Motor control circuit for starting and running without phase loss

The control circuit for starting and running without missing a phase is shown in the schematic diagram. Just by adding a few intermediate relays and connecting the circuit according to the diagram, it can be ensured that the three-phase motor will never be missing a phase when starting.

Working principle:

① Starting principle: Press the start button SB1, the coil of the intermediate relay KA1 is energized and attracted, and at the same time, the coil of the intermediate relay KA2 is energized and attracted through the KA1 contact, the KM main contact is attracted, the motor terminals U, V, and W are energized, and the motor starts to run. The KM auxiliary contact energizes and attracts the coil of the intermediate relay KA3, and the KA3 contact maintains the power supply of the main contactor coil through the button SB2. The power supply path of the KM coil is U terminal-SB2-KA3 contact--KM coil-FR contact-FU2-V terminal. Even if SB1 is reset, KA1 and KA2 are released, and the main contactor KM is still attracted, so the motor is started and starts running.

②Stop principle: Press the stop button SB2, the power supply path of the main contactor KM coil is cut off, and KM is released. The KM auxiliary contact de-energizes the KA3 coil, and KA3 is released. Even if SB2 is reset, KM cannot be re-engaged because the KA3 contact is disconnected, and the motor stops running.

Working principle of never missing phase

① Never start with phase loss: In the circuit, if any phase of L1 or L2 is missing, when SB1 is pressed, the coil of the intermediate relay KA1 has no voltage, KA1 does not attract, and the motor cannot be started; if the L3 phase is missing, when SB1 is pressed, although KA1 attracts, KA2 cannot attract because the coil of KA2 has no voltage, and the motor cannot be started. Thus, the motor will never start when a phase is missing.

②Never lose phase operation: From the circuit, we can know that if the motor has started and run normally, during the operation, if the main circuit loses phase, the motor will automatically stop. Because the rated working voltage of all intermediate relays and main contactor coils in the circuit is 380V before they are attracted. In the figure, the power supply of the main contactor KM coil is taken from the U and V terminals through the SB2 and KA3 contacts, and the power supply of the KA3 coil is taken from the V and W terminals through the KM auxiliary contact. If the U phase or V phase (for the motor terminal) is missing, the main contactor KM coil will lose power and release, the three main contact points of KM will be disconnected, and the motor will automatically stop; if the W phase is missing, the intermediate relay KA3 coil will lose power and release, and the KA3 contact will be disconnected, causing the KM coil to lose power and release, and the motor will also automatically stop. In addition, the disconnection of the fuse FU2 can also cause the motor to stop automatically, thus realizing the operation without losing phase.