A conventional controller of an A.C. generator for vehicles controlled a duty ratio of a switching element for intermittently controlling a field current of a generator to thereby limit the field current so as for the field current not to exceed a predetermined limitation value. Note that, the predetermined limitation value is selected so as to become a value equal to or larger than a value of a field current which is to be caused to flow in order to obtain a predetermined output from the generator when a temperature of a generator has risen up to a predetermined value (refer to JP 6-38720 B (page 2 and FIG. 1)).
In accordance with such a conventional controller for an A.C. generator for vehicles, there are shown generator output characteristics in which a power transistor is usually held in a conduction state as long as a generated voltage is lower than a predetermined value after a maximum vehicle electrical load has been put on a generator, and hence an output is also increased in accordance with an increase in rotational speed of the generator. That is to say, there was encountered a problem such that temperatures of an armature coil and a rectifier rose along with an increase in output of the generator to become difficult to be suppressed within an allowable temperature, which resulted in reduction of quality. In addition, there was encountered a problem that a large cooling fan was required as means for enhancing a cooling property in order to suppress rise of a temperature, which resulted in that a generator could not be miniaturized.