Three-phase motor protection circuit

Phase failure and overload operation are the main reasons for motor burnout. The motor protector introduced in this example can
promptly cut off the working power supply and protect the motor from damage when the motor has phase failure or overcurrent operation .
    Circuit working principle; the motor protector circuit consists of a power supply circuit, a current detection circuit and a protection control circuit, as
shown in the figure.
    The power circuit is composed of power transformer T, rectifier bridge stack UR, filter capacitor Cs, current limiting resistor R3 and Zener diode vs.
    The current detection circuit consists of current transformer TA, diode VD1, capacitor G and potentiometers RP1 and RP2.
    The protection control circuit is composed of time base integrated circuit IC, resistors Ri, R2, diode VD2, capacitors C2~C4, relay K and AC contactor
KM.
    Sl is the stop button and S2 is the start button.
    When S2 is pressed, the AC contactor KM energizes the suction table, its normally open contacts KIVⅡ~KM4 are connected, and the motor M is energized and runs. The working voltage of KM
is also stepped down by T, rectified by UR and filtered by C5. After current limiting and voltage stabilization, +12V CVce) working power is provided for the IC. After releasing
S2, the KM4 contact and K's normally closed contact lock the power supply, and KM remains in the pull-in state.
    When the motor M is running normally. The induced voltage on the W2 winding of TA is low, the voltage of pin ⑥ of LC is lower than 2Vw/3, pin ③ outputs low
level, and K does not pull in.
    When the Ll phase or L2 phase voltage is out of phase, the L3 phase current will increase rapidly (about 1.73 times the normal current), causing the
induced voltage of the W2 winding of TA to increase, and the ⑥ pin voltage of IC will be higher than 2Va/3. ③The pin changes from high level to low level, K pulls in, its normally closed contact is disconnected, KM
is released when power is off, and its normally open contacts KM1~KM4 cut off the working power of motor M. If the L3 phase voltage loses phase, TA has no induced voltage output
, which is equivalent to the ④ pin of IC adding a low level, and its ③ pin outputs a low level, causing K to pull in, KM to release, and the motor M to stop running.
    When the drag load of the motor M becomes heavier and exceeds the rated power of the motor, the currents on the three-phase lines Ll~L3 increase simultaneously. When the current
increases to about 1.2 times the rated current, the voltage of pin ⑥ of the IC will be higher than 2V"/3, pin ③ outputs low level, K pulls in, KM releases, and motor
M stops for protection.