Three-phase power supply phase failure protection circuit

For AC contactors with a coil operating voltage of 380V,
the coil can only be connected to any two phases of the three-phase phase lines. If
any one of these two phases is disconnected, the contactor coil will lose power and
release, cutting off the working power of the motor. However, if the circuit break occurs
in another phase other than these two phases, the AC contactor will continue
to work, causing the motor to run without phase, and in severe cases, the motor may
burn out. If a phase failure protection circuit is used, such
an accident can be prevented.
    The phase failure protection circuit using thyristor is as shown in the figure.
Close the power switch QS. When button SBZ is pressed, the coil of AC contactor
KM is electrically closed, its main contact is closed, and the motor starts and
runs. The current transformer TA has an induction signal output, and the bidirectional thyristor
VTH is triggered to conduct, which plays
the role of self-locking of the contactor's auxiliary contacts. After releasing SB2, contactor KM still maintains the suction table and motor M continues to run.
    When the L3 phase in the three-phase AC power supply is disconnected, the thyristor loses the trigger signal and turns off, KM loses power and is released, and the working source of the motor M
is cut off to achieve phase failure protection.
  If the L-phase or R-phase is open circuit, the coil of the contactor KM will lose its working condition, causing the KM coil to lose power and release, cutting off the motor
power supply, and completing the task of phase failure protection .