PURPOSE: To achieve smooth and stable operation of a vehicle by a method wherein, in case a generator driven by a prime mover is used as a driving force, an exciting current of the generator is controlled in the pattern according to the revolving speed of a prime mover and, in case where a ground power supply is used as a driving force, a power controller is controlled as in the same way.

CONSTITUTION: In case an output of generator 2 is used as a driving source, a trolley pole 32 is in the down position and also an excitation changeover switch 33 is placed on the A side. A controller 27 outputs a deviation signal of an actual output of generator 2 from a set output of the generator corresponding to the revolving speed of engine 1. Moreover, in case a ground power supply is used as a driving source, the trolley pole 32 is in the up position and also the excitation changeover switch 33 is put on the B side to control a power controller 30 by the output of the controller 27.