Demodulation

[mcdm]

Hi Oli ,you replied to the reliability magazine board forum on 11 16 2003 (Thanks Walt for your input, Demod on vibration that normally is for bearing problem detection has a standard setup 2-10KHz w. Fmax 1000Hz in this sytem.)
If you have the time could you answer a few questions about Demod?

Would you use the same parameter setting for run of the mill 2/4/6 pole ac induction motors Using standard bearings? (Not tapered or special)

Why not 1-10KHZ or any of the other parameter sets available?

If there are not any haystacks on the spectrum Commtest advise a standard setup 2-10 KHz.
If when collecting data after say six months you start to see haystack activity would you alter your Demod Settings to suit?

Do you select your demod fmax and number of lines to be the same as your velocity or acceleration fmax?

For example velocity 1000Hz Fmax 800 lines
Demod 2-10KHz Fmax 1000Hz 800 lines

Best regards mcdm

[Oli]

Hi mcdm, Sure only my opinion and the way I usually use it.....
My reason for using demod on motors are mainly for finding bad bearings.
I am currently not making any special difference for the motor speed.
Normal is 2-10KHz, I also have good results using 1-10KHz, good thing about 2-10KHz is that other unwanted things may be suppressed, good thing with 1-10KHz is that we have found, and others when looking at standard things from any make... that above 1-1200Hz most bearings have a resonance that is excited by/enhance bearing problems. That is one reason for the haystack you sometimes see in the normal spectrum.
So the reason in this case that it was 2-10KHz is that I copied a existing machine as the setup was made in haste as it was not supposed to be more than a electrical analysis...... You know. By this I can say I normally like 1-10KHz range. Going in and alter setup is not normally feasible if you don´t have plenty of time? I think it is better to stick to a setting to get as good trend as possible.

For an electrical motor I would recommend a setup for normal vibration
upp to at least 15-1600 Hz to cover electrical problems that pop up at rotor bar passage and or stator slot passage, 1000Hz is not enough on motors. 800Lines is a fair compromise for route purpose until a problem pop up..... As you saw I pushed up to 3KHz and 3200 lines to see if there was any more multiples or stuff to see when seing problems.

So I would suggest:

Velocity 2-1500Hz 800 lines,
Demod 1-10KHz 800 lines

So I am only human, not always doing what I prefere....
Hope you got some input on how i view this, any other suggestions?
Olov

I can´t help, if you have motors to monitor, we made a gadget to attach to monitor the flux when doing vibration route collection.
Actually sold one to NZ.

[mcdm]

Hi Oli
Some very good information on demod. I think I will try the 1-10KHz range. Most of the demod readings i have taken have proved to be far from reliable the activity and trends are usually very erratic , mainly because i'm using a 4 inch probe on most of the machines i monitor(large route based systems)
Data collection and report time restraints play a big factor we seem to be offering to monitor more and more machines on route based systems.
There has to be a trade off and something has to give, as there is simply not enough time to monitor and analyze machines as I would like to

[Oli]

There is one way you can reduce collection time, reduce number of points.
If you have collected data for a reasonable time so you have a fair share of data then you can reduce to only collect in one position per bearing, normally horisontal and only collect the others when problems pop up. It may not be suitable on all things but those that "behave" can be covered this way if really needed. Olov