Preface: When installing and maintaining electrical equipment, we frequently encounter AC contactors. Primarily used for long-distance connection and disconnection of AC circuits, they are widely used in industrial and mining enterprises. Some malfunctions are unavoidable, while others are caused by improper use. The following analysis of their malfunctions will clarify the issues that should be considered during their use. Analysis of AC contactor malfunctions and issues to be aware of during use include: 1. Reducing the capacity when controlling lighting loads A repair shop had a lighting circuit with a capacity of 4kW per phase, controlled by a CJ10-20 AC contactor. However, after a short period of use, malfunctions such as contact sticking occurred. A new contactor was installed, but it also failed after a few days. Inspection revealed no problems with the lighting circuit. Calculating the capacity, the 4kW lighting load current was only 18A. How could the CJ10-20 AC contactor, with a rated current of 20A, have overloaded and burned out? Later, a CJ10-40 AC contactor was used, which operated without problems for a long time. The above situation was caused by a lack of understanding of the difference between power loads and lighting loads. In addition to being able to carry the rated operating current, the main contacts of an AC contactor must also be able to withstand the starting current of the electrical equipment. The starting current that an AC contactor can withstand is generally 6 to 7 times its rated current. This is to control AC motors, whose starting current is also 6 to 7 times the design current. Since the cold-state resistance of an incandescent lamp is only a fraction of its hot-state resistance, the initial current is several times higher than the normal operating current, far exceeding the starting current that the AC contactor can withstand, easily causing it to burn out. Therefore, when using an AC contactor to control lighting loads, its rated current should be reduced so that the contactor can withstand the starting current of the incandescent lamp. 2. Do not frequently file the contacts. After operation, it is very common for the contact surface of an AC contactor to become rough, develop spots, or even cracks. There is no need to repair the contact surface. When the contacts are made of silver alloy, a black film often forms on the surface under natural conditions due to sulfide gases in the atmosphere or due to the switching of current. This film will disappear on its own due to heat or mechanical action during operation, and frequent cleaning is unnecessary; otherwise, it will shorten the contact's lifespan. The normal use of the contacts depends on their wear condition and the amount of remaining contact material. It can be said that this film on the surface of the material is beneficial in reducing contact wear. When the contacts are made of copper, they do not need to be repaired if the burn damage is not too severe. Generally, filing cannot restore the original flatness. Utilizing a slightly uneven surface with minor burn damage actually provides better contact. Only when the contacts are severely burned should they be filed, but fine sandpaper should not be used to avoid leaving quartz sand particles between the contacts, affecting contact. 3. The grease on the iron core must be thoroughly cleaned. In the electrical maintenance of AC hoist rooms, it is common to encounter situations where the circuit is intact, but the contactor does not release after being engaged. No matter how many times the stop button is pressed, or even if the power supply is burned out, the contactor remains firmly engaged, easily causing an electrical accident. This is mainly because, to prevent the contactor core from rusting during transportation and storage, a layer of grease is often applied to the end face of the core when the contactor leaves the factory. This grease must be thoroughly cleaned before operation; otherwise, over time, dust will accumulate, and the grease will turn into a highly viscous grease layer, preventing the contactor from releasing properly. 4. Reduce interference when coexisting with low-voltage systems In the CNC device of a machine tool in a certain factory, in order to reduce the size of the control cabinet, in addition to electronic components, an AC contactor was also installed, causing low-voltage and high-voltage circuits to coexist. Due to interference, it is very troublesome to use. After repeatedly observing the circuit's operation with an oscilloscope, it was found that interference pulses sometimes appear when the AC contactor is engaged. The more times the AC contactor is engaged, the more times interference signals appear, and the interference pulses randomly appear at the engagement and disengagement times of the AC contactor. For general CNC devices, the most common interference is industrial interference. Any circuit that is intermittently switched on and off, causing drastic changes in current and voltage, is the source of interference. If sparks and arcs are generated during switching, the interference is even more severe. Therefore, AC contactors are a source of interference. The interference waves generated by the source are generally transmitted to the CNC device through electrostatic coupling caused by parasitic capacitance and electromagnetic coupling caused by mutual inductance. Research on CNC devices reveals that they use dual NOT gate integrated circuits with a high output level greater than 2.4V, a low output level less than 0.4V, an open gate level less than 2V, and a closed gate level greater than 0.8V. Because interference is superimposed on the normal input voltage, if the circuit output is at a high level (greater than 2.4V), the positive interference generated when the AC contactor engages and disengages causes the input voltage to exceed the closed gate level (greater than 0.8V), causing the circuit state to flip, and the output voltage jumps from high to low. After the interference subsides, the output voltage returns to its normal high level. If the circuit output is at a low level (less than 0.4V), the amplitude of negative interference may cause the input voltage to fall below the gate opening level (less than 2V), causing the circuit state to flip, and the output to jump from low to high. After the interference subsides, the output voltage returns to the normal low level, preventing the CNC device from operating normally. One solution to this is to move the AC contactor outside the control cabinet to reduce interference. Another method is to simply modify the connection between the AC contactor and the CNC device. After multiple experimental analyses, the interference mainly originates from within the power supply, caused by the excessive rate of change of AC power supply current when the AC contactor coil is energized, with a coil voltage of 220V. Therefore, an RC circuit (its principle is only briefly described here) was connected across the coil terminals and the NAND gate input. Practice has shown that this effectively solved the interference problem. Conclusion Faults frequently occur in actual work. Through observation and experience during installations over the past few years, it is believed that mastering the correct usage and maintenance methods can reduce the failure rate of contactors, allowing this highly functional electrical appliance to be used more effectively.