Three-phase motor phase failure overcurrent protection circuit

As shown in the figure is a more practical circuit. Press ST, KM pulls in, and motor M starts and runs. Current transformer TA outputs current through the secondary side, which is rectified by VD1, divided by RP and R1, forming a voltage signal that is added to the base of VT1 through R2 and VD6, and another signal is added to the base of VT3 through RP. VT1, VT2, and VT3 form an emitter-coupled bistable circuit. Normally, VT1 is cut off, VT2 and VT3 are saturated and conductive, relay K is closed, and motor M operates normally. When a phase of a three-phase motor is out of phase, the current must be much higher than normal, or the current may be greatly increased due to motor coil short circuit, mechanical jamming, etc. At this time, the current on the secondary side of TA must also be greatly increased and added to VT1 The base voltage also increases greatly, causing VT1 and VT3 to be turned on from saturation, VT2 is cut off, the K coil loses power and is released, the KM coil loses power and is released one after another, and the motor M loses power.