PURPOSE: To efficiently radiate the heat generated by a coil section to the outside by providing iron core keeping plates or plate springs incorporating springs to iron core guides installed to a casing in corresponding to each laminated iron core block.

CONSTITUTION: Laminated iron core blocks 1 are arranged at nearly regular intervals in the circumferential direction in a casing 9 and, in each block 1, numerous coils 2 are arranged in the axial direction and outer and inner ducts 4 and 5 forming an annular flow passage 3 are arranged in a double circumferential duct structure. In addition, iron core guides 12 are provided in the casing 9 at every block 1 and iron core keeping plates 13 are arranged at nearly regular intervals in the longitudinal direction in the grooves of the guides 12. Therefore, the heat generated by the coils 2 can be efficiently radiated to the outside and the temperature rise of the coils 2 can be suppressed, since the blocks 1 can be always press-contacted with the outer ducts 4 by means of the plates 13.