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bsr201283

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Dear all, 

This is regarding doubt on Vibration FFT spectrum.

I have mounted MEMS accelerometer on DC shunt motor which is rotating with 1500Hz. SO the rotating frequency (1X) will be "25 Hz " , 2X near at 50Hz and 3X at 75 apprxomately. 

But in the FFT waveform I can see only 2X and 3X (attached screenshot ).

Vibration.png  why there is no 1X present in the waveform ???




MarkL

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Reply with quote  #2 
Maybe your auto scaling is emphasising the 2x and 3x ? 
ivibr8

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Can you post a Time Waveform?
OLi

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How far down in frequency is the system speced to go? Is it one of those that barely reach 1000Hz in the other end? I don't know what limits they have in the lower end. It may also be that your motor are in a very good balanced state and the FFT calc is a integer one and that small peak remaining is as said before "autoscaled" out of that calculation. If someone saved calculation power that way it sometimes make the analysis complicated, just suggesting but seen it sort of before.
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ruben

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[COTIZACIÓN = bsr201283] Estimados todos, 

Esto se refiere a la duda sobre el espectro Vibration FFT.

He montado el acelerómetro MEMS en un motor de derivación de CC que gira a 1500Hz. ASÍ, la frecuencia de rotación (1X) será "25 Hz", 2X cercana a 50Hz y 3X a 75 aproximadamente. 

Pero en la forma de onda FFT solo puedo ver 2X y 3X (captura de pantalla adjunta).

Vibración.png  ¿Por qué no hay 1X presente en la forma de onda?



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ruben

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I think first you should sure the real 1x with a strobe light.
Second if you don´t use triaxial acelerometer change the measurement axis, your motor could have asymmetric stiffness so in the axis of higher stiffness the 1x could be lower than in the minor axis stiffness.
OLi

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Reply with quote  #7 
It is 1500RPM not 1500Hz?
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Danny Harvey

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Reply with quote  #8 
It appears that your resolution is sufficient to recognize the difference between 50 hz and 52 hz and with about .1 (or less) mm/s at defined running speed of 25 hz, I doubt if those are 2 and 3 x rpm. That is, assuming 1500 rpm, not hz and assuming that the y scale is mm/s.

I don't think I could say that there was anything running at 1500 rpm. The other two peaks could be background vibration.

Is it possible that the motor was not running and you couldn't tell?  It's happened to me before. Sometimes you can't tell.

Can you go back and collect data again after verifying the operating speed?  If you have a tuneable strobe it could be helpful in finding background vibrations as well as running speed.

I would also check to make sure that your resolution is sufficient.

If it is indeed 2 x rpm, then I would think you would see pretty sharp impacting in the twf.
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