One of the zero sequence voltage phase failure protection circuits

The zero-sequence voltage (current) phase failure protection wire is a phase failure protection circuit designed based on the phenomenon that the midpoint voltage of the motor winding shifts when one phase of the motor power supply is missing . The circuit is shown in Figure 4-33. It uses a linkage button to better prevent malfunctions caused     by inconsistent actions of the three main contacts of the contactor when the motor is started . When the power supply loses phase, the relay KA is closed, the contactor KM is released, and the motor starts to rotate.