Design of intelligent AC contactor with anti-electricity shaking

Abstract: An anti-swaying intelligent AC contactor control module based on a switching power supply, with a single-chip microcomputer as the core and a backup power system has been designed and completed. The dynamic process control of the contactor is realized through the intelligent control module, forming a new type of electromagnetic intelligent contactor product with long life and high reliability, which can be widely used in petrochemical, steel, mining and other enterprises with high requirements for continuous production.

0 Introduction

As a special fault, power shaking is very harmful, especially for enterprises that operate continuously. The current methods to prevent power shaking can be summarized into the following four categories:

(1) Use power-off delay relays and motor restarters.

Through the timing relationship, the main contacts of the contactor are re-closed after the power swing ends (disconnected during the power swing), so that the motor can be restarted.

The characteristic of this anti-power swing method is that the main contact is disconnected during the power swing, and the motor is restarted after the voltage is restored. The impact current generated by the motor restart is large, the control circuit principle is complex, and the cost of the motor restarter is very high.

(2) Use energy storage delay elements to continue to provide energy to the contactor coil during the power swing period to ensure the main contact is closed. This anti-power swing method has the disadvantages of inflexible selection, small selection range, and increased complexity of the control circuit.

(3) Delayed locking head device, after the contactor is closed, the coil switches to power saving mode, and the main contacts are kept connected by the locking action of the locking head. When power shaking occurs, the main contacts of the contactor do not disconnect, and the main contacts are disconnected after normal shutdown operations. However, this locking head can only be used in conjunction with specially designed special contactors, and in the event of a power outage, the main contacts locked by the locking head need an independent power supply to disconnect. For contactors > 170 A, there is no matching locking device.

(4) The dual power supply mode is costly and complex. Currently, there are a small number of anti-sway AC contactor products on the market, but the price is high. The price of the same capacity anti-sway AC contactor is about 5 times that of the original AC contactor product.

Therefore, this paper develops a new type of anti-swaying intelligent AC contactor product, which adopts the structure of contactor body and intelligent control module. The intelligent control module has the advantages of compact size, precise control, strong versatility and low cost; at the same time, by intelligently controlling the starting and breaking process of the contactor, the mechanical life and electrical life of the contactor are greatly improved. With different contactor bodies, a series of anti-swaying intelligent AC contactor products can be formed, which have a high cost performance.

1 Working Principle

In addition to the functions of intelligent AC contactor, the anti-electrical shaking intelligent AC contactor also has the functions of anti-electrical shaking and remote control. Its control principle is shown in Figure 1.

Figure 1 Hardware block diagram of the anti-power-sway intelligent AC contactor.

The power supply voltage is sampled through the sampling circuit. When the sampled voltage is higher than 0.7 Ue (Ue is the rated voltage value) and less than 1.15Ue, the single-chip control system sends a signal to the isolation circuit 1, the strong excitation starting circuit is connected, and the contactor starts at a high voltage. The single-chip computer sends a drive signal to the isolation circuit 2, the low-voltage holding circuit is connected, the contactor is held at a low voltage, and then the drive circuit 1 is disconnected and exits operation.

When the sampling voltage is lower than 0.6 Ue, it is regarded as an electric shock fault. The single-chip microcomputer sends a signal to isolation circuit 3 and isolation circuit 2, and successively opens the anti-electric shock holding circuit and disconnects the low-voltage holding circuit. The fully automatic backup power supply system supplies power to the single-chip microcomputer system and the coil to keep the contactor in the attracted state. At this time, the sampling circuit continues to sample the power supply voltage. If the electric shock ends within the specified time and the voltage returns to normal, the single-chip microcomputer sends a signal to isolation circuit 2 and isolation circuit 3, and successively opens the low-voltage holding circuit and disconnects the anti-electric shock holding circuit to restore the normal holding state of the contactor. If the voltage has not recovered after N ms (N is adjustable), the single-chip microcomputer sends a signal to isolation circuit 3 to disconnect the anti-electric shock holding circuit and the contactor is disconnected.

If the contactor is manually disconnected when the power supply is normal, the normal disconnection detection circuit will detect the manual disconnection signal. At this time, the microcontroller shields the anti-power shaking program and disconnects the contactor immediately.

The fully automatic backup power supply system consists of a nickel-metal hydride rechargeable battery and a fully automatic charging circuit. When the anti-sway module is working, the charging circuit detects the battery voltage. When the voltage is lower than the set minimum value, the charging circuit starts to charge the battery. When the voltage reaches the set maximum value, it stops charging the battery and continues to detect the battery voltage in a cycle. When the power supply is normal, the anti-sway time can be adjusted through the anti-sway time adjustment circuit and displayed on the two-digit eight-segment digital tube. The microcontroller automatically stores the set time. When it is started next time, the last set value is loaded by default. The time adjustment range and adjustment gradient can be set according to user requirements. When the anti-sway time is adjusted to 0, the system turns off the anti-sway function and is used as a common intelligent contactor.

The intelligent module can work in two modes: independent and remote.

In independent working mode, it does not have communication function. In remote working mode, it can communicate with the host computer in two directions. The host computer can remotely control and adjust the connection, disconnection and anti-sway time of the intelligent control module, and the lower computer can transmit the current contactor status, anti-sway time, whether there is a shake and other signals to the host computer.

2 Software Design

The microcontroller software part of this article is programmed in C language, and the compiler selected is CCS PICC compiler. This compiler has rich internal functions, supports a variety of peripheral devices, and has standard input/output functions. It is convenient to program. CCS C is integrated into mplab for debugging, burning, and running of the program.

Figure 2 Main program flow chart of anti-sway intelligent AC contactor

The software flow is shown in Figure 2. After debugging, the software realizes the overall control function of the anti-electricity shaking intelligent AC contactor. After completing the anti-electricity shaking initial value loading, working mode judgment, threshold judgment, normal high-voltage starting, and low-voltage holding process of the contactor, the electric shaking detection program is started to detect the power supply voltage cyclically. If electric shaking is detected, timer 1 is turned on as a dedicated anti-electricity shaking timer and interrupts are turned on. The periodic interrupt of timer 1 is used to execute the timing of the anti-electricity shaking delay time in the timing interrupt subroutine. When the electric shaking time exceeds the set value, the anti-electricity shaking circuit is disconnected. When the power supply returns to normal within the set anti-electricity shaking time, the contactor enters the normal holding state and continues to detect electric shaking; the time adjustment subroutine uses an interrupt form to increase or decrease the anti-electricity shaking time, and the adjusted time is stored in the E2PROM for the next startup.

The communication receiving program also adopts the interrupt form. The operation interface of the serial port debugging assistant is shown in Figure 3. It can send operation instructions to the lower computer to perform on-off control and adjust the anti-electrical shaking time, and can monitor the current working status of the contactor in real time. The text box is used to display the instructions sent by the upper computer and the instructions received from the lower computer. It can also display the currently set anti-electrical shaking time and the current state value of the contactor; the normal disconnection detection module also adopts the interrupt form. It uses the capture function of the built-in * module of the single-chip microcomputer to capture a normal disconnection signal and immediately enter the interrupt to execute the normal disconnection program.

Figure 3 Serial port debugging assistant.

3 Debugging and test data

This paper uses proteus software as the simulation debugging tool. Proteus is a highly compatible software that can be integrated into the MPLab environment. MPLab calls proteus and supports breakpoints and single-step debugging in the MPLab environment, which can reflect the running process of the program. The simulation waveform of the virtual oscilloscope in proteus is shown in Figure 4.

In order to simulate the shaking voltage, a DC segmented pulse excitation source is selected to connect to the RA1 analog channel of the microcontroller. The first curve in the virtual oscilloscope represents the DC segmented pulse excitation source, simulating the sampling voltage; the second represents the strong excitation circuit, the third represents the low-voltage holding circuit; the fourth represents the anti-sway holding circuit; the vertical line ① represents the sampling voltage reaching the threshold, the vertical line ② represents the moment of shaking, and the vertical lines ③ and ④ represent the voltage recovery moment. It can be seen from Figure 4 that after the DC pulse excitation source reaches the threshold, the strong excitation circuit is opened for a very short time (about 15 ms) and then closed, and then the low-voltage holding circuit is opened (the third curve is set to a high level), and the contactor completes the normal starting and holding process; after that, although the voltage fluctuates, it is above the shaking limit, and the contactor is still in a low-voltage holding state (the third curve is kept at a high level). The power module in this paper adopts a single-chip switching power supply with the characteristics of wide voltage input. Therefore, even if the voltage drops slightly, it can still output a stable voltage in practice, so that the actual contactor is in a stable low-voltage holding state.

As shown in Figure 4, when the voltage drops to 60% of the rated value, it is considered that an electric shock occurs. At this time, the anti-electric shock circuit is opened (the second curve is set to a high level at the moment of electric shock, that is, time ②), and the low-voltage holding circuit is closed (the third curve is set to a low level at the moment of electric shock, that is, time ②); when the electric shock ends within the set time and the voltage returns to normal, the low-voltage holding circuit is opened (the third curve is set to a high level at the moment of voltage recovery, that is, time ③), and the anti-electric shock circuit is closed (the second curve is set to a low level at the moment of voltage recovery, that is, time ③). In the electric shock before time ④, the electric shock occurred for a period of time exceeding the preset anti-electric shock delay time. Within the entire delay time range, the anti-electric shock holding circuit is set to a high level, and immediately jumps to a low level after the delay time exceeds the preset value, and the contactor is disconnected at this time. From the waveform diagram of the virtual oscilloscope, it can be seen that the intelligent control module program has an anti-electric shock function.

On this basis, the installation and debugging of the overall hardware circuit were completed. The prototype hardware is shown in Figure 5. It can be seen that the anti-swaying intelligent control module is composed of three PCB boards and can work stably with an AC contactor below 160 A.

Figure 5: Physical picture of the prototype hardware.

The drive circuit test waveform is shown in Figure 6. Figure 6 (a) and Figure 6 (b) are the sampling voltage waveform, strong excitation drive signal, and low voltage holding drive signal. By comparing these three waveforms, it can be concluded that:

At the moment the system is powered on, the sampling voltage reaches the pull-in threshold, the control end of the strong excitation circuit is instantly set to a high level, the strong excitation circuit is connected, the contactor coil is connected to a 220 V high voltage, and the pull-in action is performed; after the pull-in process is completed, the control end of the low-voltage holding circuit is set to a high level, the control end of the strong excitation circuit jumps to a low level, the low-voltage holding circuit is connected, and the strong excitation starting circuit is disconnected at the same time. The contactor remains in the pulled-in state under low voltage, completing the high-voltage starting and low-voltage holding process control of the contactor.

In Figure 6 (c), when there is continuous power shaking (the voltage of the first curve drops) and the time of each power shaking is within the set value, the anti-power shaking circuit can be opened in time (the second curve is set to a high level when the first curve drops). When the voltage returns to normal, the anti-power shaking circuit can be cut off in time (the second curve is set to a low level when the voltage of the first curve recovers). In Figure 6 (d), when there is continuous power shaking, the signal waveforms of the low-voltage holding circuit and the anti-power shaking circuit are complementary; when power shaking occurs and the power shaking time is within the set value, the low-voltage holding circuit is cut off and the anti-power shaking circuit is opened; when the voltage recovers, the low-voltage holding circuit is opened and the anti-power shaking circuit is cut off; in Figure 6 (d), when the power shakes for the last time, the power shaking time exceeds the preset value, the low-voltage holding circuit is cut off, the anti-power shaking holding circuit is opened and maintained for the preset time before being cut off. At this time, the low-voltage holding circuit is still in the cut-off state, the contactor coil loses power, and the contactor is disconnected. At this point, the overall debugging of the anti-electrical shaking intelligent AC contactor has been completed, forming a new type of intelligent electrical appliance with the characteristics of high-voltage DC starting, low-voltage DC holding, anti-electrical shaking, fault delay time setting, energy-saving and silent operation, etc. It has the characteristics of stable operation and high cost performance.

Figure 6: Driving circuit test waveform.

4 Conclusion

The module has functions such as wide voltage input, universal AC and DC, DC high voltage starting, DC low voltage holding, energy-saving and silent operation, delayed disconnection against power outage, immediate disconnection upon power failure, and communication. By combining it with different contactor bodies, a series of intelligent AC contactor products against power outage can be formed.

Figure 4 Proteus simulation waveform.