Electrical discharge to earth of motor bearings
Electrical discharge to earth of motor bearings
(OP)
I'm currently undertaking ultrasound inspections of some 6600 volt soft start motors and appear to be getting indications of fluting whereby i might be getting a discharge to earth through the motor bearings.
Iknow VFD controlled motors can cause an induced voltage on the motor shaft but not aware if soft start motors would do the same. I'd be grateful for some advice on the subject please.
Iknow VFD controlled motors can cause an induced voltage on the motor shaft but not aware if soft start motors would do the same. I'd be grateful for some advice on the subject please.





RE: Electrical discharge to earth of motor bearings
I can imagine that you get problems if there's no by-pass contactor/breaker that closes after start is complete.
Running with SS at 100 % can bring problems, especially if it isn't an SCR softstarter.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Electrical discharge to earth of motor bearings
RE: Electrical discharge to earth of motor bearings
Measuring earth current might be a way to understand whether your damaging currents are internally or externally sourced, but I think you'll have a hard time making a call just by "use a voltmeter and test motor casing to earth". Unfortunately I'm out of my depth here. Hopefully someone with some actual field experience will chime in.
RE: Electrical discharge to earth of motor bearings
Then you should look not only at motor frame voltage to solid ground (if you can find it) but also check voltage across bearings. Measure from shaft to frame. If you have more than 500 mV to 1 V RMS, you may have a problem that needs to be eliminated.
These days, DOL motors are known to have problem with PWM residues coming from other drives with PWM inverters in the vincinity. To check that, you need an oscilloscope with at least 20 MHz BW to measure shaft to frame voltage. If you can see HF components with a couple of volts or more, peak, then there's the cause for the fluting.
To find the remedy needs deeper investigation.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Electrical discharge to earth of motor bearings
RE: Electrical discharge to earth of motor bearings
We had about 150 metres of 300 sq mm singles Cable feeding a 400 amp FLC motor.
The earth conductor ran neatly alongside the same phase conductor almost the whole length.
The motor suffered a bearing failure after about a year (on Christmas Eve, when else?).
The repairer said we've got earth currents.
We tong tested the earth conductor: 90 amps! After we relocated it away from the triad, down to next to nothing.
This may be of use to someone, not necessarily with the 6600 volt problem..
RE: Electrical discharge to earth of motor bearings
What are the symptoms of fluting using Ultrasound?
thanks
Dan T
RE: Electrical discharge to earth of motor bearings
RE: Electrical discharge to earth of motor bearings
The 100 Hz may also come from the mains voltage. At 50 Hz and force being current squared, you will see lots of 100 Hz and multiples thereof.
Fluting at exactly 100 Hz is not very common. BPF is all over the place, but not very often right on double mains frequency. And given that the fluting period and BPF interact, I would say that a bearing in a machine where you have 100 Hz and its multiples is a healthy machine.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Electrical discharge to earth of motor bearings
RE: Electrical discharge to earth of motor bearings
The first thing to look for is if your grid has any imbalance. Already at one or two percent difference between phase voltages, you will get noticeable 100 Hz vibrations. And more at higher imbalance levels.
Harmonics do not usually cause this kind of problem, so do not pay too much attention to them.
Are the two motors fed from the same transformer?
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Electrical discharge to earth of motor bearings
RE: Electrical discharge to earth of motor bearings
Would i be correct in assuming that if this was a supply problem then i would see this on all the other motors. 1 of the motors i have now been told has indicated a vibration alert so any advice you have i'd be grateful as this is more indepth than i normally would be expected to deal with.
RE: Electrical discharge to earth of motor bearings
RE: Electrical discharge to earth of motor bearings
Peter53
- Are these 2-pole (i.e. ~3000 rpm) motors ?
- What is measured value of mains voltage ?
RE: Electrical discharge to earth of motor bearings
RE: Electrical discharge to earth of motor bearings
Peter53
Excellent!
2900 rpm is indeed a 2-pole motor ....
i.e.,3000 rpm sync. speed (-) 100 rpm (nominal)slip at full load.
2-pole motors are notorious for their vibration problems.
One only needs to do a search— i.e., Google — on ...
Vibration in 2-pole motors .....
You'll find several articles / papers on the subject, some in PDF format. There are also some IEEE papers which, unfortunately are not free, even for those of us that are members thereof.
As a Prologue to my comments below, I offer excerpts from Neil Mulji's excellent article ... entitled— Vibration in Electric Motors
Comments:
In re-reading your post of 8 Jun 12 ... (quoted below) —
— I would take umbrage with the conclusions drawn as well as the methodology by which the testing was performed.
The 50-Hz and 100-Hz components from which you draw conclusions are most likely related to— 1X & 2X— Mains Supply Frequency, consequent to testing while mains excitation was applied to the MUT (Motor Under Test). The better way to conduct these tests is while the motors are coasting-down immediately upon removing excitation therefrom. In this mode, you eliminate any and all influences from the magnetics— (electromagnetic & electromechanical)—consequent to motor excitation.
Application Notes: Vibration Diagnostics for Industrial Electric Motor Drives
An Analytical Approach to Solving Motor Vibration Problems
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Bearing fluting resulting from dV/dt of the source voltage is not as likely on medium-voltage 50-Hz supplies as 60-Hz supplies because 50-Hz produces a lower value of dV/dt. Somewhere in my literature collection there is a reference to dV/dt causing bearing fluting on medium voltage 60-Hz supply back in the 1920 era. Also it is more likely on ball type bearings than on roller bearings. (Check with UR MTR MFGR. to see what type bearings are installed in affected machines of your concern.)
With induced electromechanical vibration, one is more likely to see bearing problems from brinelling as opposed to fluting. Most positive way to be sure is to remove the bearing exhibiting vibration during coast-down ... cutting it open and inspecting the races. Expensive... but informative; particularly where more than one motor is affected.
Other causes of low frequency vibration can be attributed to installation practices. Here are but a few for consideration:
1) Misalignment
2) Soft Footing
3) Weak Foundation
4) Shaft Coupling Eccentricities
5) Loose bolts of pump, gear box etc.
6) Bearing loose in housing (The bearing housing will be heated up)
Take a look at this article, which offers diagnostic advice for pursuing the above installation problems leading to vibration issues.
Basic Motor Vibration Troubleshooting Tips
Hope this helps..
\'scusaMe/
RE: Electrical discharge to earth of motor bearings
I don't know a lot about ultrasound, but the frequency band is generally high (audible range is up to 20khz, so ultrasound is generally higher?). I don't think twice line frequency vib would show on ultrasound.
There are however a variety of higher frequency electromagnetic slot-related vibrations. For example rotor bar pass frequency +/- 2*LF. The signal processing that you are using might output to the user some frequency which is different than the raw data. For example envelope detection of RBPF+/- 2*LF vibration (similar to Entek Spike Energy or CSI Peakvue) yields 2*LF output. I seem to recall that some ultrasound instruments use something like heterodyne, which is different than envelope detection but I'm guessing it might somehow have a similar effect? Just at thought. In any case the data presented is very sketchy...need more dat.
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(2B)+(2B)' ?
RE: Electrical discharge to earth of motor bearings
electricpete, I'm hooking up later with a trainer from UESystems to help me analyse some of my issues and will post further information later next week. You are correct in that ultrasound 'heterodynes' the sounds so they can be displayed in a visual form in their software package.
I attended a conference recently regarding ultrasound and one of the speakers mentioned that if you get multiples of 50hz frequency readings basically meaning high peaks at 100hz then a small peak inbetween at 50hz this would indicate an electrical problem. He went on to say that it would indicate an electrical discharge through the motor bearings.
The issue i now face is that the shaft of the motor has a ceramic coating precisly designed to stop any discharge through the NDE bearings of the motor so i'm not convinced that it is actual discharge causing the read outs.
RE: Electrical discharge to earth of motor bearings
Peter53
A couple of points I neglected in my previous post:
1. Voltage imbalance phase-to-phase can also be a cause manifested by vibration and bearing problems.
2. Harmonic Voltage Distortion must also be looked for and if present, will also cause vibration and consequent bearing problems.
/s