PURPOSE: To improve heat radiation effect without leading in cooling medium from the outside and to enlarge usable range in view point of thermal capacity by fixing a heat pipe having identical profile to that of a pole groove section to at least one pole groove section of inside or outside pole structure formed on coupling main body.

CONSTITUTION: Since a heat pipe 15 having cross section approximately identical to that of a recess 3b, i.e., a pole groove section, is sticked to the recess 3b in an outside pole structure 1b of a yoke 1, Joule's heat produced through excitation of an exciting coil 2 or frictional heat produced through slippage is transmitted to a sealed pipe 12 adhered to the recess 3b thus heating and evaporating the enclosed working liquid 13. The vapor advances through the cavity of the sealed pipe 12 toward a non-heated portion, i.e., the outer end portion, where it is subjected to heat exchange with the outer air and condensed thus radiating condensation heat. The condensed working liquid 13 is transferred to the side of the outside pole structure 1b where it is heated again and evaporated.