PURPOSE: To make uniform the flow in a cooling duct and to prevent local temperature rise in the winding by a method wherein a cooling medium inlet nozzle for letting the cooling medium jet in the longitudinal direction of a header is installed in the header of the cooling duct.

CONSTITUTION: A T-shaped cooling medium inlet nozzle 19 is installed in a header 15 of a cooling duct 18. The nozzle 19 thus constructed allows a cooling medium 7 supplied to the header 15 to jet out in the longitudinal direction of the header once and then to uniformly flow in a parallel flow passage 17 of the cooling duct. The uniform flow of the medium is secured as far as the outlet by means of the reinforcing rib, so that the cooling capacity of the cooling duct can be made uniform. Accordingly, the local temperature rise in the winding is thus prevented.