PURPOSE: To get always favorable rectification and improve operation efficiency without letting a futile current flow to an auxiliary pole coil.

CONSTITUTION: The current to be let flow to an auxiliary pole coil 2 is controlled by an FET 12 so that the geometrically neutral axis between field cores may be zero, based on the calculation data being brought out by calculating the magnetic flux of the gap between the magnetic field cores from the current flowing in an armature coil 1 and the revolution of the armature coil 1. By doing it this way, a geometrically neutral axis and an electrically neural axis cease to slid from each other, and favorable rectification can be gotten. Moreover, even in case that a current above a magnetic saturation current flows to the armature coil 1, it is restrained to let a futile current flow to the auxiliary pole coil 2.