Automotive alternator including annular core having protrusions and recesses alternately formed on its outer surface

An automotive alternator comprising:

• a stator including a stator core and a stator winding wound around the stator core;
• and
• a rotor surrounded by the stator, the rotor including a rotary shaft, a pair of first and second magnetic pole cores, a field winding, and an annular core,
• each of the first and second magnetic pole cores having a hollow cylindrical boss portion, a disc portion, and a plurality of magnetic pole claws, the boss portion being fit on the rotary shaft so as to rotate along with the rotary shaft, the disc portion extending radially outward from an axially outer part of the boss portion, each of the magnetic pole claws axially extending from a radially outer part of the disc portion, the magnetic pole claws of the first magnetic pole core being interleaved with the magnetic pole claws of the second magnetic pole core,
• the field winding being wound around both the boss portions of the first and second magnetic pole cores so that the field winding is surrounded by the boss portions, disc portions, and magnetic pole claws of the magnetic pole cores,
• the annular core being disposed to surround radially outer peripheries of all the magnetic pole claws of the first and second magnetic pole cores, the annular core having, on an outer surface thereof, a plurality of protrusions, a plurality of recesses, and a plurality of slopes, each of the protrusions being so formed and located as to be coincident with a corresponding one of the magnetic pole claws of the first and second magnetic pole cores in a circumferential direction of the rotary shaft, each of the recesses being alternately formed with the protrusions so as to be coincident with a corresponding one of air gaps formed between the magnetic pole claws of the first and second magnetic pole cores in the circumferential direction of the rotary shaft, each of the slopes being inclined to a radial direction of the rotary shaft and connecting a circumferentially adjacent pair of one of the protrusions and one of the recesses.