1500-hp Motor Failures

We thought that this was the case, upon teardown inspection however, there was no signs of electrolysis. The bearings are insulated and we tested the running systems for shaft currents.

So what is the failure mode of the motors then? You say "bearing/seal problems" but it is not related to shaft currents (you think). We could guess back and forth forever. Can you describe the "problems" in more detail?

The bearing overheats and destroys the motor. Oil analysis shows no to minimal contamination, vibration analysis shows nothing. I agree that it points to shaft current, but we haven't been able to say that this is the case at all times. I was asking the question to find out if there is a manufacturing problem associated with this size motor that we are not aware of.

Its ok to insulate the bearing, but is the motor fitted with a shaft earthing brush which will eliminate all shaft currents. Larger machines driven by VSD normally generates circulating currents which flows from the shaft to the earth (motor frame via the bearings)

Normally the brush is fitted at the end of the bearing housing (where the shaft protrudes from the bearing) with the brush spring loaded on to the shaft, the tail end of the brush is fixed to the motor frame (earth). Its advisable to fit the brush on the insulated bearing. I assume only 1 of the bearings is insulated?

Hi,

please post a photo of the failed bearing - cleaned from any oil residuals.

This would make diagnosis much easier.

RHABE

One of the comments below is very pertinent - that the shaft and the base must be earthed well to ensure there is no intermittent arcing across the balls in the bearings.

Is it possible that the grease used in these new bearings is more insulating than the last ones you have been using - that could create the difference. Even some moisture in the grease could make it slightly conductive and help disspiate the charge.

I have come across this before in automobiles and some instrumentation bearings.

Is there welding done around the equipment in question? Have the welders been trained in proper grounding procedures for welding to ensure they are not arcing across the bearings?

Has the application changed at all from new to older motors? Is side load higher? Are the bearings between old and new motors same design? I am not overly familiar with these motors but would check that bearing clearance and preload are the same as old units. Does the system have autolubrication system installed.overlube or underlube are both siginifcant sources of this type of failure. Just some thoughts to check out.

Aside from checking your cables and their length to insure that they are proper is any consideration being given as to the load being driven, the type of coupling between the motor and load, what axial movement could occur at different speeds and any excessive thrust considerations that might develop at certain speeds?

Hi,

once more: please post some photos!

If necessary to avoid future damage I have a system that is measuring the ehd-film thickness.

This will pretty soon see the problem coming up so that you may stop your motor at the very first signs of failure that are not visible by conventional methods.

This is especially helpful if no electrical failures exist.

There are a lot more problems that may exist.

RHABE

I have a couple of additional questions for you.

Can you describe the lubrication system? In your post you mention oil analysis, are the bearings babbit bearings or anti-friction bearings? When you say catastrophic failure of the bearing/seal it leaves many questions. What kind of seal? Laby, carbide faced? Or do you mean sealed ball bearings (doesn't make sense with an oil system). Better yet, how about the model # for the motor and I can look for myself.

Anyhow, are there any obvious physical differences between the 2 (old and new motor)? Are the old frames cast iron versus aluminum for the new ones (different thermal growth characteristics)? What is the speed and what are your vibration numbers?