US Patent 7705503 Rotating electrical machine

Included are a ring-shaped stator and a ring-shaped rotor arranged inside the stator; the stator includes a stator core with armature windings; the rotor includes a rotor core in which a plurality of permanent magnets are inserted and cooling holes are formed, a coolant flowing in each of the cooling holes; and each of the cooling holes is formed so as to have a sectional view which is a convex toward the outer periphery thereof.