Motor bearing fluting

Electrical motor bearing fluting has been defined as damage due to electrical current discharge through a motor bearing. Bearing race metal is electroplated onto the bearing balls. Close inspection reveals microscopic pits on bearing ball and race surfaces. A DC motor frame was completely isolated from ground except by a single grounding conductor with applied armature and field voltage. No motor shaft load was connected to the test motor. Motor bearing fluting ground currents were measured and stored with a high speed oscilloscope (50-mHz bandwidth). Waveforms were recorded photographically. With the motor running parallel with the paper machine front drum winder motor (known to have a history of fluting problems), field voltage transients on the field modules were also applied to the isolated test motor. A Dranetz meter was applied to the DC field supply module to verify the transient voltage magnitude and duration. An oscilloscope was connected across the ground shunt. Motor bearing currents were recorded with waveforms close to those expected under actual motor conditions. These meters recorded the wave shape, level, and ground current pulse duration that actually flowed through the motor bearings while rotating at operating speed.<>