基于ATMEGA16型单片机的智能无弧交流接触器控制方法

本文针对传统变流接触器分合阐过程中产生的电弧对触头侵蚀的问题，提出了智能无弧交流接触器全新的控制方法。该接触器以传统交流接触器为主体，实现三相电路的灵活控制，在主电路中每相触头并联一个单向晶闸管，以此来实现接触器在吸合和分断过程中的无弧控制，大大提高了接触器电寿命。并且将智能结构的理念带入了传统交流接触器中，采用ATMEGA16单片机作为控制核心，使智能无弧交流接触器集数据采集，故障诊断与一体，实现了交流接触器状态的实时监测。
    传统交流接触器在分断电路的过程中产生的强烈电弧不仅造成触头磨损，降低了接触器的电寿命；同时交流接触器激磁系统多由交流电控制，不仅产生很大的交流噪声，而且线圈功耗特别大。本文针对传统交流接触器激磁系统及灭弧系统提出了一种简易、可靠的解决方案。根据国内外对智能交流接触器的研究总结，本文采用了无触点分断方案，简单来讲就是在接触器触头两端分别并联一个单相晶闸管，使其在接触器分合闸过程中起到分流作用进而达到无弧分合闸的目的。同时针对传统交流接触器励磁线圈采用交流励磁功耗大的缺陷提出了直流稳压激磁、直流小电压保持方案，不仅可以保证触头稳定闭合，而且还可以减小接触器二次弹跳几率及线圈功耗，提高其使用寿命。
    同时，随着计算机、单片机及通信技术的发展，为智能无弧交流接触器的控制系统提供了强大的技术支持。本文除了对传统交流接触器激磁系统及触头灭弧系统加以改进外，还引入了智能结构的思想，将传感器、驱动器、控制器集于一体，通过实时数据的采集及数据处理，对接触器的运行状态进行及时判断，提高了接触器的实时性。同时还会对接触器各相电压、电流等参数进行实时检测与判断，从而能准确地对接触器的运行状态及故障实现实时检测。

1 系统总体方案及其工作原理
    智能无弧交流接触器将传统的交流接触器与电力电子器件相结合，对接触器触头系统进行改进，具体改进方法如图1所示。同时实现励磁线圈电压自动控制、运行状态在线自动监测、实时故障自诊断及故障定位。采用以ATMEGA16单片机为控制核心，通过相应传感器对关键参数进行实时采集与分析，进而对接触器运行状况进行实时准确的判断。系统县体实现原理如图2所示。

    
    图2中KM为普通交流接触器，选择工作场合为工频50Hz，相电压220V，线电压380V。通过对负载各相电压的监测判断，即可知道系统是否处于过压、欠压及缺相(由缺相保护电路检测)运行。若发现负载正在缺相运行，可立即封锁PWM号，使系统停止运行并给出故障信息。若系统处于欠压状态可以给出故障报警和实际电压值，根据检测的三相电压值计算三相负荷不平衡度，若在运行范围内即不影响正常工作时，负载保持运行状态；当三相不平衡度超过限定值则停止系统运行进行故障检修，并给出报警信号。

1．1 无弧通断工作原理
1．1. 1 接触器吸合过程导通区域分析
    根据图1所示主电路接线方式的分析，只有SCR1承受正向电压，SCR2承受正向电压，SCR3承受负向电压时才能保证三个晶闸管同时满足闭合基础条件(即Uscr1>0，User2>0，Usor3<0)，使其在共同导通区域内完成触发操作，从而保证三相触点在共同导通区域内无弧吸合。

    
    针对图3所示，当系统接到吸合信号时，开始对接触器线圈进行激磁操作，而后通过过零检测电路检测到A相电压的左侧过零点，这之后SCR1便进入了导通区域，延时3．33ms后SCR3进入导通区域，再延时3．33ms后SCR2进入导通区域，从而可以在图中找到三个晶闸管同时满足导通条件的区域如(图中阴影部分)，所以在阴影区域同时触发三个晶闸管，从而接触器可以实现无弧吸合。
1．1．2 接触器分断过程导通区域分析
    接触器吸持过程中触头接触电阻很小，故触头两端电压很低，从而晶阿管两端电压很低。由此接触器分断过程以主回路电流为判断依据，如图4所示，分析如下。

    
    针对图4所示，当系统接到分断信号时，首先切断保持电源，而后通过过零检测电路检测到A相电流的左零点，这之后SCR1便进入导通区域，延时3．33ms后SCR3进入导通区域，再延时3．33ms后SCR2进入导通区域，根据这个条件可以在图中找到三个晶闸管同时满足导通条件的区域(如图中阴影部分)，由此可以在该区域给晶闸管触发信号使接触器完成分断过程，实现无弧分断控制。

1．2 接触器线圈激磁系统工作原理
    目前，传统交流接触器一般采用交流吸合、交流吸持和随机分断的方式。实验得知，无论是380V线圈还是220V线圈，只要加上不低于160V的直流电压，接触器均能可靠吸合，并且减少二次弹跳几率，稳定吸合时只要维持电压不低于15V，就可以稳定保持吸合状态。而且采用直流运行方式可以从根本上消除交流接触器的噪声和振动，大大降低能耗，提高使用寿命。通过实验发现，当对接触器线圈采用240V激磁电压、24V保持电压策略时，接触器不仅可以可靠闭合，触头碰撞回跳时间也很短。所以智能交流接触器采用了稳压直流可控激磁、低压直流保持的电磁系统智能化操作方案。
    本实验在传统交流接触器的基础上，对其控制回路采用智能化控制，通过对施加在接触器线圈两端的电压进行PWM控制及自适应调节，实现了智能无弧交流接触器在宽电压范围(AC240V～380V)内均能高品质可靠稳定工作，同时还解决了由于电网电压波动过大引起接触器工作不稳定的问题。激磁系统控制原理图如图5所示。

    
    由图5所示电路产生接触器激磁电压240V和保持电压24V。在此我们先讨论输出240V的情况。交流输入电压经整流电路整流后通过电力电子器件IGBT施加到接触器激磁线圈上，微控制器根据测量电路检测输入电压大小，由PWM模块产生适当频率及占空比的脉冲触发IGBT，控制IGBT的导通与截止，最后得到直流激磁电压240V加到接触器线圈两端。系统还具有线圈电压检测电路，若线圈电压高于240V，则按一定比例减小PWM占空比，同样，若线圈电压低于240V，则按一定比例增大PWM占空比，保证了接触器的稳定吸合。最后，在接触器激磁结束后，通过单片机控制系统快速调节占空比以产生保持所需要的24V直流小电压，从而接触器实现节能、无噪声吸合。

    
    Figure 6 shows the schematic diagram of the PWM pulse modulation circuit. After the input voltage passes through the rectifier circuit, it is added to both ends of the contactor coil through the IGBT switch tube. The PWM pulse generated by the modulation in the ATMEGA16 microcontroller is input through the right-end PWM. When the microcontroller outputs a low level, the optocoupler is turned on, current flows through R2, the field effect tube VT1 is turned off, VT2 is turned on, and the IGBT is forward biased and turned on; when the microcontroller outputs a high level, the optocoupler is turned off, no current flows through R2, the field effect tube VT1 is turned on, the VT3 tube is turned on, and the -5V voltage is added between the gate and emitter of the IGBT through R4, so that the IGBT is quickly turned off. Thus, the voltage at both ends of the contactor coil is controlled. The diode VD in the figure provides a freewheeling channel for the contactor coil.

2 Software Control Flow
    The software control flow chart of the intelligent arc-free AC contactor is shown in Figure 7. After the system is powered on, the single-chip microcomputer initializes the system first. At this time, if the single-chip microcomputer receives the pull-in button interrupt information, it will enter the pull-in program. According to the DC excitation scheme introduced in Section 1.2, the coil is excited, and the AC contactor enters the pull-in stage. According to Section 1.1, the zero point of the A-phase main circuit voltage is determined, the common conduction area of ​​the thyristor is found, and the unidirectional thyristor at both ends of the contact is triggered, so that the contact is arc-free in the common conduction area of ​​the thyristor. At this time, the 24V DC holding voltage has been added to both ends of the coil to ensure low-energy operation of the contactor.

    
    If the single-chip microcomputer receives the interrupt information of the disconnect button, the duty cycle is adjusted to zero through the PWM modulation circuit, that is, the power supply to the coil is interrupted, and the contactor enters the disconnection process. Then the single-chip microcomputer detects the zero-crossing point of the current in the A-phase main circuit through the current sampling circuit, and after finding the common conduction area of ​​the three thyristors, it triggers the thyristors to turn on. Thereby, the contactor is disconnected without arc. 3.

Upper computer software design
    The intelligent arc-free AC contactor has communication function and supports PC remote control. In order to realize the contactor status display and data processing, the system designs the upper computer part, and the user can easily monitor the real-time operation status of the contactor. The upper computer part of this system is designed with Labview2011 software, and the main interface includes the display part and the control part. The main interface is shown in Figure 8.

    
    The main interface mainly includes information query part, contactor control part, operation prompt and communication status display. The query part can query the voltage and current of the main circuit; at the same time, it can also graphically display the arc current waveform and voltage waveform. The contactor control part can control the opening and closing of the AC contactor; the status display mainly includes whether the communication status is normal and whether the contact status is normal. After laboratory experimental tests, the host computer designed by this system can communicate with the contactor correctly and complete the corresponding operations through the serial communication interface.

4 Conclusion
    The intelligent arc-free AC contactor control system designed with ATMEGA16 type single-chip microcomputer as the control core is a transformation of the traditional AC contactor. Only a unidirectional thyristor is connected in parallel at both ends of the contact to achieve arc-free control. At the same time, this system also uses PWM technology to adaptively adjust the excitation coil voltage, so that the contactor can work stably and reliably with high quality in a wide range of AC input voltage (AC 240V~380V), while achieving energy saving and noise-free operation. It has the advantages of versatility, intelligence, controllability, and communication. Therefore, this system has a very broad application prospect.