Can DC Motor Bearings have Fluting from Shaft Current?

[roryrobb]

A customer reports finding first high freq vibration in the 130 to 180kcpm range, with one of the high peaks at 2BSF.

He suspected fluting and then discovered it upon disassembly. I am somewhat familiar with shaft currents in AC Induction motors, but haven't encountered it in DC.

This is a 300 hp DC motor with a 300 volt field and 500 volt armature, stab-shunt design.

Would you check for shaft voltage inside the bearings in the same way prescribed for AC, or could the cause be something entirely different?

 

[Petri]

Gday
Yes EDM ( Electrical Discharge Machining ) is quite common on DC Motors as they are on AC variable frequency drive motors. Just today I inspected a bearing from a 85KW GE motor that was showing outer race vibration peaks. What was found on the bearing was a large fluted section on the bearing outer race with the flutes being about 2mm apart.
What I generally find is that the Non Drive End bearing or the bearing closest to the commutator shows the EDM. The grease is sometimes blackened and burnt.
It's a nuisance and I believe it's due to the converter high switch rate as per an AC inverter drive.
Solutions are pretty varied but our engineers have been fitting simple earth brushes onto the shafts near the bearings that are failing. NOT A ROOT CAUSE SOLUTION.
Not sure of the long term effectivness of this method. Smoothing the converter output would be a better approach but probably more costly.
Using a brush probe ( CSI type ) like you would on an AC machine to measure shaft voltage is definitely worth trying out. let me know if it works and I'll get one too.
Good luck

 

[GregPerry]

I have little experience with eletric motors but could the fluting not be as a result of vibration damage. If the motor has been transported over a long distance with no bearing support or has stood still on site with vibrating machinery close by the bearings can be damaged.

 

[Petri]

Yes, this is commonly called "false brinelling" as opposed to "true brinelling" which is created by bearing shock or impact during operation. I have seen many cases of false brinelling even on machines that have been installed with good bearings ( bearing changed in site ). The machines are in environments where there is a lot of surrounding vibration and no damping or isolation has been provided to bases and pipework etc.
Motors must have a locking collar on the shaft to prevent shaft vibration if they are to be transported a long distance.
With EDM you may notice that the rolling elements will have a frosted appearance. Fluting marks will have a pitted or frosted appearance at the edges and they will be very evenly spaced ( approx 2mm ). False brinelling usually leaves a more random marking on raceways and there is usually a favoured area on the bearing where the shaft has been sitting for prolonged periods. This will leave some pronounced markings at distances equal to ball separation.
If you can measure the voltages on the shaft to frame during running and find that it is greater than about 10 volts you may have a problem. This voltage will produce a capacitive discharge across the bearing element interfaces. Typically a bearing grease lubricant film would have a dielectric strength of just a few volts.
I have heard it suggested that using a more conductive grease will reduce the capacitive effect and reduce the discharge, but I am not sure if a product such as this would be easily accepted by the maintenance community.

 

[pdmguy]

I've experienced fluting in DC motors throughout our plant, but it seems only on tandem motors. Oddly enough, I've seen it most prevalant in the center motors in 3 motor tandem stands. The motors are isolated from each other in terms of drive electrics, etc. We installed shunts from shaft to earth, but it seems that some of the electrically induced energy rose in the motors. The physical appearance of the damaged bearings have a tight marking the width of the race on the outer race, and the rolling elements and inner race have a dark, tarnished appearance (microscopic pitting). We've given the motors/drives a clean bill of health, but still see the shaft current damage to bearings.