PURPOSE: To reduce easily higher harmonic of each phase by loading an appropriate conductor on wiring of each phase of the inverter output separately so that each wiring can penetrate the inside of a proper insulator in its axial direction through it.

CONSTITUTION: AC power 6 is rectified by the rectifier element 1b constituting a transistor inverter 1 and is smoothed by a condenser 1c, and an induction motor 2 is driven by variable frequency AC generated by switching a switching transistor 1a. (One phase is only shown in the fig.) An appropriate cylindrical conductor 5 is loaded on the wiring 4 of each phase separately so that the wiring 4 of each phase can penetrate the center of a proper insulator in its axial direction through it. The conductor 5 has higher resistance against the higher harmonic contained in the output current by skin effect. Accordingly, the oscillation from the higher harmonic component is prevented by a simple device.