Zero sequence voltage phase failure protection circuit

As shown in the figure, when the motor is running normally, after the three-phase power supply is connected through three capacitors Y with equal capacitance, the potential of its node E is 0V, and the secondary side of the transformer T2 has no voltage output. Therefore, VT is cut off slightly, K is in the released state, its normally closed contact is in the normally closed state, and the KM coil works normally. When any one phase of the three-phase power supply is powered off, due to the imbalance of the three phases, there will be a voltage value of tens of volts at point E, which is transformed by T2, VD1 rectifier, C4, C5 filter, VD2 voltage regulator, C6 and resistor R1 delay Finally, it is added to the VT base, causing VT to conduct, K acts, the normally closed contact K opens, the KM circuit is cut off, and the motor M loses power and stops.