A conventional voltage control device for a vehicle AC generator detects an output voltage of a generator and stops supplying a field current to weaken a field flux when the detected output voltage is higher than a reference value. On the other hand, when the detected output voltage is lower than the reference value, the voltage control device reinforces the field flux. Thus, the output voltage is controlled to be substantially constant.
However, even when the supply of the field current is stopped, an inductance component exists in a field coil and an excessively high voltage is generated by this inductance component when shutting down the field current. Therefore, the voltage control device can be damaged.
Particularly in a recent vehicle AC generator with small size and high output, since the resistance of the field coil tends to be small and the inductance tends to be large, the overvoltage continuation time becomes longer accordingly.
Therefore, it has been proposed to switch to a circuit that consumes the energy of the field current by using resistance and diode in order to quickly reduce the field current, for example, as disclosed in Patent Reference 1.
Patent Reference 1: JP-A-2003-174799