Single phase electric motor 500Hz noise 2

[noisun]

I'm studying the airborne noise emitted by a single-phase small (less than 100W) electric motor for dishwasher machines.
It is a 2 poles motor with capacitor connected to an auxiliary (2 poles) winding very similar to the main winding (capacitor and auxiliary winding are always connected during motor running).
Geometrically, main and auxiliary winding has an 90 degrees angle (their directions are perpendicular).
On stator stack( 25mm height), there are 24 slots;
on rotor (squirrel cage, same height, 51mm diameter), there are 32 slots.

OK, the problem is:
noise (microphone) meas put in evidence a sound pressure peak at EXACTLY 500 Hz, when the motor is supplied at 50 Hz (no load, whatever voltage, 115V or 230V).
The same motor supplied at 60Hz DOESN'T produce any relevant sound pressure peak at 500 or 600Hz (no load, whatever voltage, 115V or 230V).
In my opinion it’s an electric noise (not related to structure resonances or to mechanical orders), but my competence in electrical harmonics is rather low…

Why 50Hz frequency produces such an evident sound peak?

Thank you in advance for whatever input.

 

[GregLocock]

Silly question - can you hear this noise yourself? I ask this as you can often get mains hum in the instrumentation itself. OK, that doesn't explain why 60 Hz doesn't produce it.

Next question - how good is your 50 Hz power supply? Check its harmonic content. (Statement of the obvious - be careful, you are dealing with mains voltages)



Cheers

Greg Locock

 

[electricpete]

I agree likely electrical noise. Particularly if it is exactly 500hz, and not 10 times running speed (would be slightly less than 500hz for induction motor due to slip).

Why at 50 hz and not 60 hz? Two thoughts:
1 - There may be a resonance involved in the vibration, perhaps stator deformation resonance.
2 - If voltage did not change going from 50 to 60 hz, then flux (proportional to V/f) was reduced by 5/6 and EM for is expected to reduce approxiamtely (5/6)^2

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[noisun]

Dear GregLocock, thank you for your inputs. I took my time to try what you said.

I can't hear 500Hz component noise by myself (I tried, referring my ear to a pure 500Hz sine),
because it isn't predominant in the overall sound pressure spectrum...more important are 49 Hz (running speed), 100Hz (pulsating torque),
1570 and sidebands (rotor bar frequency), 447 and 894Hz (rotor cooling blades and harmonics).
I can say that the amplitude of this 500Hz peak is similar to 447Hz (rotor cooling blades), but not predominant.

I checked my instrumentation...I have 12 mics in the semianechoic room (6 condenser mics and 6 ICP mics)
and two different analyzers; crossing mics, cables and analyzers, I have the same 500Hz peak, but I discovered that the direction of mics
is relevant (mics looking radially at stator stack are LESS sensitive to 500 Hz peak).
Moreover, 500Hz is not present in the room when motor is switched off.


Finally, I checked the frequency content of my 50Hz supply current and I didn't note anything to link directy to 500Hz:
I saw 50Hz, 150Hz, 250Hz in decreasing amplitude order (is it normal?).

Furtherly, I managed to disconnect the capacitor during running (that is: auxiliary winding off)...
500Hz peak amplitude decreases, but it is present all the same.
This means that 500Hz ISN'T related to auxiliary winding or to its interaction with main winding.

I repeated all above mentioned trials with 60 Hz supply frequency and I confirm that there isn't any 500 or 600 Hz peak.
My 60Hz line current seems to have more noise and harmonics than 50Hz line.
Of course, I have a shifting of all peaks related to rotor running speed (59 Hz).


Dear Electricpete, I agree with you...the slip at no load running is very low, but FFT resolution is
enough to distinguish running speed related orders and exact round electric frequencies
(except 49 and 50 Hz, that are very close).

Regarding to V/f ratio, at the beginning I tried to modify voltage according to supply frequency shift (280 V for 60 Hz), but I didn't found evident relations to voltage.

I need more time to investigate structural resonances of the motor (by some accelerometers)..it seems to be a good idea.

Thank you

 

[GregLocock]

OK, sounds like you have a real issue, not an instrumentation gremlin. Unfortunately that exhausts everything I know about (noise) AND (electric motors).

Cheers

Greg Locock

 

[electricpete]

A few more thoughts.

The effect of increasing flux density due to higher volts/hz is not only higher force, but more saturation which changes the harmonic picture.

Some things to check:
- check for rotor eccentricity which often increases noise.
- check variation of noise with voltage.

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[noisun]

Dear Electricpete, thank you for your thoughts...your first input was the right one (unfortunately)!
I tried with an impact test (accelerometer on stator stack, hammer at about the same point, motor still) and a very sharp peak structural resonance at 520 Hz came out.
Repeating the same impact test with motor running (2950 rpm), the resonance peak moves at 500 Hz (probably because of gyroscopic effects in ball bearings).
Looking at similar our motors designs, this resonance could be related to rotor flectional shape (constrained by ball bearings).


CONCLUSION (in my opinion):
The fifth harmonic of pulsating torque (it's a single phase motor...) is STRONGLY exalted by this SHARP resonance.
The mic hears only the fundamental (100Hz) and the fifth.
At 60 Hz supply frequency, the fourth harmonic is at 480 Hz (by the way, in a structure anti-resonance) and the fifth harmonic is at 600 Hz, far from 500Hz resonance.

However, could please anybody suggest books/articles about:
-rotor eccentricity harmonics;
-slots harmonics;
-saturation harmonics;

for SINGLE PHASE electric motors?
I have something about three phases motor harmonics...which is the difference?