Quote Originally Posted by ot1 View Post
My thoughts on this with another possibility, the bearings which are electrically coupled to the motor shaft which is collared to the fan and blades pick up an electrostatic discharge from the friction of air, finding its way to ground through the bearings and pitting them.
I've never heard of static charge being induced on a blade set in clean air. I've seen environments such as flour silos where the contaminants in the air can cause static charge but usually in these special locations the motor would be Ex rated and bonding would be applied to any and all components.

I have very little knowledge of cooling towers but I would imagine if the air is very humid or saturated with spray then potential differences or static charges would be even less likely as moisture would allow any differing potentials to equalise by leakage.