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In an apparatus for detecting a rotation by utilizing a residual magnetism remaining in a core, when a field current attenuates gradually through a current return diode during a deceleration period for the stop of rotation, there occurs an AC demagnetizing action which causes the field current to at

In an apparatus for detecting a rotation by utilizing a residual magnetism remaining in a core, when a field current attenuates gradually through a current return diode during a deceleration period for the stop of rotation, there occurs an AC demagnetizing action which causes the field current to attenuate, so that also during the deceleration period, the supply of electric power to a control circuit and field current to a field coil are continued.

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1. A voltage control unit for a vehicular AC generator, comprising:a rotor having a field core including a plurality of field poles formed thereon; a field coil to which a field current for magnetizing said field poles is supplied; a diode for current return connected in parallel with said field coi

1. A voltage control unit for a vehicular AC generator, comprising:a rotor having a field core including a plurality of field poles formed thereon; a field coil to which a field current for magnetizing said field poles is supplied; a diode for current return connected in parallel with said field coil; an armature having an armature core including a polyphase coil applied thereon; a rectifier circuit for rectifying a voltage generated from said polyphase coil and outputting the voltage as a DC output voltage, the DC output voltage being controlled by adjusting the field current; switch means for switching between ON and OFF of the field current; a control circuit for controlling said switch means in accordance with a signal associated with the DC output voltage; a power supply circuit for supplying electric power to said control circuit; and power supply hold means for holding the supply of electric power to said control circuit by said power supply circuit for a period from just before stop of rotation of said rotor until lapse of a predetermined time after the stop of rotation of said rotor.