PURPOSE: To prevent an armature core from overheating by providing the split face of a thin sector-shaped iron plate on the top of the supporting slot of the core.

CONSTITUTION: An armature is composed of an armature core 1, armature coils 2 protruding from both ends of the core 1, and spiders 3 for supporting the core 1. The core 1 is formed by laminating thin sector-shaped iron plates 4. The plate has a plurality of slots 5 to which the coils are inserted, and a plurality of supporting slots 7 coincident with the protrusions 6 of the spiders on its inner periphery. The slot 7 uses a dovetail shape. The sector-shaped split part of the plate for the core is provided on the top of the dovetail. Thus, its magnetic resistance is reduced, thereby preventing the core from overheating.