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RustyCas

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Reply with quote  #16 
It looks like this, doesn't it?  Wire machine coiler motor.  Two of them, about 30 hp each.  Have been running about 2 years.  They both look like this.

EDM Fluting.png 


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Noknroll

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Reply with quote  #17 
Rusty, that could be my data.
spectrum in Nov 2017 was my first survey on brand new site and recorded 50g's It was great to have this low hanging fruit on my first visit.
EDM.png

MarkL

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Reply with quote  #18 
Oh, Looky...I had this yesterday.
Been tracking the last while back they are waiting to get a window to overhaul motor.


 
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pdf Fluting.pdf (31.46 KB, 19 views)

Vibe-Rater

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Reply with quote  #19 
Hi All,

Insulated bearings do not always solve fluting issues.  The coating is so thin - think microns - that a sufficiently high charge can still jump. rgds

RustyCas

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Reply with quote  #20 
Mark, how many Hz is that? [wink]
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Noknroll

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Reply with quote  #21 
CPM Hz my head and puts my brain out of order
MarkL

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Reply with quote  #22 
Hey I learned in Cpm, and my monkey brain relates better to shaft speed than hz.
My mental arithmetic is shoddy at best and saves on pulling out my calculator mid survey like a twit.

:-)
fburgos

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Reply with quote  #23 
I like rpm, comfortable using hz, rpm or CPM feels like gigantic numbers but any will do
jvoitl

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Reply with quote  #24 
One sometimes problem with insulated bearings.  If that is all you use and your motor/driven machine have couplings with metal to metal contact you can sometimes move the problem from the motor bearings to the driven machine bearings. 
Aubrey

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Reply with quote  #25 
Quote:
Originally Posted by MarkL
Hey I learned in Cpm, and my monkey brain relates better to shaft speed than hz.
My mental arithmetic is shoddy at best and saves on pulling out my calculator mid survey like a twit.

:-)


I taught myself to use orders. The numbers don't seem to be near as large. Makes my head hurt less.

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RustyCas

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Reply with quote  #26 
I just use whatever makes sense for the situation.  And I'll do my analysis using all 3, for different purposes.  Looking at a high frequency plot labeled 0 - 300000 doesn't make much sense to me.... is that 300,000? 3,000,000? What?  (I have SAS work order numbers... all those leading zeros)  If it's 5000 Hz I can easily "visually" divide that into 5 segments, and easily tell where that haystack is centered.  Ease of "thinking" about what you're looking at is important.  That's why I use 'mils' when dealing with balance issues.  It's whole numbers, and people relate to that much better.  I address "bearing energy" using g's.... if 4 g's is normal, and 10 g's is high, it's pretty obvious that 25, or 30, or 50 g's is awful.  And I call out "overall vibration" as velocity.  I think it causes less confusion to use different units for different problems.

Same for frequency. Orders of course for mechanical faults.  RPM for sidebands (better resolution than Hz).  And RPM for a multi-spindle machine like a wire machine (59 spindles with 39 different shaft speeds) -- I set up a spreadsheet and divide each of the largest peak values by each spindle RPM to give me the "order" of that peak for each of the spindles.  So there I'm using RPM and orders together.

Why just use one?  That sounds like trying to fix everything using only a hammer.

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electricpete

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Reply with quote  #27 

The rule that insulated bearings are needed above a certain size motor applies to non-vfd motors where we are trying to interrupt line frequency circulating current from magnetic asymmetries.  Those magnetic asymmetries are much larger on large motors when segmented laminations are used. And for this scenario on the NDE needs to be insulated.

For high frequency from vfd, capacitive effects are much more relevant (impedance Z = 1/[2*Pi*f*C] gets small at high frequency).  Therefore insulated bearings with a thin coating  on the outside of the bearing is not effective against this high frequency as Vibe-Rater mentioned because the thin insulation means high capacitance (C = epsilon * A/d where d is thickness).  I would think ceramic bearings (or hybrid bearings with ceramic rolling elements) would work a lot better than insulated bearings with that thin insulation.

There are other ways to skin the cat and as mentioned Ron talks about the cable configuration in the linked thread. 
Noknroll

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Reply with quote  #28 
Thanks Pete
Your first paragraph directly addresses my question about "only 100Kw and above needing insulated bearings" So with VFD's this "rule" goes out the window. Yes?
electricpete

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Reply with quote  #29 

Yes that's what I was saying. The rule seems incorrect to me in the sense that susceptibility to pwm common mode voltages from vfd does not depend on motor size (unlike susceptibility to line frequency circulating currents from magnetic asymmetry which does depend on motor size… it is only a problem when the motor is big enough to require segmented core laminations rather than one-piece laminations). I would say there are a few sublteties involved though....

Here’s a link for vfd motors that sort of resembles the rule you quoted (insulated bearing only on large motors) .  But the bolded part resembles the logic I stated.  The answer to this conflict is in the context. In the quote below there is an assumption that a grounding brush is present in all cases.  The grounding brush is supposed to protect from vfd pwm damage (all size motors) and the ODE insulated bearing protects from line frequency circulating currents (large motors only).  The insulated bearing is not protecting from vfd damage.

 https://www.designworldonline.com/how-to-protect-bearings-in-inverter-duty-motors/

Quote:
For motors above 100 hp, where both circulating currents and common mode voltages could cause bearing damage, combining an insulated bearing on one end with a shaft grounding ring on the opposite end provides the best protection from electrical bearing damage [Figure 4]

For motors up to 100 hp, where common mode voltages could cause bearing damage, adding a shaft grounding ring to the motor, either inside the motor or externally, provides effective protection against bearing currents for motor bearings as well as attached equipment [Figure 5].

 Two other things to note: 1 – I’m not positive grounding brush is a guarantee for preventing vfd bearing damage; 2 – the cutoff for what is a large motor susceptible to line frequency circulating currents really depends on the core size (is it too big to be made out of a standard size lamination steel square).  It is typically somewhere up around 500hp for 4-pole, a little higher for 2-pole, a little lower for 6 pole.

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