Fault detection in the manufacturing process of form-wound coils by means of dissipation factor and hipot tests

Abstract

The resin rich process is quite commonly used in rewinding tasks of medium voltage rotating electric machines. This process involves the application of mica paper tapes, impregnated with epoxy resin, to the coils to be manufactured. These pre-impregnated tapes are then consolidated by confining coils in molds while applying heat and pressure. During the manufacturing processes defects may appear in the wrapping of the insulating tape. These defects can be caused by bad quality of the tape, irregularities and incorrect control of pressure or temperature during the pressing and curing process and errors during the manual wrapping process. As it will be analysed in this paper, these defects might not be detected by the usual electric tests applied in the quality control processes. As a consequence, the coil's useful life, and its resistance to the different stresses applied to the machine during operation, would be reduced. Then, this paper presents a preliminary study carried out on form-wound coils with mica-epoxy insulation. Measurements obtained from electric tests, as dissipation factor or hipot tests, reveals that they are not able to detect some defects in the coil insulation.