Electrical discharge to earth of motor bearings

[peter53]

I'm currently undertaking ultrasound inspections of some 6600 volt soft start motors and appear to be getting indications of fluting whereby i might be getting a discharge to earth through the motor bearings.
Iknow VFD controlled motors can cause an induced voltage on the motor shaft but not aware if soft start motors would do the same. I'd be grateful for some advice on the subject please.

 

[ScusaMe]

Peter53

A couple of points I neglected in my previous post:

1. Voltage imbalance phase-to-phase can also be a cause manifested by vibration and bearing problems.

2. Harmonic Voltage Distortion must also be looked for and if present, will also cause vibration and consequent bearing problems.

/s