Comparison of the effect of varnish treatment processes on the moisture resistance of electrical insulation systems for random wound motors

Moisture and associated contaminates such as dirt and salt, can cause failure of electrical insulation by bridging cracks in embrittled insulation or cause failure by providing paths to ground. Previous Navy work reported by Flaherty and Walker 1 showed that failures can be moved into the end turns of windings by lap folding the slot cells to provide a long path to ground. In the 1961 report insulation systems were given a rating in accordance with their moisture resistance with the totally encapsulated winding providing the best protection. This study was followed by later work by Flaherty, Tobin and Walker^2-3 in 1968 which compared varnish procedures using conventional solvent type varnishes and 100% solids varnishes using the trickle process. This work showed that the trickle process provided moisture protection equivalent to three dips and bakes in the solvent type varnish.