Strains on Motor Endwindings

Stator winding failures are second in frequency of occurrence only to bearing problems in the categories of ac motor troubles and can lead to operational losses many times the cost of the complete motor. There are a number of causes of failure, but one of the more likely is inadequate bracing of the stator endwinding to withstand the high electromagnetic forces experienced during starting. The use of a finite element technique to predict stator endwinding forces and endwinding movement of large induction motors is described. The determination of the mechanical properties of the coils and bracing system components is discussed, as is the life testing of the insulation with mechanical vibration and electrical checks. The use of lasers to determine endwinding movement during the starting of several large 13.8-kV motors is also described.