Patent ID: 8704497

Claim:
A control device for a vehicle AC generator, in which a DC voltage obtained by rectifying an AC output voltage of an armature coil in said vehicle AC generator that comprises said armature coil and a field coil with a rectifier is applied to said field coil to obtain a field current, and the field current is controlled by ON/OFF control of a switching element connected in series to said field coil, the control device for the vehicle AC generator comprising: a minimum value selector which compares on-duty of said switching element indicated by a power generation command inputted from a component located on the outside of a body of said control device to on-duty of said switching element calculated inside the body of said control device, selects a smaller on-duty of both on-duties to perform the ON/OFF control of said switching element, and transmits the selected on-duty to the component located on the outside, wherein the on-duty of said switching element calculated inside the body of said control device is an upper limit on-duty capable of continuing energization of said switching element, wherein said minimum value selector compares the on-duty of said switching element indicated by the power generation command to the upper limit on-duty of said switching element calculated inside the body of said control device, and selects the smaller on-duty of both on-duties to perform the ON/OFF control of said switching element, and wherein the upper limit on-duty of said switching element is controlled to be zero when a rotational speed of said AC generator is detected and the DC voltage divided by the rotational speed is not lower than a set value.