PURPOSE: To obtain control characteristics of a smooth load current always stably by constituting a chopper from a plurality of unit choppers connected in parallel with each other and by applying power alternately to said unit choppers.

CONSTITUTION: A chopper change-over part 22 causes power to be applied alternately to respective unit choppers 10a and 10b and, with the lapse of further period III, IV..., the conduction ratio γA* of an armature chopper gradually increases and reaches a point where γA*=1. However, because power is alternately applied to both unit choppers 10a and 10b, one unit chopper is surely out of operation, while power is applied to the other unit chopper. Thus, the armature chopper conduction ratio γA* can be increased continuously to 1, as a sufficient OFF-time is secured. Therefore, the control for the full conduction change-over for discontinuously increasing the armature chopper conduction ratio γA* from the maximum conduction ratio γM to 1 is made unnecessary so that passengers will feel comfortable.