Motor+Fan, BPF(+ - 1*RPM)

[skan99]

I am a pretty begineer in the area of motor and fan system. It has 3 blades and 60Hz BLDC motor. The operating speed is 800RPM. There are the higher tonal noises at each BPF and sidebands(320+-13Hz(=operating speed)) of 8th harmonics of the BPF. Is there anyone who can explain about the causes of the sideband tonal noises and why sideband tonal noises only appear near 8th harmonics ?
Thanks for your help in advance.
skan

 

[GregLocock]

Resonances

Cheers

Greg Locock

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[skan99]

Could you explain a little bit in detail? Thanks

 

[GregLocock]

No, go and get a book on dynamics.

Cheers

Greg Locock

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[electricpete]

I think what Greg is saying is that you have a resonance near the 8th harmonic which is amplifying both the 8th harmonic and it's sidebands. (correct my if I'm wrong).

If this is the case, you should be able to put it on a log scale and look at the lower order harmonics and see sidebands on those as well.

It is not clear what the cause of your sidebands is.

Also I am curious how you determined this is 8th harmonic of blade pass frequency rather than 24th harmonic of running speed. Additionally if this machine operates at syncronous speed (I don't know anything about BLDC motors), then perhaps the frequency may be related to a harmonic of the line frequency.

You will find a lot of fantastic assistance on rotating equipment vibration questions at maintenanceforums.com

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[vanstoja]

I agree with Electicpete's opinion that 1X sidebands around the 8th Impeller Blade Passing (IBP) harmonic probably infer that all IBP harmonic peaks have 1X sidebands which are caused by amplitude modulation related to blade cutwater clearance variation associated with impeller wobbling or orbital motion variation. Overhung impellers are notorious for impeller wobble-induced rotational sidebands. It is unusual to find a dominant IBP harmonic as high as the 8th since the broadband source level of IBP peaks drops off at a rate of something like 6dB per octave. Consequently, I am inclined to agree with Greg Locock that structural or hydroacoustic resonance is probably elevating an 8th IBP harmonic.

 

[vanstoja]

A possible explanation for a dominant 8th IBP harmonic is blade-vane interaction. If there is a vaned diffuser downstream of an impeller then the number of impeller blades and diffuser vanes controls the relative intensities (signal to noise ratios) of individual IBP harmonics along with the blade-vane cutwater clearance. The stationary vanes act like efficient radiation zones (ERZ) such that blade harmonics close to vane harmonics tend to be amplified. The so-called Tyler-Sofrin equation from compressor design technology gives an intensity parameter
I=mB+/-nV where B and V are blade and vane numbers and m and n are integers, ie, harmonic numbers. Then I=0,1,2...,etc rank the highest to lowest amplification factors. For example a 6-vane diffuser downstream of a 3-blade impeller would yield I=0 for the 2nd blade harmonic interacting with the 1st vane harmonic and for the 8th blade harmonic interacting with the 4th vane harmonic.