Three-phase asynchronous motor self-locking control circuit

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As shown in Figure 3-2, the main circuit of the self-locking control line of the three-phase asynchronous motor is substantially the same as the main circuit of the jog control, but a stop button SB1 is connected in series in the control circuit, and two of the start button SB2 are connected. A pair of normally open auxiliary contacts of the contactor KM are connected in parallel. The contactor self-locking forward rotation control circuit not only enables the motor to run continuously, but also has an important feature that it has undervoltage and pressure loss (or zero pressure) protection. It is mainly composed of push button switch SB (starting and stopping motor), AC contactor KM (used to turn on and off the motor power supply and voltage loss and undervoltage protection), thermal relay (used as overload protection of the motor) and so on.

Undervoltage protection: “Undervoltage” means that the line voltage is lower than the rated voltage that the motor should add. “Undervoltage protection” refers to a kind of protection that the motor can automatically stop off from the power supply voltage when the line voltage drops to a certain value to avoid the motor running under undervoltage. Because when the line voltage drops, the torque of the motor decreases, and the speed of the motor also decreases, which increases the operating current of the motor, affecting the normal operation of the motor, and causing "blocking" when the voltage drops seriously. (ie, the motor is powered on but does not rotate), causing damage to the motor. The use of the contactor self-locking forward control circuit can avoid the undervoltage operation of the motor. This is because when the line voltage drops to a certain value (generally below 85% of the rated voltage), the voltage across the contactor coil also drops to the same level. A certain value, so that the magnetic flux of the contactor coil is weakened, and the generated electromagnetic attraction is reduced. When the electromagnetic suction force is reduced to less than the pulling force of the reaction spring, the moving iron core is forced to release, driving the main contact and the self-locking contact to be disconnected at the same time, automatically cutting off the main circuit and the control circuit, and the motor loses power and stops to achieve undervoltage protection. the goal of.

Loss of pressure (or zero pressure) protection: Loss of pressure protection means that the motor can automatically cut off the motor power when it is in normal operation and suddenly powers off due to some external cause. When re-powering, ensure that the motor can not start by itself, to avoid equipment and personal injury. The contactor self-locking control circuit is used, and since the contactor self-locking contact and the main contact are disconnected when the power is turned off, the control circuit and the main circuit cannot be connected. Therefore, when the power supply is restored, the motor cannot start running by itself, ensuring the safety of the person and equipment.

Control principle of self-locking control circuit for three-phase asynchronous motor

When the start button SB2 is pressed, the power supply U1 phase passes through the thermal relay FR break contact point, the stop button SB1 break contact point, the start button SB2 move the contact point, and the AC contactor KM coil turns on the power supply V1 phase, so that the AC contactor The coil is energized and acts, and its main contact is closed to rotate the motor. At the same time, the normally open auxiliary contact of the AC contactor KM is shorted to the moving contact of the start button SB2, and the AC contactor coil is always in a charged state, which is called self-locking (self-protection). The normally open auxiliary contact that is self-locking in parallel with the start button SB2 is called a self-locking contact (or a self-protecting contact).