Phase lack detection device for double power supply automatic transfer equipment

The utility model discloses a default-phase detection device for a power supply. The device comprises a voltage-reduction rectifying filter circuit connected with a three-phase power supply, an impulse-signal conversion circuit and a detection circuit, wherein, the circuits are connected sequentially; and signals output by the rectifying circuit, after passing through the filter circuit, are processed by the impulse-signal conversion circuit, and then signals to be detected are obtained and sent to the detection circuit to detect whether the power supply suffers default-phase, and the signals are connected to a control power supply through the detection circuit to switch the operation of equipment automatically. The utility model can be used for not only a three-phase four-wire power supply but also a three-phase three-wire power supply, and has the advantages of simple circuit and detection process, stable performance, and the like. The utility model can be widely applied to power supply systems in cities, c