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Danny Harvey

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Reply with quote  #1 
Here in the states, we worry about scr firing frequency in DC motors at 360 hz but here at my shop we call it 21,600 cpm.

What do y'all (The southern US) make of a significant peak at 21,600 cpm with sidebands of 448 cpm which is running speed? It there at low amplitude in the velocity spectrum but much more prominent in the PV data with a 60000 cpm fmax and a 1000 hz hp filter.
Shoveldr

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Reply with quote  #2 
Your HP Filter in Peakvue is too low.  You are probably seeing electrical noise from the drive that is having sidebands at running speed above the HP filter.  If you take a normal spectrum up to 5K (300,000 cpm) you will see it in the normal vibration.  I normally go with a 5000 Hz HP filter for DC motors.
Ralph Stewart

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Reply with quote  #3 
Quote:
What do y'all (The southern US) make of a significant peak at 21,600 cpm with sidebands of 448 cpm which is running speed? It there at low amplitude in the velocity spectrum but much more prominent in the PV data with a 60000 cpm fmax and a 1000 hz hp filter.




I am assuming you are looking at the same order of the 360 Hz (4th order) in your velocity data that Peakvue sees with the 1K HP filter since it (Peakvue) begins saving data passed 1K Hz with the 1K HP Filter?
Or are you referring and comparing the fundamental 360 Hz Frequency in the velocity data to the 4th order of the 360 Hz, which is the 1st order seen by Peakvue and is in acceleration units? How many harmonics of the 360 Hz does Peakvue show in its allotted 1K Fmax?

What does the velocity, non Peakvue data of the 4th order, look like in conventional acceleration when compared to Peakvue?

This doesn't answer your question of what us Southerners make of you current problem. Sorry. [smile]

Just asking. [smile]

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Walt Strong

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Reply with quote  #4 
Ralph,

6 x 60 = 360 Hz so it is 6th order and not 4th order, or am I incorrect? I have seen 2x line frequency (2 x 60-Hz) present for many motors when measuring with PeakVue. I guess periodic electrical excitation produces  a modulation whether for AC or DC motors.

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Ralph Stewart

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Reply with quote  #5 
Quote:
Ralph,

6 x 60 = 360 Hz so it is 6th order and not 4th order, or am I incorrect? I have seen 2x line frequency (2 x 60-Hz) present for many motors when measuring with PeakVue. I guess periodic electrical excitation produces  a modulation whether for AC or DC motors.

Walt


Walt,

I was speaking of the 4th order of the actual 360 Hz Frequency position which would be the fundamental frequency of the frequency Danny is referring to, which would be the resulting fundamental position of the 60 Hz LF x the 6 Rectifier firings, which you are seemingly referring to. [confused]

I may be wrong in what the original question was about. [confused] I think the Peakvue is looking at the 4th order of the actual fundamental of the 360 Hz (60 x 6) when it (Peakvue) is having to use the 1K HP Filter.

Sorry if my reply was not easily understood for what ever reason, be it incorrect or be it not well written. [smile]

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Danny Harvey

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Reply with quote  #6 
It took me a while, but I get what Ralph is referring to.

And I understand that I can filter it out and make it go away.

My question is why would vibration at running speed (the sidebands) cause vibration in the scr's (the effect)?
Ralph Stewart

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Reply with quote  #7 
Quote:
Danny Harvey quote:
It took me a while, but I get what Ralph is referring to.

And I understand that I can filter it out and make it go away.

My question is why would vibration at running speed (the sidebands) cause vibration in the scr's (the effect)?


Danny,

Did you get "Filter it out and make it go away" out of what I said?  I hope not. But if you did, I guess I really have you confused. [smile]

I was just trying to determine if the 4th order (360 x 4 = 1440 Hz position) in velocity, has a higher amplitude than the fundamental (1st order) of the 360 Hz position in velocity.

You said:
Quote:
low amplitude in the velocity spectrum but much more prominent in the PV data with a 60000 cpm fmax and a 1000 hz hp filter.


To me, the Peakvue data is giving us, at the least, a picture of the (360 Hz x 4) position (4th order) and is not comparable to the system's fundamental 360 Hz (1st order) displayed in velocity.

As far as being modulated by running speed, this possibly is a problem with one or more of the 6 firing "cards" which is causing the speed to vary slightly. Ever what the cause, it is not a common problem and probably will take an experienced SCR electrical person to determine exactly what the problem is.

Just my Opinion.

Thanks,
Ralph

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David Eason

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Reply with quote  #8 
Danny,
You said it has six firing cards? Imagine what would happen if you have a six-cylinder engine in your car and you pull one of the spark plug wires loose. The engine develops a "miss" or a stutter when that particular spark plug is supposed to fire. This would be a stutter at the running speed of the engine. What would happen if one of the fuses for the firing cards was blown? Wouldn't it develop a stutter at running speed? Would this explain your sidebands at 488? I wouldn't think it would be very hard for somebody to check the fuses and rule this out.

David Eason
Danny Harvey

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Reply with quote  #9 
Ralph,

No, filtering it out would be the change to a 5000 hz hp filter that shoveldoc suggested. I rarely use the 5000 hz hp filter but it might be best just to make it go away.

Your point that PV would be looking at the 4th order of scr firing frequency makes sense. Unfortunately, the velocity spectrum is only out to 60,000 cpm due to the maximum speed of 600 rpm.

I already did like David and Ralph have suggested and asked them to check for loose fuses or firing cards. I'll post again if I find out what they find.

Thanks to all for the help.


dnk

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Reply with quote  #10 
I have seen this several times. Twice I found loose connections at card.
RRS_Dave

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Reply with quote  #11 

David has the best explanation I've heard. I have seen this a couple or three times, and in each instance we found problems with a firing card. Don't know exact problem with card, in each instance it was a swap parts and see if it fixes it. It did, and that was end of feedback as to what problem detail actually was.

D

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