Motor protection circuit

During use, the three-phase AC motor burns out due to phase failure (because after a phase failure, the motor load remains unchanged,
the current of one winding of the phase failure is zero, and the operating current of the remaining two phases will increase a lot. ) phenomenon is relatively common. This example introduces a motor protector
made of discrete components , which has the characteristics of simple production and high reliability .     Circuit working principle: The motor protector circuit consists of a starting control circuit, a phase failure detection circuit and a protection execution circuit , as shown in the figure. The start control circuit is composed of start button S2 and stop button Sl.     The phase failure detection circuit is composed of current mutual inductance coil TA, rectifier diode VD1, resistor, capacitor C and potentiometers RP1 and RP2.     The protection execution circuit is composed of transistors VT1 ~ VT3, relay K, AC contactor KM, etc.     After pressing the start button S2 (linkage of S2a and S2b), the AC contactor KM is energized and closed, and its normally open contacts KM1 to KM4 are all connected. The motor M starts and runs, generating an induced voltage of 1.2V in TA. This voltage is After VD1 is rectified, VT2 is saturated and turned on. At this time, since S2b is disconnected, VT1 is cut off. VT3 is turned on, relay K pulls in, and its normally open contact K is turned on. After the hand leaves the start button S2, the motor M can still maintain normal operation.     When the U phase or V phase of the three-phase alternating current is disconnected, an induced signal voltage L 73 times larger than the normal value (1.2V voltage) will be generated at both ends of TA, causing VT1 to conduct, VT3 to cut off, and the relay K to release . The normally open contact K is disconnected, causing the AC contactor KM to release, cutting off the working power of the motor M, thus protecting the motor from burning due to phase failure.     If phase C is out of phase, the induced voltage at both ends of TA disappears, causing VT3 and VT2 to cut off, relay K and AC contactor KM are both released, and motor M stops running. ,     adjusting the resistance of RP1 and RP2 can change the sensitivity of VT1 action.     If VT3, VD3 and R3 are removed from this circuit, relay K is connected in parallel to the collector and emitter of VT1 (boards c and e). , the circuit can also work normally.