PURPOSE:To prevent increase of the outer diameter of core and to improve cooling effect by mounting a tubular magnetic body made of thin board onto an armature core, then folding the end section of the tubular body inwardly, and arranging a cooling blade while furthermore making a vent axially through the core section at the inner circumferential side of coil. CONSTITUTION:A thin magnetic board such as a tubular body 10 formed of an iron board through deep drawing is mounted on the outer circumferential face of a core 4. The end section of the tubular body 10 at the opposite side from a commutator is folded inwardly in order to contain the coil end section 18, and a blade 12 is formed on the end section 11. A plurality of cooling vents 13 are made axially through the core 4 in order to cool the interior of the core 4, the spaced surrounded by an insulator 6, a coil 5, the core 4 and a shaft 3, and the bag space 14.
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