PURPOSE: To prevent a field winding from damage to the insulation to earth by arranging a spacer material interposed to the clearance between the field winding and a coil presser foot on the side of the field winding and a porous material on the side of the coil presser foot.

CONSTITUTION: A winding conductor 1a is wound over in required number of times, on the outside of which an insulation to the earth 1b is applied and the whole is impregnated in the vacuum with epoxy varnish to fabricate a field winding 1. A field core 2 is equipped with the field winding 1. Then, in installing a coil presser foot 3, a spacer material 5 on the side of the field winding 1 and on the side of the coil presser foot 3 a porous material impregnated beforehand with liquid polyester varnish are interposed, so that a coil presser foot bolt 4 is applied to fix. Thus the thermal elongation of the field winding is not affected to the porous material.