Three-coil voltage relay balance control phase failure protection circuit

In the circuit shown in the figure, the motor M is running normally, the three coils of the relay KV are respectively connected to the three-phase AC voltage, the total combined suction force of the three coils is zero, and the contacts are normally closed. When one phase is disconnected, the three-phase voltage is unbalanced, causing the voltage relay to operate, cutting off the power supply of the KM coil, and the motor M stops running. The disadvantage of this circuit is that it cannot protect the internal winding of the motor M from phase failure.