Stress grading performance of form wound motor coil under initial heat cycle load

A stator coil end of a rotating machine has a stress grading (SG) system to reduce the electric field. The SG system is made of two tapes: stress grading tape (SGT), which has a field-dependent resistance, and low-resistance corona armor tape (CAT). However, the effects of components other than CAT and SGT on the SG system are not known. In addition, since a rotating machine is operated at a temperature above 100°C, the effect of temperature on the SG system should be understood. In this paper, we investigate two effects on the SG system: initial heat cycle load and attached position of adhesive insulating tape on the CAT end. The potential distribution on the coil bar surface is measured using a Pockels sensor. It turns out that the SG system performance degrades in two heating cycles and reaches a steady-state after three cycles. It is also found that the adhesive tape has a negative effect on the SG system performance when it is attached directly to the main insulation of the coil conductor.