 160kW, so I turned on their "Whisper Wave" feature which varies the carrier frequency within each cycle as skogsgurra mentioned. I had to derate the drives a little as sed2developer said, but they were over sized for altitude to begin with and I figured they are only operating in winter when it's very cold, so it wasn't going to be a problem. The noise disappeared (to us Homo Sapiens at least) and I was a hero.

Of course, I made it out to be a much more involved process than it was. One must never pass up the opportunity to look good!

JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[Skogsgurra]

Jeff. Don't spill any more beans! We need to keep our little secrets of the guild to ourselves ;-)

Gunnar Englund

http://www.gke.org

 

[HYDL]

Thanks for all the input. Very helpful.