Can DC Motor Bearings have Fluting from Shaft Current?
Can DC Motor Bearings have Fluting from Shaft Current?
(OP)
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?
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?





RE: Can DC Motor Bearings have Fluting from Shaft Current?
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
RE: Can DC Motor Bearings have Fluting from Shaft Current?
RE: Can DC Motor Bearings have Fluting from Shaft Current?
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.
RE: Can DC Motor Bearings have Fluting from Shaft Current?