Conventionally, in a motor such as a small synchronous motor, a stepping motor or the like, a stator is used in which a plurality of pole teeth is arranged in a circumferential direction. In a case of a stepping motor, the pole teeth are commonly formed in a trapezoid shape (see Japanese Patent Laid-Open No. 2000-350431). This is because, when the pole teeth are formed in a rectangular shape, the detent torque becomes stronger, the cogging is increased and a smooth rotation is not attained. Therefore, the pole teeth are formed in a trapezoid shape.
In other words, as shown in FIGS. 12 and 13, the stator yoke 101 in a conventional stepping motor is provided with a plurality of pole teeth 102, which is formed in a trapezoid shape and which is juxtaposedly arranged in a circumferential direction. In this example, the pole teeth 102 are juxtaposed in the circumferential direction so as to surround the center line 104 which passes the center of a circular space 103 surrounded by the plurality of the pole teeth 102. A rotor is disposed in the circular space 103 and an exciting coil is wound around an outer periphery of the pole teeth 102. Further, a circumferential direction face 105 which is a side face of each of the pole teeth 102 formed in a trapezoid shape is, as shown in FIG. 13, formed in a face coincided with a normal line 106 with respect to the center line 104.
FIG. 14 shows a state which one of the pole teeth 102 is viewed from the rotor side. As shown in FIG. 14, in a case when the circumferential direction faces 105 of the pole tooth 102 are formed as faces coincided with the normal line 106 with respect to the center line 104, an external shape of the rotor side face of the pole tooth 102 is a complete trapezoid and an external shape of its exciting coil side face is also a complete trapezoid. The face on the exciting coil side of the pole tooth 102 is located to the outer peripheral side of the rotor side face and thus, when the pole tooth 102 is viewed from the rotor side, both the circumferential direction faces 105 of the pole tooth 102 can be seen on the outer sides in the circumferential direction of the rotor side face. In FIG. 14, a circumferential direction width “W62” which is a width dimension in the circumferential direction of the circumferential direction face 105 of a tip end part 108 side of a pole tooth 102 is formed to be smaller than a circumferential direction width “W61” of its root part 107 side, and the circumferential direction width “W61” becomes narrower proportionally as it goes to the tip end side and the width of the tip end part 108 is the circumferential direction width “W62”.
However, in the case of the pole teeth 102 having a trapezoid shape, when the pole teeth 102 are set to make thinner for reducing the size in the radial direction of the motor, magnetic saturation is easily occurred and a torque (rotational force) of the motor is not increased. Moreover, the experiments of the present inventor have cleared that, when the pole teeth 102 are formed in a trapezoid shape and made thinner as they are, magnetic saturation is occurred and the detent torque which is a stationary torque of the motor is liable to be larger.
In order to avoid the magnetic saturation, when the whole size of the pole teeth 102 in a trapezoid shape is increased, both of the root part 107 and the tip end part 108 of the pole tooth 102 are widened in the circumferential direction. As a result, the width of the root part 107 becomes wider than the width of one magnetic pole of the rotor and thus the root part 107 is extended over two magnetic poles. In this state, a torque down of the motor is occurred.
Further, in order to increase a torque of the motor (rotational force of the motor), when the pole teeth are formed in a rectangular shape instead of a trapezoid shape, the detent torque becomes larger to occur cogging as described above and smooth rotation is not attained.