PURPOSE: To prevent spark discharge of a stator winding caused by vibration by bringing outer corona preventing layers surrounding the main insulating layer to the earth of stator coils in a slot into direct contact with one wall of the slot in a conductive state and indirect contact with the other wall of the slot through semiconductor side liners and filling the void of the slot with a semiconductive rubber filler.

CONSTITUTION: Two stator coils 3B and 3A are inserted into a slot 2 by using a wedge 8 and liners 7C-7A. Outer corona preventing layers 6 are put around the coils 3A and 3B with main insulating layers 5 to the earth in between. The layers 6 are brought into direct contact with one wall of the slot 2 in a conductive state and the other wall of the slot 2 through semiconductive side liners 9B and 9A. Voice formed among cores, the layers 6, and the liners 9B and 9A in the slot 2 are eliminated by filling the voids with a conduction rubber filler. Therefore, the occurrence of spark discharge caused by vibration can be prevented and the coils can be used for a long period.