120 u00b0 u03b8w u03b8v between those outputs v and w and 120 u00b0 u03b8v between those on outputs w and u the embodiment under discussion owing to the special devices empliyed in the construction and connection of the transformers t1 to t3 brings about an effect of decreasing the deviation in phase difference between the output voltage phases during the operation of an unbalanced load to about one half of the deviation involved in the conventional prior art without requiring any reduction in the reactance of the series reactors evidently the circuit of fig1 can be realized by using diport transformers which are provided with magnetic shunts one example of this configuration is illustrated in fig3 in this diagram the same symbols as used in fig1 denote identical or equivalent parts ts1 to ts3 stand for diport transformers provided respectively with magnetic shunts these diport transformers contribute to simplifying the configuration by obviating the necessity for using series reactors as external circuit elements since they have entirely the same operation as those of fig1 the explanation thereof will be omitted the circuit having the configuration of fig1 can be applied to a two way uninterruptible ac power supply using an inverter output as well as the conventional commercial ac power supply as inputs one example of the application is illustrated in fig4 in the diagram the same symbols.