FIG. 5 is a circuit diagram illustrating a structure of the power supply circuit 23. The s coils 7 are annularly connected. In FIG. 5, three coils 7 are connected by a delta connection method (triangle connection method). In addition, the c (six in FIG. 5) commutator pieces 9 are short-circuit connected to an annular coil circuit every 360/p degrees (every 180 degrees in FIG. 5) with respect to the rotation angle of the rotor 6. Note that C₁ to C₆ in FIG. 5 represent six commutator pieces 9. For example, on the annular coil circuit illustrated in FIG. 5, a point P₁ between the first coil and the second coil is short-circuit connected to the commutator pieces C₁ and C₄. Positions of these commutator pieces C₁ and C₄ are shifted by 180 degrees with respect to the rotation axis C. Therefore, a potential at the point P₁ becomes identical every time the rotation axis C makes a half turn. Similarly, a point P₂ between the second coil and the third coil is short-circuit connected to the commutator pieces C₂ and C₅, and a point P₃ between the third coil and the first coil is short-circuit connected to the commutator pieces C₃ and C₆.