VFD (Variable-frequency Drive) control motor leakage problem how to do

1. The cause of the leakage problem

In some sites, the use of VFD (Variable-frequency Drive) to control the motor will cause leakage problems, and the leakage voltage ranges from tens of volts to 200 volts. Here, the theoretical analysis and explanation of the cause of this failure are as follows.

According to the functional diagram of VFD (Variable-frequency Drive) control motor operation (Figure 1), the three-phase power is rectified and supplied to the inverter bridge (IGBT) through the filter capacitor, and then the three-phase power with adjustable frequency and voltage is output through the inverter. To control the operation of the motor.

We all know that the three-phase stator winding of the motor flows through the current to generate a rotating magnetic field. According to the principle of magnetoelectric induction, the shell of the motor will generate an induced electromotive force. The magnitude of this electromotive force depends on the switching frequency of the VFD (Variable-frequency Drive) IGBT Because the high-performance control requires a high switching frequency, the switching speed is very fast, so the DV/DT is too large, and at the same time, the induced electromotive force is too large, and people feel electric shock when they touch it. In theory, the faster the switching speed of the IGBT, the higher the induced electromotive force on the motor casing, and the higher the control accuracy and response of the VFD (Variable-frequency Drive) to the motor, the higher the feeling of being electrified after touching, and vice versa , The switching frequency of IGBT is slow, the induction current is small, and the feeling of human touch is small, so the switching frequency of the low-end VFD (Variable-frequency Drive) design in China is low, the induction current is small after controlling the motor, and there is no touch. It feels good, but its control is poor and its dynamic response is slow. Our company's VFD (Variable-frequency Drive) has better performance and dynamic response, so the switching frequency and speed of our company's IGBT are higher, and the induced electromotive force will be larger accordingly. Because the frequency of the motor running at power frequency is only 50HZ, there will be no feeling of electric leakage under normal circumstances, but in the case of frequency conversion control, the switching frequency is high, and the motor casing will feel electric leakage.

2. Solution to leakage problem

In order to avoid this problem, an induction surge filter circuit (its equivalent circuit is shown in Figure 1) was added during hardware design, and the ground terminal of the surge filter was connected to the VFD (Variable-frequency Drive) shell, and in the VFD (Variable-frequency Drive) wiring instructions, it is required to connect the ground terminal of the motor to the ground B of the VFD (Variable-frequency Drive), and connect the ground (earth) of the input power to the VFD (Variable-frequency Drive) is connected to the ground A, so that the induction electricity of the motor passes through the motor and the ground of the VFD (Variable-frequency Drive), and the VFD (Variable-frequency Drive) forms a loop with the ground wire of the power supply, so that the ground VFD of the motor The ground of the (Variable-frequency Drive) and the ground of the power supply are at the same potential, and the potential difference between them is 0 volts, so that people standing on the ground can touch the casing of the motor, the frame of the equipment, and the VFD (Variable-frequency Drive) Frequency Drive) shell will not have the feeling of being charged.

However, for the convenience of wiring in some factories, the ground wire is not pulled into the workshop in the high-voltage power distribution room, and even mistakenly think that the earth is the ground wire. This kind of thinking is wrong. You may wish to think about it. Then why do we need to pull the ground wire of the N wire box in our daily life? The N wire in the power station is also connected to the ground wire? Don't we save a lot of wires if we don't need to pull the ground wire of the N wire box? Why waste manpower, material resources and time? However, in reality, many factories do not pull the ground wire of the power supply, the equipment cannot find the grounding point, and the motor has induction leakage during use. In this case, we provide two solutions:

Solution 1: Connect the ground terminal of the motor, the frame shell, and the ground terminal of the VFD (Variable-frequency Drive) together

After the three wires of the motor, VFD (Variable-frequency Drive) and frame are connected together, they are at the same potential, and through the surge absorption and discharge inside the VFD (Variable-frequency Drive), the induced voltage is greatly reduced , so as not to make people feel electric shock again, that is to say, it doesn’t matter if there is no ground wire, as long as a few grounds are connected together, so that the surge filter inside the VFD (Variable-frequency Drive) will work. function to function.

Scheme 2: Under normal circumstances, after the treatment of Scheme 1, there will be no phenomenon of electrification, but due to special reasons, the induced voltage is still relatively high, and it can still be electrified. Then, on the premise of Scheme 1, VFD ( Variable-frequency Drive) Add an induction surge filter to the input power supply end.

And connect the ground of the induction surge filter with the ground of the motor and the ground of the VFD (Variable-frequency Drive) (as shown by the red line in Figure 4), so that the induction surge filter can once again control the ground of the motor. Inductive electricity is absorbed and discharged to further reduce the induced voltage to achieve the purpose of preventing electric leakage. The circuit principle of the added induction surge filter is the same as that of the surge filter circuit inside the VFD (Variable-frequency Drive), because it is too large to be designed and installed in the internal circuit of the VFD (Variable-frequency Drive) , so it is made into an external connection.

We have done a lot of experiments to prove that through the on-site rectification of the connection method of Scheme 2, the induced voltage generated by the operation of the motor can be reduced to below 20V in the application where there is no ground wire connected to the power supply, ensuring that the on-site operators The safety, no longer have the feeling of electric leakage. However, if the ground wire of the power line is connected in the second scheme, then there is no need to connect an external induction surge filter.

In addition, if there are multiple VFDs (Variable-frequency Drive) on site to control the operation of the motor, and it is inconvenient to install multiple induction surge filters, it is not necessarily required that each VFD (Variable-frequency Drive) be equipped with Check the induction surge filter, or connect only one or two induction surge filters, and connect the ground terminal of the filter with the ground terminal of several VFDs (Variable-frequency Drive) on site and the ground terminal of the motor on site. , The equipment racks are connected together, as shown in Figure 5: Since each VFD (Variable-frequency Drive) has an induction surge filter circuit inside, if the ground wire of the motor is not connected back to the VFD (Variable-frequency Drive), If the ground terminal of Drive) is removed, the induction surge filter will not work, so the ground terminal of the motor must be connected with the ground terminal of VFD (Variable-frequency Drive) in the field application. Of course, some equipment will not have the feeling of leakage if the motor is not grounded in some occasions. This is different from what I said earlier in this article. , The resistance value is also different in size" The principle is the same. However, according to the correct electrical safety regulations, it is required that the motor be well grounded, but if the conditions are not allowed (such as no power grounding terminal), the ground of the motor, the shell of the electric cabinet and the ground of the VFD (Variable-frequency Drive) can always be connected to together.