PURPOSE: To improve the strength of an apparatus by forming only the connection and its vicinity of an armature coil having a rectangular section with a commutator in to a round section and by connecting the connection with the riser groove of the commutator.

CONSTITUTION: An armature coil 3 having a plurality of different phases is wound round a grooved core 2 fixed round a shaft 1. Also, one side of the core 3 is fitted with a commutator 4 and the end of the armature coil 3 is inserted into and connected with a plurality of slots 6 formed in the riser part 5 of the commutator. In this case, the section of the part of the armature coil 3 inserted into the core 2 is rectangular, while the part thereof projecting from the core is round. Thus, when the round coil is connected with the riser part, it is possible to reduce the required outside diameter of the riser part of the commutator and as a result to decrease a rotational centrifugal force.