Electronic relay phase failure protection circuit

As shown in the circuit in the figure, when the motor is running normally, in the three-phase Y-connected balanced circuit, the voltage value of the neutral point "E" is not high, so VD1 is not conducting, the composite tube (VT1, VT22) is cut off, and the relay K is released status; when a phase failure occurs, the voltage of point "E" to the neutral line increases. After rectification by VD1, the emitter of the composite tube is forward biased and turned on, K is attracted, and its normally closed contact is disconnected, causing the contactor KM to lose power and release. Motor M stops.