What is an AC contactor?
The contactor is an intermediate control element, which has the advantages of frequent on and off lines and large current control with a small current. Working with the thermal relay can also provide a certain overload protection for the load device. Because it relies on electromagnetic field suction to open and close work, it is more efficient and more flexible than manual switching and closing circuit. It can divide and load multiple load lines at the same time, and has self-locking function. After picking up, it can enter the self-locking state and continue to work.
AC contactors are the most commonly used low voltage control appliances in electric drag and automatic control systems. As an actuator, it is used to switch on or off the line, or to frequently control the operation of the motor and other equipment. It consists of dynamic and static main contacts, arc-extinguishing cover, moving and static iron core, auxiliary contacts and bracket housing. After the electromagnetic coil is energized, the moving iron core is attracted by the electromagnetic force, and the moving contact is in contact with the static contact directly or through the lever transmission, and the circuit is connected. After the electromagnetic coil is powered off, the moving iron core automatically returns under the action of the return spring, which is commonly called release, the contacts are separated, and the circuit is broken.
More than 90% of the automation control power systems use contactors, which shows how wide it is.
How AC contactors work
When the coil is energized, the static iron core generates electromagnetic attraction, and the moving iron core is sucked. Since the contact system is linked with the moving iron core, the moving iron core drives the three moving contact pieces to simultaneously act, the main contact is closed, and the main The auxiliary normally closed contact that is mechanically connected to the contact is opened, and the auxiliary normally open contact is closed, thereby turning on the power. When the coil is de-energized, the suction force disappears, and the moving iron core linkage part is separated by the reaction force of the spring, so that the main contact is disconnected, the auxiliary normally closed contact mechanically connected with the main contact is closed, and the auxiliary normally open contact is disconnected. , thus cutting off the power.
One: The general three-phase contactor has a total of 8 points, three inputs, three outputs, and two control points. The output and input are corresponding and can be easily seen. If you want to add a lock, you need to connect the line from the terminal of the output point to the control point.
Second: First of all, you should know the principle of the AC contactor. He uses an external power source to apply to the coil to generate an electromagnetic field. After the power is applied, the contact point is disconnected after the power is turned off. Knowing the principle, you should figure out the contact point of the external power supply, that is, the two contacts of the coil, generally in the lower part of the contactor, and on each side. The other few inputs and outputs are generally in the upper part and are known at a glance. Also pay attention to the voltage of the external power supply (220V or 380V), which is generally marked. Also note that the contact point is normally closed or normally open. If there is self-locking control, the line can be based on the principle.
The AC contactor has a large carrying current, generally an internal pull-in coil to control its action, and the control coil is operated by various types of relays connected in series with it, and the trip is caused by the exchange after thunder. The relay of the contactor has overcurrent or grounding protection function. When there is lightning on the line, it will control the action of the AC contactor and cut off the load power supply to protect the equipment from high voltage, high current damage or grounding. .
Characteristics of the AC contactor:
1. Application of AC contactor in direct starting circuit of motor
Direct start-up is the most basic starting method for low-voltage motors. It has a wide range of applications. Generally, small and medium-sized enterprises and rural agricultural and sideline products use this kind of starting method. The so-called low voltage motor. Usually refers to an asynchronous motor rated at 380V or 660V. The motor with power of 22kW or less can adopt the direct start mode, and the AC contactor is used as the main switch. It is not recommended to use the rubber cover switch to close the start. Such safety is poor, and there have been accidents of arc burning.
The primary circuit and the secondary circuit that the motor directly starts are shown in Figure 1 and Figure 2, respectively.
The so-called primary circuit is the circuit component and the wire through which the working current of the motor winding passes: the secondary circuit is an auxiliary circuit that is indispensable for ensuring the normal operation of the device. The main functions of the secondary circuit are control, measurement, signal and protection. The circuit that causes the motor to start and stop running is the control function circuit of the secondary circuit; the measurement of the electrical parameters such as voltage, current, power and power factor is the measurement function; the running and stop indicator lights, the abnormal alarm sound, etc. are secondary The circuit components of the signal circuit: thermal relays, motor protectors and other components can achieve motor protection. The following is a detailed analysis of the working process of the motor direct start circuit.
In Fig. 1, the live lines (phase lines) L1, L2 and L3 of the three-phase power supply are connected to the upper end of the isolating knife switch QS. The role of QS is to disconnect the power supply during maintenance. There is an obvious disconnection point between the repaired circuit and the power supply to ensure the safety of the maintenance personnel. FU is a protective fuse for primary circuits. When preparing to start the motor, first close the knife switch QS, then if the main contact of the AC contactor KM is closed, the motor is powered: the contactor main contact is open and the motor stops running. Contactor contacts are closed or not. Then it is controlled by the secondary circuit.
In Figure 2. FUl and FU2 are secondary fuses. SBl is the stop button. SB2 is the start button. FH is the protective output contact of the thermal relay. Press SB2. The coil of the AC contactor KM1 is energized, its main contact is closed and the motor starts to run. At the same time, the auxiliary contact KMl-1 of the contactor is also closed. It allows the contactor coil to be continuously powered and the contactor's pull-in state is maintained. It is customary to refer to the auxiliary contact KMl-1 as a self-protecting (holding) contact.
The motor is running. If an abnormal situation such as overcurrent or short circuit occurs due to any abnormality, the bimetal inside the thermal relay FH (see Fig. 1) will be thermally deformed due to excessive current, and its protective contact FH (see Fig. 2) will operate within a certain time limit. Disconnected, causing the contactor coil to lose power, the contactor main contact is disconnected, the motor stops running, and the motor is protected from overcurrent. After the protection action, the auxiliary contact KMl-1 of the contactor is disconnected and the motor remains in the stopped state.
If the motor is running, press SBl. The motor will also stop running, and its action will be the same as the thermal protection.
The stop indication green light HG and the operation indication red light HR are respectively controlled by the normally closed (open) or normally open (moving) auxiliary contacts KMl-2, KMl-3 of the contactor for use as signal indication. The secondary coil of the current transformer TA is connected in series with the current meter PA, and the voltmeter PV is directly connected to the power line, which performs the running current and voltage of the motor.
In Fig. 2, there is no "·" in the place where the wire "D" is connected. There is generally no "·" in the drawings of electricians. In fact, whether or not to draw a dot or not is in line with the requirements of the national standard GB4728 "Graphic Symbols for Electrical Drawings", but it has its own drawing habits in different application fields.
2. Motor reversible operation control circuit
In order to enable the motor to rotate forward and reverse, two contactors KM1 and KM2 can be used to change the phase sequence of the three-phase power supply of the motor, but the two contactors cannot be sucked. If the suction is simultaneously, the short-circuit accident of the power supply will occur. To prevent such accidents, reliable interlocking should be adopted in the circuit. The above figure shows the control circuit running in the forward and reverse directions of the motor with double interlocking of the button and the contactor.
The line analysis is as follows:
First, the positive start:
1. Close the air switch QF to connect the three-phase power supply.
2. Press the forward start button SB3, KM1 energizes and absorbs the self-locking, and the main contact closes to turn on the motor. The phase sequence of the motor at this time is L1, L2, L3, that is, forward running.
Second, reverse start:
1. Close the air switch QF to connect the three-phase power supply.
2. Press the reverse start button SB2, KM2 energizes and absorbs and self-locks through the auxiliary contact. The normally open main contact closes and replaces the three-phase power phase sequence of the motor. At this time, the phase sequence of the motor is L3, L2, L1. , that is, running in reverse.
Third, the interlocking link: has a prohibition function to play a security role in the line
1. Contactor interlock: KM1 coil circuit is connected to the normally closed auxiliary contact of KM2, and KM2 coil circuit is connected to the normally closed contact of KM1. When the coil of the forward contactor KM1 is energized, the auxiliary normally closed contact of KM1 is disconnected from the KM2 coil circuit. If the KM1 is electrically connected, the KM2 must be powered off first, and the auxiliary normally closed contact is reset. This prevents the KM1 and KM2 from simultaneously attracting and causing phase-to-phase short circuit. This line is called the interlocking link.
2. Button interlock: In the circuit, the forward and reverse transmission control circuit is operated by the control button. The buttons SB2 and SB3 both have a pair of normally open contacts and a pair of normally closed contacts, and the two contacts are respectively associated with KM1. KM2 coil circuit connection. For example, the normally open contact of button SB2 is in series with the contactor KM2 coil, while the normally closed contact is in series with the contactor KM1 coil loop. The normally open contact of button SB3 is in series with the contactor KM1 coil, while the normally closed contact pressure KM2 coil circuit is connected in series. Thus, when SB2 is pressed, only the coil of contactor KM2 can be energized and KM1 can be powered off. When SB3 is pressed, only the coil of contactor KM1 can be energized and KM2 can be powered off. If SB2 and SB3 are simultaneously pressed, then two None of the contactor coils can be energized. This plays the role of interlocking.
4. After the motor is running in the forward (or reverse) direction, you do not need to press the stop button to stop the motor. You can directly press the reverse (or forward) start button to make the motor run in the opposite direction.
5. The overload protection of the motor is completed by the thermal relay FR.
Motor reversible operation control wiring diagram
Commissioning of motor reversible operation control circuit
1. Check whether the wiring of the main circuit is correct. In order to ensure that the phase sequence of the motor can be reliably replaced when the two contactors are in operation, the upper port of the contactor should be kept consistent during wiring, and the phase of the contactor should be phased.
2. After checking the wiring is correct, the power-on test, in order to prevent accidents during the power-on test, the motor wiring should be disconnected first.
Pre-processing of fault phenomena;
1. Do not start; one of the reasons is to check whether the control insurance FU is open circuit, whether the thermal relay FR contact is wrong or bad contact, and whether the normally closed contact of the SB1 button is bad. The reason for the second button interlock is incorrect.
2. When the device is activated, the contactor “Do not suck” is not sucked; this is because the interlocking connection of the normally closed contact of the contactor is wrong, the interlocking contact is connected to lock itself, and the closed contact is the contact when starting. The electric coil of the coil is closed, the normally closed contact is disconnected after the contactor is sucked, the contactor coil is discharged again, the normally closed contact is released, the contactor is turned on and the contact is closed again, and the contact is disconnected, so哒” The contactor does not attract.
3, can not be self-locking and lift the hand contactor to disconnect, this is because the self-locking contact wiring is wrong.
