Harmonics

[Zogzog]

Sorry, i am not a big PQ guy, we have this new fan in our shop

http://www.bigassfans.com

It is making some high pitched noise that varies with speed and is not mechanical. I hooked up my Arbiter 928A and am seeing 26V on the 3rd harmonic regardless of 1st harmonic voltage (Speed), based on speed the supply voltage varies from 30V-something higher than 100V, I cant turn the thing past speed 5 (Goes up to 10) or I will have a huge mess to clean up io the shop (24' diameter fan).

Any PQ gurus out there who can explain the source of the noise?

 

[Skogsgurra]

I guess that you have a Variable Frequency Drive (VFD) feeding it.

That sound is from the VFD. You may try to adjust the carrier (switching) frequency so you won't hear it any more. But, if you are relatively young, you will need to set frequency to about 16 kHz. And that usually means you have to degrade the drive.

All this only if there is a VFD, of course.

Gunnar Englund

http://www.gke.org

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

 

[Zogzog]

Sorry forgot to mention there is a VFD. When you say the sound is from the VFD you dont mean physically from the VFD right? You mean the sound is from the motor and is being caused by the VFD. Please correct me if I am wrong here.

The Big Ass Technical support (Thats what they are really called so dont red flasg me) had me adjust the carrier freq as high as the VFD would go and we are still having the problem.

 

[Humble2000]

They are usually filters at output of VFD drives to protect the motor from high frequency PWM waveshapes. The sound is the nature of the filter . Manufacturing rules allows the noise level to be up to the certain level. Does it exceed that level? usually 50-70 db.

 

[Skogsgurra]

Yes, of course, Zog^2. I meant that the PWM voltage from the VFD causes the magnetic circuits in the motor to vibrate and send out the sound. How high could you set it? Not at 16+ kHz, I assume?

Gunnar Englund

http://www.gke.org

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

 

[Zogzog]

I dont know how high I can set it, it is a code, it dosent say what the kHz are.

I dont have db meters, but it is audible over some fairly loud classic rock on the radio.

 

[itsmoked]

Just so we are sure this is the correct subject. Did this noise change character when you changed the carrier frequency?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Zogzog]

No (to human ears anyways) it was the same

 

[itsmoked]

It sure should've..

What say you now gentlemen??

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Tmoose]

I'd try a basic microphone to record a wav file to a PC at a few different test speeds. Then use one of the freeware FFT programs to extract the frequency info.

Using a cheap sound level meter set to A weighting makes it easier to see what I'm hearing.

 

[alehman]

Sometimes it's hard to judge pitch of very high frequency sounds by ear. Perhaps as tmoose suggested, measurement might be worthwhile.

 

[Zogzog]

Little update, Big Ass Fans had a Big Ass Tech (Real names, not cussing here) come out and the motor is bad, they are replacing next week.

 

[LionelHutz]

So it was a mechanical noise?

 

[Zogzog]

No, it is electrical, the noise goes away when you turm off power and the fan is spinning, but the problem was found in the motor.

 

[LionelHutz]

OK, but a noise under power doesn't necessarily mean an electrical problem. Those fans have a gear reducer that could make noise when motoring but not when coasting. Similar to a live axle under a car or truck that can whine when under power yet be silent when coasting.

 

[Zogzog]

Maybe that is the problem, I will let you know what the BA tech says this week.