Analysis on the application of computer and microprocessor technology in low voltage electrical appliance market

0 Introduction

With the substantial improvement of the performance-price ratio of single-chip microcomputers, the application of computer and microprocessor technology to industrial control, especially to increase the intelligent functions of low-voltage electrical appliances, has great market economic potential. We have done a lot of research and feasibility analysis in the intelligent control scheme and specific implementation of AC contactors, and developed a device with intelligent functions. By combining this device with AC contactors, the intelligent functions of AC contactors can be increased. It has the functions of simple setting, reliable use, energy-saving control, online setting change and display. When the single-chip microcomputer receives a closing or breaking command, it can control the three contacts to break and close at zero according to the optimal breaking and closing phase angles, reducing the spark energy. By using its two-way communication with the central control computer, a local 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. Contactor intelligent content and working principle

The contactors, circuit breakers and protectors (such as thermal relays) currently used in my country are all mechanical non-intelligent. Generally, they are AC pull-in, AC holding and random disconnection, and the coil voltage is divided into 220V and 380V. Experiments tell us that whether it is a 220V or 380V coil, as long as a DC voltage of not less than 160V is added, the contactor can be reliably pulled in and will not produce one or two bounces. At this time, as long as the holding voltage is maintained at not less than DC 15V, the pull-in state can be stably maintained. Once the disconnection process occurs, it is inevitably accompanied by the generation of an arc. The only principle to determine when the disconnection process occurs is to minimize the total energy of the arc under the premise of time permitting. For a single-phase electromagnetic circuit, the best time for the contact to close and break should be when the main circuit current passes zero, while for a three-phase electromagnetic circuit, if the disconnection process occurs at the moment when a phase current passes zero, the total energy of the three-phase arc should be the minimum at this time. Controlling the zero-crossing disconnection of the three contacts in turn can make them have the same service life. Depending on the control object and requirements, this solution uses relays or thyristors as components to control the on and off of the contactor coil. Under the control of the single-chip microcomputer, the system runs and displays according to the set requirements. The entire circuit is simple in structure and runs smoothly. When the startup process is completed, the high voltage is automatically removed and the freewheeling circuit remains in the energized state.

After the power is turned on, the rectifier circuit, the voltage-stabilizing power circuit and the single-chip microcomputer system are powered. After the phase current is set, the single-chip microcomputer starts sampling and comparing with the set value, and the start circuit and the freewheeling circuit system are in standby state. While the single-chip microcomputer continuously samples, compares and records the power supply voltage and phase, it waits for the start signal. If a signal is received, a corresponding high voltage will be added to the coil package through the start circuit in a timely manner within a time of no more than 20ms. Under the suction force generated by strong excitation, the moving contact overcomes the spring thrust and inertia and quickly moves toward the static contact. The single-chip microcomputer can judge the contact's pull-in condition through the sensor and control the freewheeling circuit to provide a suitable holding voltage in time. Once it is found that the power supply voltage is less than the release voltage, the single-chip microcomputer immediately selects the appropriate phase, stops supplying power to the coil package, and the contact is reset under the action of the spring. Obviously, during the startup process, the phase current will surge or even exceed the set value. The degree of current surge and the duration of the surge are related to the load carried by the motor. This can be done by adding relevant procedures according to ministerial standards and industry requirements, so that different load conditions and even short circuits can be distinguished well and the corresponding starting protection time can be automatically selected, so that the motor can be started under load.

2. Selection of MCU and design of basic hardware

For the convenience of installation and use, the input voltage can be designed to be adaptive. The sensor is a winding on a magnetic ring or silicon steel sheet and sent to the single-chip microcomputer through a rectifier. After the single-chip microcomputer samples and compares through the sensor, it finally outputs a level signal or a pulse width signal of a certain frequency to drive the actuator. However, strong electric and magnetic interference often causes malfunction. For this reason, it is necessary to take a number of measures to resist interference. In addition to traditional measures such as shielding and photoelectric isolation, interference can also be suppressed by bandpass filtering and sampling algorithms. The single-chip microcomputers are respectively selected from the M68HC series of Motorola Company and the PIC series of Microchip Company. Motorola's single-chip microcomputers are widely used in automobiles and have strong anti-interference performance. Microchip's PIC series single-chip microcomputers have internal A/D, so their peripheral circuits are relatively simple. Since these single-chip microcomputers have relatively rich internal resources and use time-sharing dynamic scanning mode for display, their external components are very few, which not only reduces costs but also greatly improves reliability.

3 MCU main program block diagram

The degree of intelligence of the AC contactor mainly depends on the selection of the control scheme and the compilation of the software.

When the program is executed, it first samples the grid voltage. If the unbalance of the grid voltage is greater than 30%, the program refuses to continue and displays this item with a light-emitting diode. After this test is passed, the microcontroller selects the next contact as the target contact according to the phase synchronization signal and the record of the last zero-crossing contact that was closed and disconnected. At the same time, according to the value of the sampled voltage, it selects the appropriate closing phase angle and enters the control standby state. As soon as the closing command arrives, the microcontroller immediately executes the closing subroutine. When the closing is completed, the microcontroller turns off the main control device, and the system automatically enters the holding stage. The coil maintains the excitation closing state with low voltage and low current.

4 Experimental studies

The feasibility experiment was conducted on the development device. Considering the anti-interference ability and easy operation of the system, the 89C52 single-chip microcomputer with easy erasing and programming was selected, and combined with the JZ7 intermediate relay (380V, 5A) produced by Suzhou Machine Tool Electric Factory. A large number of experiments were carried out, and the basic situation is as follows.

(1) Closing and disconnecting process

The working voltage of the contactor winding is 380V, and it cannot be closed under the condition of AC 220V. Combined with the intelligent device, the closing is completed in one time under the condition of 220V. Observed by an oscilloscope, the maximum time error between the closing instruction and the actuator does not exceed 40ms, and more than 76% is within 20ms. Under the premise of no primary or secondary bounce, the closing current is 0.12A, and the holding current is only 6mA, which is quite ideal. When the external voltage drops to 160V, the contact is automatically disconnected, and the single-chip microcomputer is in the sampling and standby state. A large number of experiments tell us that for the same type of contactors and circuit breakers, their spring elastic force and moving point mass are basically the same, so they have the same inertia. The fluctuation of the grid voltage makes the time for the magnetic force to overcome the inertia and move the same distance different, so the conduction phase angle and conduction time should also be different. The best closing phase angle and closing time of different types of contactors and circuit breakers under different voltages are tabulated, and the single-chip microcomputer controls them by looking up the table, so that the contactor can work in the best state. The breaking process has similar conclusions. Here the single chip microcomputer has to do two things: replace the sampling contact and determine the breaking moment. The latter task 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 breaking, and the relevant data in the table are automatically updated through the self-learning function to improve the intelligence of the module.

(2) Suction process

After the start-up process is completed, the coil works in a low voltage and low current state. While maintaining this state, the single-chip microcomputer monitors the pull-in voltage and the imbalance of the power grid. Once there is a short circuit, a phase failure, the imbalance of the power grid voltage exceeds 30%, or the power grid voltage is lower than 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 failure, thereby protecting the safety of the equipment.

(3) Communication networking: The serial port of the single-chip microcomputer is connected to the microcomputer RS232, and the control parameters of the single-chip microcomputer can be modified from the microcomputer keyboard or mouse. At the same time, the current acquisition values ​​of each channel of the single-chip microcomputer are read back to the microcomputer display. In this way, after networking, the operating status of each controlled object can be monitored in real time, which is easy to operate.

5 Conclusion

By choosing different single-chip microcomputers, products with strong anti-interference ability and reliable operation can be formed. Although its anti-interference ability is not as good as PLC, its high performance-price ratio and satisfactory operation effect are still achieved. Combined with new sensor technology, intelligent products with learning functions including monitoring contact temperature can be added. Combined with a large number of contactors and circuit breakers in my country, it will be a favorable opportunity for my country's low-voltage electrical appliances to get rid of backwardness and get out of the trough, and will definitely contribute to improving the grade of my country's low-voltage electrical appliances.