Application and Maintenance of AC Contactors | Comprehensive Analysis of Common Fault Handling, Master It in One Article

1 Analysis of Key Components of AC Contactors

An AC contactor is an automated electromagnetic switch used for long-term, high-frequency switching of AC main circuits and control circuits. It features advantages such as automatic operation, under-voltage and no-voltage protection, high-capacity operation, strong stability, and low maintenance requirements. In the electrical control circuits of machine tools, AC contactors are mainly used to control electric motors and other loads.

The key components of an AC contactor include the electromagnetic system, contact system, and arc-extinguishing device, etc. It is mainly composed of structural parts such as main contacts, moving iron core, coil, static iron core, and auxiliary contacts.

1.1 Electromagnetic System

The electromagnetic system of an AC contactor mainly consists of a coil, moving iron core, static iron core, and short-circuit ring. When the control coil is energized or de-energized, it will complete the pulling-in or releasing action respectively, which can keep the moving contacts and static contacts in the open or closed state respectively, so as to achieve the purpose of switching the circuit.

To reduce eddy current and hysteresis losses, the iron core and armature of an AC contactor are mainly made by laminating E-shaped silicon steel sheets during production. To increase the heat dissipation area and avoid burning out, the coil is made into a thick and small cylinder wound on an insulating frame, with a certain distance maintained between it and the iron core to prevent overlap. The E-shaped iron core reserves an air gap of 0.1 - 0.2 mm on the end face of the middle cylinder to reduce the effect of residual magnetic field and prevent the armature from jamming. 

When the AC contactor is working, the alternating current in the coil forms an alternating magnetic field in the iron core, causing the armature to oscillate and generate noise. A groove is provided at each end of the iron core and armature, and a short-circuit ring made of copper or nickel-chromium alloy is embedded in the groove to solve the above problem. After installing a short-circuit ring, when alternating current flows through a winding, magnetic fluxes Φ₁ and Φ₂ with different phases will be formed, thereby ensuring that there is always an attractive force between the iron core and the armature, greatly reducing vibration and noise.

1.2 Contact System

There are three types of AC contactor contacts, namely point contact type, line contact type, and surface contact type, as shown in the following figure. According to the structural form, they can be divided into bridge contacts and finger contacts. Bridge contacts include point-contact bridge type and surface-contact bridge type, which are suitable for different current occasions. Finger contacts are mostly in line contact mode, and their contact area is a straight line, which is suitable for frequent and large-current occasions. According to the making-and-breaking capacity, they can be divided into main contacts and auxiliary contacts. Main contacts are suitable for large-current main circuits, and there are generally 3 pairs of normally open contacts. Auxiliary contacts are suitable for small-current control circuits, and there are generally 2 pairs of normally open contacts and 2 pairs of normally closed contacts.

1.3 Arc-Extinguishing Device

For high-current or high-voltage circuits, arcs will inevitably occur when AC contactors open, causing contact burning, damage to the device, affecting its service life, and even interfering with the circuit breaking time; in severe cases, it may lead to fires. For safety reasons, all contactors with a capacity exceeding 10 A must be equipped with an arc-extinguishing device. The arc-extinguishing methods commonly used in AC contactors include double-break electric force arc extinguishing, longitudinal slot arc extinguishing, and grid arc extinguishing.

The double-break electric force arc-extinguishing device divides the arc into two parts, and stretches the arc through the electric force of the contact circuit itself, so as to realize the heat dissipation and cooling of the arc and achieve the purpose of extinguishing it. The longitudinal slot arc-extinguishing device is made of arc-resistant clay, asbestos cement and other materials, with one or more longitudinal slots on its inner side, which can expand the contact area between the arc and the wall of the arc-extinguishing chamber, and achieve the effect of extinguishing the arc by compressing it. When the contacts are in the separated state, the arc is sent into the slots through an external magnetic field or electric force, and the heat energy is transferred to the wall of the arc-extinguishing chamber, so that the arc is extinguished quickly.

On this basis, a new type of grid arc extinguisher structure is proposed. The metal grid adopts herringbone copper-plated or galvanized iron sheets and is inserted into the arc-extinguishing cover. The arc formed by the contact breaking generates a strong magnetic field, and the existence of magnetic resistance makes the electric field intensity in this area uneven, thereby pulling the arc into the gaps of the grid to form short arcs. Each grid acts as an electrode, dividing the entire arc voltage drop into several sections, and the arc voltage between each section is less than the arc ignition voltage. At the same time, the grid dissipates heat to eliminate the arc quickly, achieving the effect of extinguishing [3-5].

1.4 Auxiliary Components

The auxiliary components of an AC contactor include a reaction spring, a buffer spring, a contact pressure spring, a transmission mechanism, a base, etc. The reaction spring pushes the armature to release energy after power failure, so that the contacts return to their original state. The buffer spring can mitigate the impact force. The contact pressure spring can greatly increase the contact pressure and reduce the contact resistance. The operating contacts are driven by the armature or the reaction spring to control them to be connected or disconnected.

2 Proper Use of AC Contactors

2.1 Selection Principles of AC Contactors

The rated voltage of the main contacts must not be less than the rated voltage of the control circuit. The rated current of the main contacts should meet the load requirements: for resistive loads, it should be equal to the rated current; for motor loads, it should be slightly greater than the rated current. The voltage of the attraction coil is selected according to the complexity of the control circuit: 380 V or 220 V can be selected for simple circuits, and 36 V or 110 V for complex circuits. The number and type of contacts must meet the basic standards of the control circuit.

2.2 Installation and Maintenance of AC Contactors

For the pre-installation inspection, it is necessary to confirm whether the technical data of the contactor (such as rated voltage, current, operating frequency, etc.) comply with the standards, check whether the appearance is damaged and the movement is flexible, and measure the DC resistance value and insulation resistance value of the coil. The installation position should be vertical, with an inclination not exceeding 5°, and the side with heat dissipation holes should face the vertical direction. During installation and wiring, prevent parts such as screws, washers, and terminals from falling, which may cause the AC contactor to be stuck or short-circuited.

After installation, it is necessary to check whether the wiring is correct. Without energizing the main contacts, energize and de-energize the contactor several times to check the movement of the main contacts and whether there is noise after the iron core is pulled in. It can be put into use only if there is no error. It is not allowed to connect the AC contactor to a DC power supply, otherwise the coil will be burned out.

3 Common Faults and Maintenance Methods of AC Contactors

3.1 Main Contact Faults

3.1.1 Severe Sparking at the Moment of Connection and Disconnection of Moving and Static Main Contacts

When the load is working normally, sparking occurs at the moment the contacts are connected and disconnected. The contact surface forms irregular small pits due to the high temperature of the arc, resulting in reduced contact area, increased current, and severe sparking. To repair the damaged contacts, it is necessary to check the degree of damage on the contact surface; the contact can be repaired only if its thickness is more than 2/3 of the original thickness. When repairing the contacts, first place fine sandpaper on a horizontal surface, then grind the damaged contacts flat on the sandpaper, check the repair situation until all damaged points are ground off, and finally deal with the burrs.

3.1.2 Melting, Burning and Adhesion of Moving and Static Main Contacts

The main causes of melting, burning and adhesion of moving and static main contacts include short circuit of the load, short circuit of the main circuit, or reduction of load impedance. Among them, the simultaneous occurrence of short circuit and main circuit short circuit is the key factor. Due to work needs, the operating frequency of the AC contactor ranges from low to high; during the frequent connection and disconnection of the contacts, the surface temperature rises, and under the action of the arc, the moving and static main contacts will eventually melt, burn and adhere.

There are generally two treatment methods: first, replace the AC contactor with one with a higher voltage and current rating; second, repair the AC contactor: replace the contacts with the same specification, clean the carbon deposits around the moving and static contacts, etc., and connect Resistor-Capacitance (RC) arc-extinguishing devices in parallel with each of the 3 pairs of main contacts.

3.2 Auxiliary Contact Faults

3.2.1 Excessively High Contact Resistance of Moving and Static Auxiliary Contacts

Excessively high contact resistance of moving and static auxiliary contacts will lead to an increase in the loop impedance of the control circuit and a decrease in voltage. There are two main reasons for this phenomenon: first, a large amount of oil stains and dust are deposited on the contacts; second, an oxide layer is formed on the contact surface. Based on the under-voltage protection mechanism of the AC contactor, when the voltage across the AC contactor coil is lower than 85% of the rated voltage, the control circuit will stop working. The solution is to take out the contacts, wipe them dry with clean gauze, and then gently treat the contact surface with fine sandpaper.

3.2.2 Severe Sparking at the Moment of Connection and Disconnection of Moving and Static Auxiliary Contacts

The main reasons for this fault may be that the controlled circuit has experienced a short circuit, or the impedance value of the energy-consuming components in the control circuit has decreased, etc.

3.3 Coil Faults

3.3.1 Coil Open Circuit

An open circuit of the AC contactor coil will cause the control circuit to fail to work. This phenomenon is relatively rare, and it is generally caused by quality problems of the contactor or improper installation during assembly.

3.3.2 Coil Short Circuit

A short circuit of the AC contactor coil will cause the fuse of the short-circuit protection in the control circuit to blow. A common situation of coil short circuit is that the AC voltage applied across the coil is not 0.85-1.05 times the rated voltage; long-term operation of the coil under low or high voltage may cause a short circuit. A damaged AC contactor coil must be replaced; when replacing the coil, attention should be paid to the coil size, rated voltage, and the specification of the AC contactor.

3.4 Faults of Moving and Static Iron Core Contact Surfaces

3.4.1 Adhesion of Moving and Static Iron Core Contact Surfaces

The main reason for this fault is the presence of oil stains on the contact surfaces of the moving and static iron cores. After pressing the start button, the motor runs normally, but when pressing the stop button, the AC contactor coil loses power, the contacts do not return to their original state, and the motor continues to run. After the hand leaves the stop button, the coil remains energized, and the motor continues to run. The treatment method is to clean the contact surfaces of the moving and static iron cores.

3.4.2 Loud Noise from the Iron Core

The main reasons for loud noise from the iron core are the breakage of the short-circuit ring, or a large amount of rust on the contact surfaces of the moving and static iron cores. For the case of a large amount of rust, fine sandpaper can be used to treat the contact surface. If the short-circuit ring is damaged, the iron core is generally replaced to repair the fault.

4 Conclusion

The proper use, fault diagnosis and maintenance skills of AC contactors are crucial to the stable operation of electrical control systems. In order to improve the service efficiency of AC contactors and extend their service life, common faults should be repaired in a timely manner to reduce the failure rate during production.