High Pitched whine after rewind

[accordlx]

We recently rewound a 100HZ single phase 10 pole synchronous generator that is driven by a six pole 60HZ synchronous motor. The generator puts out the correct voltage and frequency and for all purposes, it functions as it should. The problem is that it now emits a high pitched whine that supposedly was not noticable previous to the rewind. Does anybody have any idea as to what would be the cause?

 

[electricpete]

Some guesses:

Electromagnetic
Change in winding configuration (coil pitch) - can change slot harmonic vibration.
Change in winding configuration (total series turns) - affects flux levels and em vibration
Change in airgap eccentricity which can aggravate slot harmonics

Mechanical
Change in height of wedge below stator bore - might change some aerodynamic siren effects
Changes to rotor fan blades
Removal of sound insulation or change which allows more direct exit of sound to atmosphere

Other
unexplainable

There may be others, but my guess is the last one listed above.
You might try to determine the frequency to narrow down the cause.


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[accordlx]

The frequency indicates 10 times drive motor speed which is 1200 rpm

 

[ScottyUK]

1200rpm = 20rev/second = 20 Hz. 10x that is 200Hz, hardly a high pitched whine. To a musician that is well down into the bass end of the frequency range. Your description sounds a little odd, if you'll pardon the awful pun.


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If we learn from our mistakes I'm getting a great education!

 

[DickDV]

I don't know, ScottyUK! If all you ever hear is 60hz hum or, in your case, 50hz hum, 200hz probably sounds pretty racey!!

 

[davidbeach]

Since you don't really get 60Hz (50Hz) hum out of electrical equipment, such as transformers, you actually get 120Hz (100Hz) hum as both the positive and negative peaks of the voltage create the driving force for the noise. So he is generating 100Hz electrical and getting 200Hz acoustical, sounds about right. ;-) Also agree that it wouldn't be "high pitched" at a mere 200Hz.

 

[electricpete]

Yes, you're right about that David.

60hz into 6 pole motor gives 60*(2/6) = 20 rot/sec shaft output

20 rot/sec rpm shaft output into 10 pole generator gives 20*(10/2) = 100hz electrical output.

100hz voltage and fields in the generator creates 200hz vibration

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[electricpete]

Also I would guess that you determined the frequency from bearing measurements and as others have said it is probably not the frequency you are hearing.

If you can arrange to use a microphone and capture the data digitally for fft, that may provide a more direct info about the source of the noise.

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