Voltage stress: Not as simple as it sounds
Have you ever wondered how a lightning rod works? That small rod atop a large building extends less than a yard (meter) above the building. Yet the lightning is attracted to that sacrificial rod rather than to the building. If you have not wondered about lightning rods, Iʼll bet you are curious about what lightning rods could possibly have to do with electric motors. The common denominator between electrical windings and lightning rods is geometry. Lightning strikes the lightning rod because the voltage stresses are higher between the pointed rod and the cloud than between that same cloud and the building. The pointed shape raises the potential voltage stress at the end of the lightning rod. For the same reason, we often see winding failures affected by the geometry of a coil, a connection, or a sharp corner within a stator frame. When the subject of voltage stress comes up, many people assume that the voltage stress does not exceed the line voltage of the winding. Our early assumptions were that voltage stresses followed a linear pattern and could simply be calculated by the relationship of circuits, coils and turns per coil.