Impact of inverter drives employing fast-switching devices on form-wound AC machine stator coil stress grading

The conventional stress-grading system of form-wound motor coils has been found to perform well under sinusoidal, power-frequency voltage. However, when the motor is fed by inverter-based drives employing fast switching, a significant impact on the electric stress-grading system occurs. Simulations of the stress-grading systems using finite element analysis indicate that, during the fast rise-time, a high electric stress can develop right at the slot exit of the stator if the conductivity of the conductive armor tape (CAT) is not high enough. Increasing the conductivity of the CAT alleviates the problem but increases the eddy current loss in the stator laminations. To address this problem, the conductivity of the area just beyond the slot exit needs to be increased by coating the surface of the CAT with a high-conductive material. The stress-grading at the end of this conductive paint section then is achieved by two coordinated layers, which help to distribute the electric field and heat that is produced