The rotor is the type having, axially aligned on the rotor shaft, two pole pieces (2,3), external to the poles (11,11') projecting parallel to the axis of the shaft (8) towards the middle of the rotor, and a central pole (4) disposed between the two external polar pieces, which has a number of poles (14,15) projecting parallel to the axis of the rotor, towards the external polar pieces, each one embedded between the two adjacent poles of the external pole piece. The rotor R has magnetic poles (11,11',14,15) presenting, in the top radial view, a rectangular shape.
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