PURPOSE: To prevent the generation of an exciting rush current, by a method wherein an electrically-operated rotary machine having a large flywheel effect and adapted to excite an iron core by an induced voltage during a period from breaking of a power source to making of the same again is connected as a load to the tertiary winding of a transformer.

CONSTITUTION: A primary winding 10 of a transformer TR mounted on a car is connected to a power source 12 through a pantagraph 11, while a secondary winding 13 is connected with a car motor 15 as a load through a rectifier 14. Moreover, the transformer TR is provided with a tertiary winding 16, as an auxiliary power source, which is connected with as a load an electrically-operated rotary machine 17 having a relatively large flywheel effect. Thus, if the power source is broken, the electrically-operated rotary machine 17 continues rotation for a while and induces a voltage as a generator to excite an iron core 18 from the tertiary winding 16. Accordingly, it is possible to prevent the generation of an exciting rush current in making the power source again.