| 摘要 |
PURPOSE:To reduce switching loss of main switching element without sacrifice of higher harmonic suppressing effect by automatically resulting PWM carrier frequency of a PWM converter based on comparison result between detected higher harmonic and voltage distortion and preset operating conditions. CONSTITUTION:A carrier frequency (fc) controlling circuit 9C is preset to vary the carrier frequency on some conditions, and when a specific order harmonic is a problem because of the character of load setting is made according to the character of the system or the load. In this case, the fc controlling circuit 9C responds quickly to the conditions for increasing fc while responds to the conditions for decreasing fc such that operation is started upon continuation of that condition for a predetermined time. When a main circuit element 10 is switched with as low fc as possible within such range as voltage distortion at system side and the amount of higher harmonics to be produced from the load satisfy preset conditions, an active filter can be functioned effectively with minimum switching loss. |
