Earth brush - benefits

Is it a shaft earthing brush? If there is a possibility of significant shaft voltage then it is preferable to ground the shaft with a brush. Otherwise, the bearings will be under stress and this may lead to bearing failure. Many a times ,for larger installations (> 2 MVA)we also ask the user to monitor the current through the brush to detect incipient abnormalities.

yes it is shaft earthing brush

Do a search on the term "AC motor shaft current" and you will find papers and references to a phenomenon that was virtually unknown before the advent of inverter drives. A shaft grounding brush is one way to provide a safe path for these stray currents, as opposed to flowing across the bearings and bearing races where they cause damage.

There is another method of protecting the bearings, used mainly on older mill type motors with pedestal bearings. Both bearings are insulated from earth, but then one bearing has an earth strap to it to ground the shaft. In this way circulating eddy currents can't pass through the bearings. It has one major down side and this happened to me, a production worker propped a crow bar against the insulated bearing pedestal and left it there for several days. The bearings didn't sound very happy when I got to it!