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IE3 and IE4 motors seem to have more EDM (bearing current) problems
3

IE3 and IE4 motors seem to have more EDM (bearing current) problems

IE3 and IE4 motors seem to have more EDM (bearing current) problems

3
(OP)
I didn't really notice this a few years ago. One year ago, I started wondering and now - I feel certain that there are more problems with high-efficiency motors (IE3 and IE4). Bearing run-time is one or two years, which is a lot worse than we saw on older motors with VFDs.

Manufacturers do not want to talk about it. They mostly repeat the old "truths" about good grounding, symmetrical motor cables, 360 glands and a few other standard recommendations that they still believe in.

After a lot of measurements, I start to see a clear picture. And it does not look promising.
Facts:
- High Efficiency motors tend to be built slender
- High Efficiency motors usually have a narrow air gap
- High Efficiency motors run cooler - because they have low losses

This results in:
1. A slender motor has a longer rotor than normal. That induces a higher voltage in the rotor shaft. Peak voltages above 35 V have been measured between DE-NDE on motors as small as 132 kW.
2. A narrow air gap increases air gap capacitance.
3. A motor running at low temperatures has a thicker oil film in the bearings and that means a higher break-down voltage.

A higher shaft voltage means more break-downs of the oil film. That can be easily seen when looking at the bearing voltage with an oscilloscope.
A higher air gap capacitance increases the available energy stored in the air gap. So destruction increases with narrower air gap.
A cooler bearing means thicker oil film and, as a result, a higher break-down voltage. Energy is proportional to capacitance times voltage squared. So, the lower temperature is actually worse than the higher capacitance. Together, the two kill bearing quite quickly. And the increased induced shaft voltage supplies the necessary voltage to overcome the oil film's higher break-down voltage.

So far, my observations. What are yours?
Have you noticed an increase in bearing problems lately? Especially so with IE3 and IE4 motors? What do your motor suppliers say about it? The usual crap? Or do they help? Or ignore? Do they at all understand?

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: IE3 and IE4 motors seem to have more EDM (bearing current) problems

I am not saying your wrong but i have not seen this problem in IE3 motors but i don't have any IE4s in service.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

RE: IE3 and IE4 motors seem to have more EDM (bearing current) problems

Gunnar, that makes perfect sense to me.
I do not see enough field problems to confirm or deny your theory in practice, but the logic is exactly what I would expect.
Best regards,
Mark.

Mark Empson
Advanced Motor Control Ltd

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