The reason of motor bearing fails

With larger motors it is normal to insulate the NDE bearing. The normal reason is that the majority of movement will come from the DE. This is done by machining out the end shield to take a sleeve into which your bearing will sit. Material selection is vital. Please be aware that this insulating requirement would include any lubrication, vibration, temperature monitoring equipment and the like. If your motor is very big the insulation material sits between the base and the pedestal but again you have to work on sleeves for your bolts and insulating washers on you connections. Before you drop you shaft in you can test but that is the last opportunity.
Motor bearing failure
The other method of grounding the DE shaft. Many people play around with carbon and copper brushes and mixtures of the two. I believe that you are wasting your money. The reason your bearing fails is that the lubrication acts as an electrical insulator between the rotor and the earth of the rest of the machine. As the voltage builds up in the rotor from reasons explained by others it reaches a point where it flashes over like a lightning bolt. This happens at high speed and destroys the traveling surface causing more friction. Metal breaks away and conducts and within a short time you have failure on the bearing. The normal brushes a designed to glaze over to make a good sliding surface. It has to be a little more complex than this. Look at conducting fibre rings. Look at Impro-seal shaft grounding rings.

You can also induce voltage in your shaft from either your shaft having a magnetic area or the casing having a metal area. This is more prevalent on higher rotating shafts but I have seen it at 1500 rpm. How does this happen? On the shaft, a bump or surge will do it. It is DC. On the static parts, the favorite is the magnetic base of a clock gauge used to set up the bearings. Also vibration monitoring equipment set on your machine with a magnetic base. You will need a Gause machine to detect magnetic areas.

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