It is well known that the three phases auto-compensated power voltage stabilizer, as well as the equipment attached to the stabilizer, may be damaged when one of three phase's power supplies turns to fail with only two phase phase working. There are a lot of lack of phase protecting devices available but all of these fail to work on the three-phase power stabilizer. This is because when one phase of the power supply fails, the three-phase voltage transformer, a key device in the stabilizer, produces a new phase power in the same amplitude and phase as the lacking power has, which masks the fact that there are only two phases supplying the power that should be the duty of three phases. As a result, unbalanced current that may damage the transformer increases. In this paper we introduce a new method to detect the above lack of phase fault. A prototype of the new lack of phase detecting device is also introduced.
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