PURPOSE: To suppress abrupt variation of torque by squeezing or rising field current with a time constant during series/parallel transition.

CONSTITUTION: During series/parallel transition, P switch 9 and G switch 10 are thrown in at first based on P, G throw-in commands fed from a P, G throw-in command section 8 then S switch 5 is opened. Transistor 12 at field current control section 11 is turned ON based on the P, G throw-in commands 14 and charges are discharged with a time constant from a capacitor 13 thus squeezing the field current 16 by means of a chopper 2 through a chopper controller 4. An S switch 5 open signal 15 is fed to the field current control section 11 in order to turn the transistor 12 OFF thus charging the capacitor 13 with a time constant and increasing the field current gradually through the chopper controller 4 and the chopper 2.