1. Introduction Intelligent AC contactors are characterized by their small size, light weight, reliable operation, and long lifespan. The adoption of power electronics and microelectronics technologies significantly improves their performance; however, this also brings about interference problems. AC contactors often operate in harsh environments with diverse interference sources, such as high-order harmonics of the mains voltage, inrush currents from the mains, electrodynamic forces in the contact circuit, electric arcs, and electromagnetic interference from the surrounding environment. There is also interference from the circuit itself, such as unreasonable circuit layout and design. These interferences have negative consequences, potentially causing the contactor to malfunction, disrupting normal operating procedures, and leading to serious consequences. Effective measures must be taken to suppress or eliminate the adverse effects of interference. 2. Working Principle Analysis of Intelligent AC Contactors This intelligent contactor uses a thyristor or MOSFET as the device to control the on/off state of the excitation coil. The control signal is provided by a microcontroller. As shown in Figure 1, the microcontroller system uses control loops 1, 2, and 3 to control the main control element 1, main control element 2, and freewheeling element in real time. During startup, the microcontroller system continuously samples the power supply voltage and determines the threshold voltage for activation. If the voltage is higher than the threshold voltage, the system selects the appropriate program based on different input voltage values. At each half-wave corresponding moment (conduction phase angle), the main control element is activated through control loops 1 and 2, and the coil operates under strong excitation. After a certain activation time, the microcontroller system uses the control loops to deactivate main control elements 1 and 2, ending the startup process. During the holding phase, the holding winding of the transformer's secondary winding provides a suitable holding voltage, which, through the freewheeling circuit, allows the contactor coil to operate under a small DC current. This DC start and DC holding operation significantly improves the contactor's performance. Throughout the entire operation, the microcontroller system continuously monitors the power supply voltage. If the power supply voltage is found to be lower than the release voltage, the freewheeling element is immediately deactivated through control loop 3, and the contactor coil is de-energized. During the disconnection process, we improved the contactor's contact system. By controlling the disconnection time of the intermediate phase contacts, we can achieve synchronous disconnection of the three-phase circuit by ensuring that all three phase contacts disconnect the circuit before the current crosses zero. The key to achieving synchronous disconnection is a high-performance current sensor to detect the current in the main circuit. Compared to traditional current transformers, this type of current transformer should have stronger anti-interference performance. Under the influence of strong electromagnetic interference (including arcing and electrodynamic interference), it should be able to accurately reflect the current changes in the main circuit, especially detecting the accurate current zero point. If the zero-point detection is incorrect, the arc on the main contacts may reignite after the current crosses zero, leading to loss of control. Therefore, various interference sources directly affect the reliability of intelligent AC contactors. If these interferences prevent the contactor from operating normally according to its original design, it may cause malfunctions and disrupt the normal operation of the system. Therefore, it is essential to minimize the interference affecting the contactor and improve the reliability of intelligent AC contactors. The following is a detailed analysis of the main interference sources that may exist in the operation of intelligent AC contactors. 3. Main Interference Sources in the Operation of Intelligent AC Contactors 3.1 Power Supply Interference Since electronic circuits are connected to the power grid through power supply circuits, noise from the power grid can interfere with the electronic circuits, which is one of the main reasons for interference in electronic circuits. The most important and serious interference source in microcomputer systems also comes from power source pollution. The faster the industrial development, the more serious the power supply pollution. Many documents believe that if the anti-interference measures of the power grid are improved, half of the anti-interference problem of microcontrollers and electronic circuits will be solved. This shows the importance of suppressing power supply interference. Power supply interference can be considered in detail from the following situations: Source: Power Transmission and Distribution Equipment Network The first situation is through coupling of the power transformer. The transformer is the most common component in the power supply circuit. Its primary is connected to the power grid, and the secondary obtains various AC voltages according to the design requirements. Then, after passing through rectifier filter circuits and voltage regulator circuits, the DC voltage required for the line operation is obtained. When studying how high-frequency spike pulses of the power grid propagate through the transformer, it was found that the AC electromagnetic coupling of the transformer primary is not the main propagation path of this noise. In fact, this propagation path is caused by the distributed capacitance between the primary and secondary of the transformer. Because the primary coils of a transformer are very close together, the distributed capacitance between these two parts is typically several hundred pF. This distributed capacitance not only has a large capacitance but also excellent frequency characteristics, exhibiting very low impedance to high-frequency noise. The second scenario involves power supply noise caused by faults such as overvoltage, undervoltage, and power outages. All power supplies and transmission lines have internal resistance, and it is this internal resistance that causes noise interference. Without internal resistance, any noise would be absorbed by the power supply short-circuit, and no interference voltage would be generated in the circuit. During peak electricity consumption periods, when the load exceeds the rated power capacity of the power plant, undervoltage occurs. During off-peak periods, the load is very light, which may cause voltage rise and overvoltage. These potential interferences are called slow power supply variations, and their danger is obvious. The third scenario involves surges, sags, voltage spikes, and other power supply interference. The continuous switching on and off of high-power equipment by users, especially high-power motors, requires a large starting current at the moment of connection, which can last for hundreds of milliseconds. This results in a large voltage drop across the internal resistance of the transmission line, which is the main cause of voltage transients (surges and sags) in the power grid. These noises, superimposed on the sinusoidal AC voltage, propagate along the line, causing interference wherever they reach. If the amplitude is too large, it can damage the system. Additionally, poor equipment grounding can cause voltage drops and excessive deviations of the neutral wire from zero potential on branch lines, leading to surges and voltage dips between the live and ground wires, and between the neutral and ground wires. Transmission lines are floating, ungrounded systems; lightning strikes in their vicinity can induce voltage spikes of up to 3kV in the lines. Radiation sources such as car ignition, radio emissions, and electric arcs can all induce voltage spikes of thousands of volts in transmission lines. The switching of contacts in electrical appliances such as relays can induce voltage spikes of around 600V in the power grid. Excessively long ground wires, susceptible to interference from spatial electromagnetic fields, can also generate voltage spikes in the power grid. While these voltage spikes are short-lived and generally do not damage the system, they are extremely detrimental to the normal operation of microcontroller systems, causing logic corruption, program corruption, and even malfunctioning contactors. 3.2 Line and other interference If the circuit board design is unreasonable, the arrangement of components is not reasonable, or the layout between components in the line is unreasonable, there may be interference sources in the line itself, causing the microcontroller system to malfunction. The key to achieving synchronous disconnection is that the signal of the current sensor must be able to accurately reflect the current of the main circuit. Compared with the traditional current transformer, this current transformer should have stronger anti-interference performance. Under the influence of strong electromagnetic interference, it should be able to accurately reflect the change of the main circuit current, especially to detect the accurate current zero point. 4 Several main anti-interference measures adopted in this device 4.1 Hardware part (1) Design of reset circuit of microcontroller system Usually, microcontrollers have a reset pin for system reset. However, the reset circuit is easily affected by power supply fluctuations. When the microcontroller power supply is disturbed, the voltage drops to a low level, and the reset terminal potential also drops to a low level. Obviously, this will cause the microcontroller to reset, making the microcontroller unable to work normally. Therefore, the reset circuit is improved in the design. The reset circuit has a simple RC filter circuit connected in parallel between the reset terminal and ground, which effectively suppresses the influence of the microcontroller power supply fluctuation on the reset terminal. (2) The circuit adopts opto-isolation. The purpose of opto-isolation is to cut off the electrical connection between the two circuits. The control circuit of this contactor adopts the latest opto-device launched by Motorola Corporation of the United States - the opto-bidirectional thyristor driver as its main control element. This element can use low DC voltage and small current to control high voltage and large current. Its output is a normal wave with no waveform distortion, low electromagnetic interference and no noise. It is used to trigger the thyristor. The triggering circuit is simple and reliable, and it is an ideal driving element. This element consists of two parts: input and output. The input stage is a gallium arsenide infrared light-emitting diode (LED). Under the action of 5-15mA forward current, the diode emits enough infrared light to trigger the output part. The output stage is a photo-controlled bidirectional thyristor with zero-crossing detection. When the infrared light-emitting diode emits infrared light, the photo-controlled bidirectional thyristor is triggered and conducts, and sends a control signal to trigger the main control element. Because it uses opto-isolation and can be driven by TTL level, it is easy to interface with microcontrollers to perform real-time control of various automatic control equipment. It improves the anti-interference performance of the line. (3) There are two types of grounding: one is to ground the outer shell of the equipment for the purpose of personal or equipment safety, which is called outer shell grounding or safety grounding; the other is to provide a common potential reference point for the circuit operation, which is called working grounding. The circuit board of this intelligent AC contactor adopts working grounding. The current transformer of the intelligent AC contactor adopts an outer shell grounding system. In order to reduce the electromagnetic interference of the current signal circuit, the signal sent to the microcontroller adopts shielded twisted pair, and the shielding layer of the twisted pair is connected to the outer shell of the contactor. (4) Shielding technology High-frequency power supply, AC power supply, strong electrical equipment, electric sparks generated by electric arc, and even lightning can generate electromagnetic waves, thus becoming sources of electromagnetic interference. When the distance is close, the electromagnetic waves will couple to the signal circuit through distributed capacitance and inductance to form electromagnetic interference; when the distance is far, the electromagnetic waves will form interference in the form of radiation. The power transformer adopts a shielding layer grounding between the primary and secondary sides to solve the problem of power grid interference. Shielding structures made of metal plates, metal mesh, or metal boxes can effectively combat electromagnetic radiation interference. 4.2 Software Section When a microcontroller is interfered with, it will produce more complex situations than a general circuit. For example, a normally executing program may jump to an unpredictable address due to interference, causing the program to become chaotic and resulting in unimaginable consequences and accidents. In order to prevent the above situation from occurring, in addition to taking necessary anti-interference measures in hardware, the anti-interference capability of the software should be fully explored in the microcontroller program design, and certain measures should be taken to minimize the impact of interference. (1) Using a watchdog timer During the operation of the program, sometimes due to the influence of some noise interference, dead loops or program "random flying" may occur, thus affecting the normal operation of the system. Both of these situations can be monitored by using the watchdog timer WDT inside the PIC microcontroller. Because when the system is working normally, the program clears the counter at regular intervals, and the counter counts by clock pulses. When this time is shorter than the overflow time of the watchdog timer, the counter will never overflow. However, if the system is disturbed, the normal execution order of the program is disrupted, and the counter cannot be cleared before it overflows, thus causing the counter to overflow. Therefore, the counter overflow can be used as a sign that the system is disturbed. This intelligent AC contactor can set the timer overflow time by setting the status register, and clear the timer during program execution using the CLRWDT instruction, thereby preventing the timer from overflowing during normal program execution and resetting the system. It can effectively eliminate the influence of interference. (2) Software traps Usually, there is a lot of unused space in the ROM storage area of ​​the CPU, and when the program is disturbed, it often jumps to these addresses. In order to capture this interference, traps can be set in these areas - boot program segments. Once the program falls into this area, it will be guided to a specific processing program and restored to normal. The advantage of this measure is that the anti-interference processing is simple. The disadvantage is that it is related to the amount of unused storage of the CPU. The more unused space there is, the greater the probability of capturing interference, so it has certain limitations. (3) Software suppression of power interference Although measures can be taken in hardware to suppress power interference, in actual operation, some power noise will still enter the system, causing software reset and disrupting the normal execution order of the program. To resist such interference, a program is arranged at the beginning of the program, which can determine the closing, holding and disconnecting states of the intelligent AC contactor. Source: After initialization, the power transmission and distribution equipment network system enters the annual review state, and determines the current working state of the system based on the voltage and current signals measured by the voltage and current detection unit to ensure the reliable operation of the system. (4) Digital filtering When the microcontroller calculates the closing voltage and releasing voltage, it adopts the digital filtering method, which can eliminate the problem of malfunction caused by inaccurate sampling signals due to electromagnetic interference. 5 Conclusion The intelligent AC contactor improves the reliability of the entire system and production process, and brings the control level to a new height. In order to meet the needs of intelligent network systems, the development and research of multifunctional, intelligent, high-performance, and highly reliable intelligent AC contactors has become an urgent task. The reliability of intelligent AC contactors directly affects the promotion and operation of the product. This article is some of the work we have done in this field.