Abstract: This paper describes the intelligent control scheme and principle of AC contactors based on microcontrollers. By combining the logic judgment and communication functions of the microcontroller with the AC contactor, intelligent control can be achieved. Its main functions include ensuring that after the AC contactor is engaged, it automatically executes a low-voltage DC holding subroutine, monitors overcurrent, overvoltage, undervoltage, three-phase imbalance, and leakage current in the equipment, and minimizes the spark energy when the contacts break, based on zero-crossing detection. Networking with a host computer can form a simple DCS system. Keywords: AC contactor, intelligent control, design, application 0 Introduction With the significant improvement in the performance-price ratio of microcontrollers, the application of computer and microprocessor technology to industrial control, especially adding intelligent functions to low-voltage electrical appliances, has great market potential. We have conducted extensive research and feasibility analysis on the intelligent control scheme and specific implementation of AC contactors, and developed a device with intelligent functions. Combining this device with an AC contactor can enhance the intelligent functions of the AC contactor. It features simple setup, reliable use, energy-saving control, online setting changes, and display functions. When a microcontroller receives a closing or opening command, it can control the three contacts to perform zero-crossing opening and closing in turn according to the optimal opening and closing phase angle, reducing spark energy. Utilizing its bidirectional communication with the central control computer, a local area control network and a simple DCS system can be formed. It has broad application prospects in industries, oil fields, coal mines, rural areas (irrigation systems), and cities. 1. Intelligent Content and Working Principle of Contactors Currently, the contactors, circuit breakers, and protectors (such as thermal relays) used in my country are all mechanical and non-intelligent. They generally operate on AC activation, AC holding, and random opening, and the coil voltage is either 220V or 380V. Experiments show that regardless of whether the coil is 220V or 380V, as long as a DC voltage of not less than 160V is applied, the contactor can reliably activate without primary or secondary bouncing. At this time, as long as the holding voltage is maintained at not less than 15V DC, the activated state can be stably maintained. Once the opening process occurs, an electric arc will inevitably be generated. The only principle for determining when the breaking process occurs is to minimize the total energy of the arc, given the time constraints. For single-phase electromagnetic circuits, the optimal moment for contact breaking is when the main circuit current crosses zero. For three-phase electromagnetic circuits, if the breaking process occurs when the current in one phase crosses zero, the total energy of the three-phase arc should be minimized. Alternating control of the zero-crossing breaking of the three contacts ensures they have the same service life. Depending on the controlled object and requirements, this scheme uses relays or thyristors as the elements controlling the contactor coil's on/off state. Under the control of the microcontroller, the system operates and displays according to the set requirements. The entire circuit is simple in structure and runs smoothly. When the startup process ends, the high voltage is automatically removed, and the freewheeling circuit remains engaged. The overall control principle is shown in Figure 1. As can be seen from the block diagram, after the power is turned on, the rectifier circuit, the regulated power supply circuit, and the microcontroller system are energized. After the phase current is set, the microcontroller begins sampling and comparing it with the set value, while the startup circuit and freewheeling circuit are in standby mode. The microcontroller continuously samples, compares, and records the power supply voltage and phase while waiting for the startup signal. If a signal is received, a corresponding high voltage will be applied to the coil through the starting circuit within a time of no more than 20ms. Under the attraction force generated by strong excitation, the moving contact overcomes the spring push and inertia, and quickly moves towards the stationary contact. The microcontroller can determine the contact engagement status through the sensor and control the freewheeling circuit to provide a suitable holding voltage in a timely manner. Once it is found that the power supply voltage is less than the release voltage, the microcontroller immediately selects the appropriate phase, stops supplying power to the coil, and the contacts reset under the action of the spring. Obviously, during the startup process, the phase current will surge and even exceed the set value. The degree of current surge and the duration of the surge are related to the load of the motor. According to the departmental standards and industry requirements, relevant programs can be added to distinguish different load conditions or even short circuits and automatically select the corresponding startup protection time, so that the motor can start under load. 2 Selection of microcontroller and design of basic hardware For ease of installation and use, the input voltage can be designed to be adaptive. The sensor is a winding wound on a magnetic ring or silicon steel sheet and fed into the microcontroller through a rectifier. After sampling and comparing data from sensors, the microcontroller outputs a level signal or a pulse width signal of a certain frequency to drive the actuator. However, strong electrical and magnetic interference often causes malfunctions. Therefore, multiple measures are needed to combat interference. Besides traditional methods such as shielding and opto-isolation, bandpass filtering and sampling algorithms can also be used to suppress interference. The microcontrollers selected are the M68HC series from Motorola and the PIC series from Microchip. Motorola's microcontrollers are widely used in automobiles and have strong anti-interference capabilities, while Microchip's PIC series microcontrollers, with their internal A/D converters, have relatively simpler peripheral circuitry. Due to the rich internal resources of these microcontrollers, a time-division dynamic scanning mode is used for display, requiring very few external components, which not only reduces costs but also greatly improves reliability. 3. Microcontroller Main Program Flowchart The level of intelligence of the AC contactor mainly depends on the selection of the control scheme and the software development. The overall program flowchart is shown in Figure 2. During program execution, the mains voltage is first sampled. If the voltage imbalance exceeds 30%, the program refuses to continue execution and displays this information using an LED. After this test passes, the microcontroller selects the next contact as the target contact based on the phase synchronization signal and the record of the previous zero-crossing contact during engagement and disengagement. Simultaneously, it selects an appropriate engagement phase angle based on the sampled voltage value and enters the control standby state. Upon receiving the engagement command, the microcontroller immediately executes the engagement subroutine. After engagement, the microcontroller cuts off the main control device, and the system automatically enters the holding stage, where the coil maintains the excitation engagement state with low voltage and low current. 4. Feasibility Study The experiment was conducted on the development device. Considering the system's anti-interference capability and ease of operation, the 89C52 microcontroller, which is easy to erase and rewrite, was selected, combined with a JZ7 intermediate relay (380V, 5A) produced by Suzhou Machine Tool Electrical Factory. A large number of experiments were conducted, and the basic situation is as follows. (1) The working voltage of the contactor winding during the closing and opening process is 380V, and it cannot be closed under AC 220V conditions. Combined with intelligent devices, the closing is completed in one go under the same 220V conditions. Observed with an oscilloscope, the time error between the closing command and the actuator is no more than 40ms, and more than 76% are within 20ms. Under the premise of no first or second bounce, the closing current is 0.12A, while the holding current is only 6mA, which is quite ideal. When the external voltage drops to 160V, the contacts automatically open, and the microcontroller is in sampling and standby mode. A large number of experiments tell us that for the same type of contactor and circuit breaker, their spring force and moving point mass are basically the same, and therefore they have the same inertia. The time it takes for the magnetic force to overcome inertia and move the same distance due to the fluctuation of the grid voltage is different, so the conduction phase angle and conduction time should also be different. The optimal pull-in phase angle and pull-in time of different models of contactors and circuit breakers under different voltages are made into a table. The microcontroller controls the contactor by looking up the table, which can make the contactor work in the best state. The disconnection process has similar conclusions. Here the microcontroller has to do two things: change the sampling contact and determine the disconnection time. The latter work is also completed by looking up the table based on a large number of experiments. At present, new sensing technology and process sampling technology are being used to improve the closed-loop control of zero-crossing disconnection. The relevant data in the table is automatically updated through the self-learning function to improve the intelligence level of the module. (2) After the start-up process of the holding process ends, the coil works in a low voltage and low current state. While maintaining this state, the microcontroller monitors the pull-in voltage and the grid imbalance. Once there is a short circuit, phase loss, grid voltage imbalance exceeding 30%, or grid voltage below 160V and the starting current exceeds the set value, the control circuit immediately enters the disconnection subroutine according to the zero-crossing requirement to disconnect the power, thereby protecting the safety of the equipment. (3) Communication networking: The microcontroller is connected to the microcomputer via its serial port and RS232 interface. The microcontroller's control parameters can be modified via the microcomputer keyboard or mouse, and the current acquisition values ​​of each channel of the microcontroller can be read back to the microcomputer for display. In this way, the operating status of each controlled object can be monitored in real time after networking, which is easy to operate. 5 Conclusion: By selecting different microcontrollers, products with strong anti-interference capabilities and operational reliability can be formed. Although their anti-interference capabilities are not as good as those of PLCs, their high performance-price ratio and operating effect are still satisfactory. Combined with new sensing technologies, intelligent products with learning functions, including monitoring contact temperature, can be added. Combined with the large number of contactors and circuit breakers in my country, this will be a favorable opportunity for my country's low-voltage electrical appliances to get rid of backwardness and emerge from the trough, and will surely contribute to improving the level of my country's low-voltage electrical appliances.