VFD noise and motor frame material?

[dmalicky]

For a VFD-driven motor, does the material of the motor frame make much difference in the noise emitted from the VFD's carrier frequency? I know cast iron has higher damping than steel or aluminum, but I'm not sure how much this damping actually affects the acoustic noise in practice. If the effect is negligible, I'd prefer to get a lighter motor rather than a CI. (I know I can dial the VFD up above ~12 kHz, but some people can hear that, too.)

Thanks for any insights,
David

 

[ozmosis]

I'm not sure about the actual casing but very low cost (usually Chinese own brand) motors are worse than higher quality made motors when it comes to audible noise due to the CF of a VFD.
I guess it comes down to the amount of copper and iron within induction motors.

Don't forget, with some (most) VFD's that you will have to de-rate them if you wish to increase the CF, due to the increased heat of the VFD and therefore less efficient operation for the same output.

 

[DickDV]

I've seen large differences in audible noise between two quality cast iron motors of different brands. I'm guessing but I think it has to do with the way the magnetic steel is held in the frame. For example, I respect both Reliance and Marathon motors but, in my experience, the Marathons are much quieter than the Reliances.

 

[Skogsgurra]

Such simple things as peckerhead covers can make a big difference. A 315 kW cast-iron pump motor was whining so loudly that people couldn't work near it. I was asked to have a look at it and I can say that I needed both ear plugs and 'cups' to be able to get near it.

One could almost see how the cover vibrated. Hand in glove damped it and we later sprayed foam on the inside of it. That was all that was needed to quiet that one.

So, I think it is difficult so say in general if aluminum or cast-iron is louder. Parts that resonate can exist on both types. And it is the resonaces that create the highest sound levels.

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]

Along the lines of the peckerhead comment, I have seen at least 5 times on motors or transformers where a buzzing sound can be stopped by putting your hand on one of the many attached metal labelplates.

I agree many variables for motor noise in general and vfd introduces more

Cast iron has better material damping properties than the other materials steel and aluminum, which in general would lead us to suspect lower noise with "all other things being equal". Considering that all other things being equal requires a somewhat philosophical view since it is impossible experiment to change only the damping characteristics only ... when we change the material we also change stiffness and density which affects resonances. BUT, there are som many excitations and possible resonances that without more information one cannot say one material is more likely to be close to resonance than another. And at resonance is where the damping properties are most important.

See below including referenced posted by some guy named electricpete:

http://maintenanceforums.com/eve/forums/a/tpc/f/3751089011/m/838106265

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(2B)+(2B)' ?

 

[dmalicky]

Thanks, all, for the helpful replies! Many variables at play, I see, so it's hard to generalize. That is interesting that a small part can be the worst offender. Thanks also for the link.

Yes, I'm watching the derating curves for higher carrier freqs... fortunately the VFD I'm looking at (Hitachi WJ200-37) has no derate for my room temps.

For the motor, I think I've now narrowed it down to a Baldor Super-E (5 HP), which is offered with either a steel or cast iron frame. I'll try asking them if it makes much difference for noise.

Thanks again!