A field current limiting section includes a field current limitation instructing section for, when a field current limitation determining section determines that a determination value has reached a predetermined determination threshold value, generating a field current limiting instruction to a field current control section so as to limit field current to be equal to or smaller than a predetermined permissible value during a predetermined field current limitation time Tlim. A field current limitation releasing section outputs a field current limitation releasing instruction to the field current limiting section so as to release limitation of the field current during a predetermined field current limitation release time TC. When having received the field current limitation releasing instruction from the field current limitation releasing section, the field current limiting section releases limitation of the field current during the predetermined field current limitation release time.