1, the cause of the leakage problem
In some fields, using the inverter to control the motor will cause the leakage problem. The leakage voltage may range from tens of volts to 200 volts. The theoretical analysis and explanation of the cause of this fault are as follows.
According to the function diagram of the inverter control motor running (Figure 1), three-phase power is supplied to the inverter bridge (IGBT) after being rectified by the filter capacitor, and then the three-phase power with adjustable output voltage and voltage is used to control the operation of the motor. .
We all know that the three-phase stator windings of the motor generate a rotating magnetic field through current. According to the principle of magnetic induction, the motor's housing will generate an induced electromotive force. The magnitude of this electromotive force depends on the switching frequency of the frequency converter IGBT, due to the high The control of performance requires a high switching frequency, and its switching speed is very fast, so the DV/DT is large, and at the same time, the induced electromotive force is too large, and there is a feeling of electric shock on the human touch. In theory, the faster the switching speed of the IGBT, the higher the induced electromotive force on the motor housing, and the higher the frequency converter's control accuracy and response to the motor, the higher the sense of being touched by the human after touching, otherwise, the switching frequency of the IGBT. Slow, inductive power is small, the feeling of human touch is small, so the domestic low-end inverter design of the switching frequency is low, the control of the motor after the induction of electricity is small, people feel nothing to feel, but its control is poor, dynamic The response is slower. Our inverter performance and dynamic response are better, so our company's IGBT's switching frequency and speed are higher, and the induced electromotive force will be correspondingly larger. Because the frequency of the motor running frequency is only 50HZ, so there will be no leakage in one case, while the frequency conversion control, the switching frequency is high, the motor casing will have the feeling of leakage.
2, the solution to the problem of leakage
In order to avoid this problem, when the hardware is designed, an inductive surge filter circuit (the equivalent circuit is shown in Figure 1) is added, and the ground of the surge filter is connected to the shell of the inverter. At the same time, in the wiring description of the inverter, it is required to connect the grounding terminal of the motor with the grounding B of the inverter and connect the ground (ground) of the input power supply with the grounding A of the inverter so that the inductive power of the motor passes through the motor and The ground of the inverter and the ground wire of the inverter form a loop with the power supply, so that the ground of the ground of the motor and the ground of the power supply of the motor are at the same potential. The potential difference between them is 0 volts, so that the person stands on the ground The casing that touches the motor, the rack of the device, and the casing of the inverter will not feel like electricity.
However, in some factories, for the convenience of wiring, the ground wire was not pulled into the workshop in the high-voltage power distribution room, and even the mistaken belief that the earth was the ground wire was wrong. We might as well think about it. If the earth can be a local line, Then why should we pull the N box ground in our daily life? Is the N line in the power station also connected to the ground? Do we not need to pull the N box ground wire instead of saving a lot of wires? Why waste manpower, material resources, and time? However, in reality there are many factories that do not pull the power ground wire, and the equipment cannot find the grounding point. However, in the use of the motor, there are induced leakages. In this case, we provide two solutions:
Option 1: Connect the ground of the motor, the housing of the housing, and the ground terminal of the frequency converter together
After the three wires of the motor, frequency converter, and frame are connected together, they are at the same potential and are absorbed and discharged by the surge inside the inverter, so that the induced voltage is greatly reduced, so as not to make people feel shocked again. That is to say, there is no problem if there is no ground wire, as long as several grounds are connected together, so that the surge filter inside the inverter will play a role.
Option 2: Under normal circumstances, after the treatment of Option 1, there will be no electricity phenomenon, but due to special reasons, the induced voltage is still relatively high, you can also electricity, then under the premise of Option 1 in the inverter Add an inductive surge filter to the input power supply.
The ground of the inductive surge filter is connected to the ground of the motor and the ground of the inverter (as shown by the red line in Fig. 4). The inductive surge filter absorbs the induction of the motor again. Relieving, further reducing the induced voltage, to prevent the current leakage of electricity. The added circuit principle of the inductive surge filter is the same as the surge filter circuit inside the inverter. It is because the volume is too large to be designed and installed in the internal circuit of the inverter.
We have done a lot of experiments to prove that, through the site rectification of the scheme 2 connection method, the application of the ground wire without power supply can reduce the induced voltage generated by the motor operation to 20 V or less, ensuring the on-site operator. The safety will no longer be felt by people who have lost electricity. However, if the ground wire of the power cord is connected to the second scheme, then no external inductive surge filter can be used.
In addition, if there are multiple inverters controlling the running of the motor in the site, and it is not convenient to install multiple inductive surge filters, it is not always necessary that each inverter be equipped with an inductive surge filter. Connect only one or two inductive surge filters, and connect the ground of the filter to the ground of several frequency converters on the site, the ground of the field motor, and the equipment rack, as shown in Figure 5: Each inverter has an inductive surge filter circuit inside, but if the ground wire of the motor is not connected back to the ground terminal of the inverter, the inductive surge filter will not work, so in field applications The ground terminal of the motor must be connected to the ground terminal of the inverter. Of course, some devices may not have the feeling of leakage due to the ungrounded wire of the motor in some cases. This is the same as the “earth†mentioned above. Although the earth is also a conductor, the earth has a resistance after all, and according to the soil composition of different lands. The resistance is also different, the principle is the same. But in accordance with the correct electrical safety specification, it is required that the motor be well grounded, but the conditions are not allowed (such as no power ground), the ground of the motor, the shell of the electrical cabinet and the ground of the inverter can always be connected together.
Thread End Mills,Thread Cutting End Mill,Full Tooth Thread Milling Cutter,Multi-Tooth Coated Thread End Mill
Supal(changzhou)Precision Tools Co.,Ltd , https://www.endmillpro.com