Shandong Inverter Maintenance: Soft starter power-off solution: Some people to use soft starter to control the motor, often appear power-off situation, many people wonder how power-off, because people understand that the working voltage of power-off is more than 100 volts to 200 volts, slightly inadvertent will threaten life. So for these problems, the editor tells you why.
一、漏电问题产生的原因根据变频器控制电机运行的功能框图（图1），三相电源经过变频器整流桥整流之后，经电容滤波送到逆变桥（IGBT），再经过逆变桥输出频率、电压可调的三 相交流电去控制电机的运行。三相互差120度的交流电在电动机的三相定子线圈绕组里流过，产生旋转磁场，使电动机的转子在定子绕组旋转磁场的作用下自动旋 转起来。
First, according to the functional block diagram (fig. 1) of the converter to control the motor operation, the three-phase power supply is rectified by the converter rectifier bridge, filtered by capacitor to the IGBT, and then controlled by the three-phase alternating current with adjustable output frequency and voltage of the inverter bridge. The alternating current of 120 degrees of three-phase difference flows through the three-phase stator coil winding of the motor, which generates a rotating magnetic field, so that the rotor of the motor can rotate automatically under the action of the rotating magnetic field of the stator winding.
我们都知道，电动机的三相定子绕组流过电流之后产生了旋转磁场，而根据电磁感应的原理，电动机的外壳就会产生感应电动势。此感应电动势的大小，就取决于变 频器IGBT的开关频率的大小和C*DV/DT（与IGBT的开关的速度有关）；由于高性能的控制要求较高的开关频率，其开关速度要求较快，则DV/DT 偏大。如果这个感应电动势较大，那么人触摸到就会感觉被电击一样。理论上IGBT的开关频率越高，电机外壳的感应电动势的有效值（即感应电压）就越高，而 变频器对电机的控制精度和动态响应也就越高，人体触摸之后被电的感觉就越大；反之，IGBT的开关频率越低，电机外壳的感应动势的有效值（感应电压）就越 低，而从体触摸到之后被电的感觉就越小。所以，某些国产低端的变频器IGBT的开关频率设计得较低，控制电机运行之后，电机外壳的感应电压较低，但其控制 性能较差、动态响应较慢。我司变频器的性能和动态响应都较好，因而我司变频器IGBT的开关频率和开关速度都较高，感应电动势相对也就会大些。
As we all know, the three-phase stator winding of the motor generates a rotating magnetic field after flowing through the current. According to the principle of electromagnetic induction, the shell of the motor generates an inductive electromotive force. The magnitude of the induced EMF depends on the switching frequency of IGBT and the switching speed of C*DV/DT (which is related to the switching speed of IGBT). Because of the high switching frequency required by high performance control, the switching speed of IGBT is faster, and the switching speed of DV/DT is larger. If the induction electromotive force is large, people will feel shocked when they touch it. In theory, the higher the switching frequency of IGBT is, the higher the effective value of inductive electromotive force (i.e. inductive voltage) of the motor housing is. The higher the control accuracy and dynamic response of the converter to the motor, the greater the feeling of being electrically charged after human touch. Conversely, the lower the switching frequency of IGBT, the lower the effective value of inductive electromotive force (inductive voltage) of the motor housing, and the lower the body touch to the body touch. The less electric you feel afterwards. Therefore, the switching frequency of IGBT, a low-end converter made in China, is designed to be low. After controlling the motor, the inductive voltage of the motor shell is low, but its control performance is poor and its dynamic response is slow. The performance and dynamic response of our converter are good, so the switching frequency and switching speed of our converter IGBT are higher, and the induction electromotive force is relatively larger.
由于异步电动机运转，电机外壳都会有感应电压（即所谓的漏电），所以，电机制造厂才会在电动机出厂的时候，在其接线盒里面安上接地端子，方便用户在应用时 连接大地以消除其感应电动势（即消除感应漏电电压），以解决人体接触电动机时被电的感觉。当然，因为工频运行电机时，工频的开关频率约为50HZ，很低， 所以一般情况下几乎不会有漏电的感觉（除非电机绝缘很差）。而变频器控制时，由于其开关频率都比工频频率高得多，所以变频器在控制电动机转动时，电机外壳 就会有漏电的感觉。
Because the asynchronous motor runs, there will be induction voltage (so-called leakage) in the motor shell. Therefore, when the motor manufacturer leaves the factory, it will install a grounding terminal in its junction box to facilitate the user to connect the earth to eliminate the induction electromotive force (i.e. eliminating the induction leakage voltage) in order to solve the feeling of electricity when the human body contacts the motor. Of course, because the switching frequency of the power frequency is about 50HZ when the motor is running at the power frequency, it is very low, so there is almost no sense of leakage (unless the insulation of the motor is very poor). The frequency converter control, because its switching frequency is much higher than the power frequency, so when the frequency converter control the motor rotation, the motor shell will have the feeling of leakage.
2. Solution to leakage problem. In order to avoid this problem, the inductor surge filter circuit (its equivalent circuit is shown in Figure 1) is added to the design of the converter hardware, and the grounding end of the inductor surge filter is connected to the housing of the converter. In the wiring description of the converter, the grounding end of the motor is required to be connected with the grounding terminal B of the converter. Connect the ground of the input power supply (i.e. the ground) to the ground terminal A of the frequency converter.
从而使得电动机运转产生的感应电流能够通过电机与变频器的接地线、变频器与电源之间的接地线形成回路，使得电动机的地、变频器的地与电源的地（即大地）都 处于同等的电位上，它们之间的电位差为0伏电压。这样，人体站在大地上面（也是电源的地）接触到旋转的电动的外壳、机械设备的机架（一般设备的机架是与大 地连接在一起的）、变频器的外壳就都不会有被电的感觉了，因为它们之间的电位差（电压差）为0伏，人体也就没法感觉出来是否有电了。
Thus, the induction current generated by the motor can form a loop through the grounding wire between the motor and the frequency converter, and the grounding wire between the frequency converter and the power supply, so that the ground of the motor, the ground of the frequency converter and the ground of the power supply (i.e. the ground) are at the same potential, and the potential difference between them is 0 volt. In this way, the human body standing on the ground (also the ground of the power supply) touches the rotating electric shell, the frame of the mechanical equipment (the frame of the general equipment is connected with the ground), and the shell of the frequency converter will not feel electrified, because the potential difference (voltage difference) between them is 0 volts, and the human body can not feel whether there is electricity or not.
当电动机的地线未能与变频器的接地端连接在一起，而电源的地线也没有与变频器的地、机械设备的外壳或者电的接地端接在一起的时候，电机的外壳、变频器的外 壳、电源的地（即大地）就不是处于同等电位了。假如在这种情况下，电动机运转产生的感应电压为100V，电动机又与机械设备的某部分机架在一起，因为电源 的地线在配电房没有拉过来，而人体的电气等效模型理论上可以等同于一个约2K欧姆的电阻（如果人体出汗、潮湿时电阻值更小，有时甚至只有几十欧姆），人体 站在大地上触摸到与电动机相连的设备金属时，电动机的感应电（如100V）就能过人体向大地进行放电，那么人体就会有电流流过，就会有被电的感觉。虽然， 理论上电机外壳与机械设备的机架是连接的，而设备的机架又是装在大地上的，按理说人站在大地上触摸到设备机架应该是不会被感应电触电到的，但是，别忘了大 地虽然也是属于导体，但大地毕竟是有阻值的，而且根据不同的土地的土壤成份，阻值也大小不一。否则，为什么国家
When the ground wire of the motor is not connected with the ground end of the frequency converter, and the ground wire of the power supply is not connected with the ground end of the frequency converter, the shell of the mechanical equipment or the ground end of the electricity, the shell of the motor, the shell of the frequency converter and the ground of the power supply (i.e. the earth) are not at the same potential. If, in this case, the induction voltage produced by the motor operation is 100V, and the motor is connected with some part of the frame of the mechanical equipment, because the ground wire of the power supply is not pulled over from the distribution room, and the electrical equivalent model of the human body can theoretically be equivalent to a resistance of about 2K ohms (if the human body sweats and is wet, the resistance value is smaller, sometimes only tens of ohms). When the human body touches the equipment metal connected with the motor on the ground, the inductive electricity of the motor (such as 100V) can be discharged through the human body to the earth, then the human body will have the current flowing through, and the feeling of being electrified. Although, theoretically, the motor shell is connected with the machine frame of the mechanical equipment, and the machine frame of the equipment is mounted on the ground, it is reasonable that people standing on the ground touch the machine frame of the equipment should not be touched by the induction electric shock, but do not forget that although the earth is also a conductor, after all, the earth has resistance value, and according to different soil components of the land, the resistance value is also large. Small differences. Otherwise, why the state