Voltage stresses in three-phase, A-C motors

Winding failures on large AC motors (100 HP and up) have been reported in increasing numbers recently by repair shops throughout the country. Although these reports involve equipment operated in varying circumstances, the evidence suggests that voltage stress on the windings has been a major cause of such failures. The author examines the kinds of voltage stress that occur in large AC motors and outlines steps that can be taken to prevent the windings from failing prematurely. It is emphasized that it is necessary to be very careful in large random-wound motors to keep the voltage per coil as low as possible in the type of connection used, and to place the turns in the slots so that they are as evenly distributed as possible. The connection and coil design should be such as not to exceed 30 V per turn