RBPF line : an aerodynamic problem ?

[dazhoid]

Hi,

I have a noisy squirrel-cage induction machine with a 115 dB line at Rotor Bar Pass Frequency (f*Zr/p where f is fundamental current frequency, Zr number of rotor bars, and p number of pole pairs) in off-load sinusoidal running conditions.

That sharp peak decreases to 90 dB when using the external cooling fan... which suggests that noise comes from a purely aerodynamic problem.

A possible explanation is that rotor teeth rise too much above rotor bars, which makes the rotor act like a fan with Zr blades. As the motor is open, this noise would easily radiate out of the motor.

Is that a well-known noise problem in induction machines ? Does this phenomenon have a name ? Have you ever met such a problem ?

Thanks for you answers,

 

[electricpete]

Can you explain the role of the external cooling fan which is apparently switchable?
Blows air on the outside of the motor or the inside?
Is this a vfd fed machine?

Traditionally, RBPF is number of bars times running speed. This differs from your f*Zr/p by a factor of (1-s). Can you clarify.

RBPF wtih 2*LF sidebands is a very common pattern in vibration resulting from electromagnetic effects.



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[dazhoid]

I don't know why the fan has been made switchable, but it blows air on the inside of the motor through the airgap, rotor and stator ventilation channels in order to cool windings, rotor and stator cores.
This is a vfd fed machine, but tests were run in sinusoidal supply (provided by an alternator).
Sorry, I forgot the slip as test were run in off-load case, the noisy frequency is f(1-s)Zr/p.
I know that RBPF line usually comes from electromagnetic effects, but as I said, the variation of 25 dB when switching on the fan clearly shows that the problem is not magnetic.
Tests have been run at several different frequencies, and RBPF line systematically decreases with the fan. It increases with speed at constant flux, which also indicates that it comes from an aerodynamic problem...

 

[electricpete]

I can't argue with your logic. I believe it is not common. "Noise of Polyphase Electric Motors", by Gieras, Wang and Lai (ISBN 0-8247-2381-3) discusses many sources of noise, but aerodynamic effects of rotor bar passing is not among them. However fan noise is certainly prominently mentioned. I guess there can be portions of the motor where the rotor bars act like fan blades such as within the vent ducts and sometimes adjacent to the endring and in these areas it seems logical that an aerodynamic bar-passing (blade passing) effect could occur. Along the length of the bar, the tops of the bars are typically recessed slightly below the iron creating a much smaller irregularity which I doublt would cause this effect, but you never know.


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