Hi all
Ok, While trying out are new motor tester,on one of are 900hp DC motors it was found to have an extremely low megger reading of .1meg and motor was changed out. Motor in service 10 years with nothing showing up in vibration spectrum but low 21600 SCR freq. .02 in/sec.
After motor changed,and drive self tuned,there has been a considerable change in levels of 21600 to .25 in/sec in velocity spectrum and waveform levels of 1.5 G's.
Also the motor has a fairly distinct hum.
Another self tune was performed on drive,no change,also ran motor at different speeds thinking possible resonance issue, no change.
This motor was just reconditioned, has grounding brush installed on shaft,and was laser aligned when installed.
Has anyone ever experienced such an issue?
Thanx for reading EB (newbee)

What motor testing equipment are you using?

PDMAmax

I have been using a PdMa for three years now and I would not have changed out a motor based on a PdMa reading alone, especially a "meggar" reading. I would want to put a "real" meggar on all three phases first to verify.
Was the shaft brush on the motor when it was tested?

I'm not sure what you mean by "self tune", since I haven't fooled with one in 6 or 8 years, but obviously there has been a change.

Ron Brook will be along directly and beat you with the shaft brush

Welcome to the forum.

Dave

Yea,We were skeptical too about the reading and did varify with regular megger and it also read .1 at the motor w/leads unhooked.
The brush was on the motor shaft,does this have an effect on the reading?Is a grounding brush a bad thing?
As far as self tuning I am not really sure,I believe its just tuning in the drive to the new motor (Avtron)but not really sure. EB

It's been awhile and I wasn't 100% involved, but a few years ago we had a large horsepower DC motor (recently rebuilt) that exhibited similar issues (high 21600 CPM vibration & noise).

After fooling around with 'tuning', among other things, we found that one of bolts that secured a field coil was loose. This was corrected on-site and the high vibration frequency at 21600 CPM and associated noise was eliminated.

If I recall Technical Associate's Vibration Diagnostic Chart referenced a 'Broken Field Winding' as the probable culprit... pretty close anyhow.
I'll see if I can't find the old report with pictures that documents exactly what we found and post it tomorrow.

The attached file shows a velocity spectrum and a picture of a motor similar to one that we saw with the loose field coil bolt. As I stated in my previous post... the bolt was tightened and the noise and vibration was essentially eliminated.

250HP_DC_Loose_Field_Windings.doc (130 Kb, 13 downloads)