This invention relates to a one for three air-conditioner, and its frequency converting control method including indoor and outdoor machines in which a frequency converting controller with three frequency branch outputs of 30 Hz-60 Hz-90Hz, 90Hz-120Hz connects the outdoor converting compression and the indoor controller. The control method is that when an indoor machine operates, the output of the controller is 30Hz-60Hz, when two indoor machines operate, the outputs is 60Hz-90Hz, when three operate, the output is 90Hz-120Hz, which saves electricity more than that of one for many air-condition and pressures of the compression system are almost the same, when the three sets operate at the same time. |