Zero sequence current phase failure protection circuit

As shown in the circuit in the figure, when the start button ST is pressed, KM is engaged and the motor M is put into normal operation. At this time, the three-phase load of the motor is balanced, the secondary current of the zero-sequence current transformer TA is equal to zero, VT1 is in the cut-off state; VT2 is in the conduction state, the relay K (JR-4 type) is closed, and KM is self-locking. When a phase failure occurs, the induced current generated by the secondary of TA is rectified by VD1, causing VT1 to flip from off to on, and VT2 to flip from on to off (the power supply of VT2 is taken out from the KM coil plus the L winding for 15~ 18V, supplied after VC rectification). When K loses power, it cuts off the power supply of the motor to achieve the purpose of phase failure protection. In order to avoid the unbalanced current during starting, a time relay KT can be added, and its delayed opening normally closed contact will temporarily short-circuit the secondary of TA during the starting process of the motor M. For small-power self-balanced three-phase motors, there is no need to increase KT.