Stray Current in Rotor Pinion? 3

[jpandf]

I have a problem where rotor pinions that are initially press fit to the rotor shaft (and keyed) are becoming loose over time. I suspect stray current is the cause but I have no practical experience in evaluating this. The attached picture shows the pinion.

Here are the details and some of my questions:

The AC induction motor is on the inside of a motorized pulley (motor and gear reducer inside pulley shell). The motor is a 50Hz motor, made in Europe, and running in the U.S. on 460V/3PH/60Hz power. The motor is 4 pole, class H, 29.7 FLA. The stator housing is aluminum.

Does the errosion shown on the pinion shaft face and end in the picture look typical of that caused by stray current corrosion?

I don't seem to have this problem on motors with ferrous stator housings. Could the aluminum stator housing be a part of the problem?

Is the solution installing a current diverter ring on the rotor?

Is there a test I can run on the motor to determine the likeliness of stray current corrosion occuring?

Thanks for the help!

 

[Ray39836]

Hello, I have never seen shaft currents in a NEMA size motor but, If there are shaft currents present a complete circuit is needed. Does the bearing on the opposite end have electrical etching? Since this is a press fit it makes more sense that the electrical arcing would take place in the gear teeth. Is your interference fit correct? Also the metallurgy needs to be correct. Sorry for asking more questions but doesn't look like electrical ecthing to me.

 

[jpandf]

Thanks for the questions. I have not looked at the opposite end bearing for etching and I will do so. I have attached a schematic of the motor drive for reference.

While a difficult path (through the gear train) one exists to ground. I assume this circuit is highly improbably though.

We see a brown thick fluid coating the motor when we also find this type of problem. The motor is cooled by a synthetic lubricant passing over the stator and drawing the heat away. The brown fluid is different from the dark oil we see when the lubricant retains some moisture. Fluid is also diffent from the gray paste that we have seen in other gear boxes that have failed mechanically. I'm not sure if this information adds to the discussion.

I'll try and get some information on the materials of construction of the shaft and pinion.

Thanks again for the help.

 

[waross]

A magnetic path is the path that the magnetic flux follows through the gear train. Ground may affect the currents causing the magnetic flux but doesn't affect the flux directly. An opening or air gap in the magnetic path will dramatically reduce the magnetic flux.
I suspect some interaction between the coolant and the aluminum. Possibly an alcohol content. Do you use methyl alcohol to trap moisture?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

I find it hard to believe you'd see a pressed together joint having electrical erosion issues without seeing pitting in the gears or bearings. Current will flow through the areas where there is direct contact and these areas wouldn't be subjected to any pitting since there is a direct low resistant connection. Pitting requires a gap where a small arc can occur.

Bearings will typically show a washboard type pattern. I'm not sure about bearings but I'd suspect you would find abnormal pitting on the contact surfaces.