Multifunctional protector circuit

The protection signal is obtained by connecting current transformers TA1, TA2 and TA3 in series. This type of transformer is made of a magnetic ring with a lower saturation magnetic density (for example,
a soft ferrite MX 0-2000 type manganese-zinc magnetic ring). When the motor is running, the magnetic ring is in a saturated state, so the induced electromotive force in the secondary winding
of the transformer has a third harmonic component in addition to the fundamental wave.
    When the motor is running normally, the three line currents are basically balanced (that is, they are equal in magnitude and 120 degrees out of phase with each other), so the fundamental electromotive force in the secondary winding of the transformer
is zero, but the third harmonic electromotive force is synthesized as each electromotive force. three times. The obtained third harmonic electromotive force
is rectified by the diode VD1, stabilized by VD2 (using the forward characteristics of the diode), filtered by the capacitor C, and then divided by Ri and the island, and then supplied to
the base of the transistor VT, causing VT to saturate and conduct. Then relay KA pulls in and its normally open contact closes. When SB2 is connected, the electromagnetic contactor coil KM
is energized and self-locked.
    When the motor power supply is disconnected, the currents
in the two wires are equal in magnitude and opposite in direction. Only two windings of the three series-connected secondary windings of the transformer induce
electromotive force , and they are equal in magnitude and opposite in direction. As a result, the total secondary winding of the transformer The electromotive
force is zero, there is neither fundamental electromotive force nor third harmonic electromotive
force t. Therefore, the base current of VT is zero, vT is cut off, the relay KA connected to the VT
collector is released, and the contactor KM coil is disconnected. Electricity, the contact cuts off the
motor power. When the motor winding
temperature     is too high due to failure or other reasons . If the temperature exceeds the allowable value, the resistance of the PTC sensitive resistor rises sharply, changing the voltage dividing ratio between Ri and the island, causing the base current of the transistor VT to drop to a very low value (actually close to zero). VT is cut off, relay KA is released, its normally open contact is disconnected, contactor KM coil loses power, and the motor is disconnected from the power supply.