Analysis of the fretting wear phenomenon on the surface coatings of form-wound coil

Electric rotating machines are prone to suffer damage of mechanical origin in some of their parts. It is the case of the fretting wear that occurs on the surface of form-wound coils of medium voltage electric motors. This fretting wear is a consequence of vibrations of very small amplitude that are produced, mainly, by magnetic forces; it can lead to the deterioration of the upper layer of the coil, constituted by the conductive armor coating that interacts with the slot wall of the stator. If the conductive coating becomes worn, electric partial discharges can appear inside the slot. In this work, the fretting wear phenomenon occurring between the conductive armor coating and the stator wall laminations was experimentally simulated by using a ball-on flat reciprocating tribometer. Additionally, the same configuration was used to determine the resistant to the fretting wear of the semiconductive stress grading coating, another coating applied at the coil end. The results show the friction coefficients achieved during the tests and the surface damage produced in these coatings. Since the semiconductive stress coating presents better performance against fretting wear, its shown how this coating can also be used in the section of the coil inside the slot in order to reduce the possibility of partial discharges due to the damage of the conductive armor coating.