In this paper, a new mehtod of reducing harmonics that involved in input AC line currents or in output DC voltage of the converter system is presented. which is due mainly to the quipment of 2-3 switching taps on interphase reactor. In case that the 3 tap method is applicable to conventional 12 pulse converter, it could not bring to an effect of reducing harmonics under firing angle 15ulcorner To solve these problems, 24 pulse or 36 pulse converter is controlled by microprocessor. The former is performed in 2 tap method when ulcorneris less than 15ulcorner the latter in 3 tap method when it is more then 15ulcorner Therefore the originality of this paper liea in microprocessor based equipment of the 2-3 switching taps on interphase reactor. I applied this method to the 12 pulse converter, and proved validity of that theoretically and experimentally.