The prior art stepping motor includes a rotor formed of ferromagnetic material having a plurality of alternately circumferentially arranged oppositely magnetized poles and a stator having a plurality of identical poles extending inwardly to the pivot point of the rotor. The stator is polarized in opposite polarities in response to driving electrical pulses to generate moment that causes the rotor to rotate in a predetermined direction through a predetermined angle in step with the applied drive pulse. It is therefore necessary that its direction of rotation be invariable irrespective of its operating conditions, whether mechanical or electrical. However, the stator of the prior art stepping motor is constructed in a symmetrical configuration so that during the interval between successive driving pulses, the direction of rotation would become indeterminate when the rotor has been displaced from the neutral position by an external shock. If the direction of the displacement is opposite to the intended direction of rotation, the subsequent drive pulse will cause the rotor to rotate in the opposite direction.
Although such reverse rotation of the rotor may be prevented by the provision of a ratchet-and-pawl arrangement, such an approach will not only increase the complexity of the motor mechanism, but also will impose some degree of restriction to the rotational moment, thereby raising the minimum operating threshold voltage of the motor. When the stepping motor is to be used in a timepiece for automotive vehicles, the invariability of the direction of rotation and the minimum operating voltage of the drive pulse are of primary concern.