Why does the inverter leak and how can it be solved?

Last Updated on 2018-04-11 Hits:2413

First, the cause of leakage problems

Some field use inverter control motor, there will be leakage problems, leakage voltage from tens of volts to 200 volts. To solve this problem, the reasons for this failure are analyzed and explained in detail here.

According to the function block diagram of the inverter control motor running (Figure 1), after the three-phase power supply is rectified by the inverter rectifier bridge, it is sent to the inverter bridge (IGBT) through capacitor filtering, and then the output frequency and voltage of the inverter bridge are adjustable. The AC power is used to control the operation of the motor. Three alternating currents with 120 degrees of difference flow through the three-phase stator coil windings of the motor to generate a rotating magnetic field, which causes the rotor of the motor to rotate automatically under the influence of the rotating magnetic field of the stator windings.

We all know that a three-phase stator winding of a motor generates a rotating magnetic field after passing a current, and according to the principle of electromagnetic induction, the motor's housing generates an induced electromotive force. The magnitude of this induced electromotive force depends on the switching frequency of the frequency converter IGBT and C*DV/DT (related to the speed of the IGBT switch); due to high-performance control requires a higher switching frequency, its switching speed requirements Fast, DV/DT is too large. If this induced electromotive force is large, people will feel like they have been struck by electric shock. Theoretically, the higher the switching frequency of the IGBT, the higher the effective value of the induced electromotive force of the motor housing (ie, the induced voltage), and the higher the frequency converter's control accuracy and dynamic response of the motor, the more the human body feels after being touched. Larger; Conversely, the lower the switching frequency of the IGBT, the lower the effective value (induction voltage) of the induced dynamic potential of the motor housing, and the smaller the sense of being electrically charged after the touch from the body. Therefore, some domestic low-end inverter IGBT switching frequency is designed to be lower. After the control motor is running, the induced voltage of the motor casing is low, but its control performance is poor and its dynamic response is slow. Our inverter's performance and dynamic response are better, so our company's inverter IGBT's switching frequency and switching speed are relatively high, and the induced electromotive force will be relatively larger.

Due to the operation of the asynchronous motor, the motor casing will have induced voltage (so-called leakage current). Therefore, the motor manufacturer will place a grounding terminal in the junction box when the motor is shipped from the factory, which is convenient for the user to connect to the earth during application to eliminate The induced electromotive force (ie, the elimination of induced leakage voltage) is used to solve the feeling that the human body is electrically connected to the motor. Of course, when the motor is running at industrial frequency, the switching frequency of power frequency is about 50 Hz, which is very low, so there is almost no sense of leakage in general (unless the motor insulation is poor). When the inverter is controlled, since its switching frequency is much higher than the frequency of the power frequency, when the inverter controls the rotation of the motor, the motor casing will have the feeling of leakage.

Second, the solution to the leakage problem

In order to avoid this problem, when the inverter hardware is designed, it adds the inductive surge filter circuit (the equivalent circuit is shown in Figure 1), and the ground terminal of the inductive surge filter and the inverter The enclosure is connected. At the same time, in the wiring description of the inverter, it is required to connect the ground terminal of the motor with the ground terminal B of the inverter, and connect the ground of the input power (ie, ground) to the ground terminal A of the inverter.

Thus, the induced current generated by the operation of the motor can form a loop through the ground wire between the motor and the inverter, and the ground wire between the inverter and the power supply, making the ground of the motor and the ground of the inverter equal to the ground of the power supply (ie, the earth). The potential difference between them is 0 volts. In this way, the human body stands on the ground (and also the ground of the power supply) and touches the rotating electric housing, the mechanical equipment rack (generally the equipment rack is connected with the earth), and the frequency converter housing will not Being electro-sensed, because the potential difference (voltage difference) between them is 0 volts, the human body cannot sense whether there is electricity.

When the ground wire of the motor fails to be connected with the ground terminal of the frequency converter and the ground wire of the power supply is not connected with the ground of the frequency converter, the casing of the mechanical device or the ground terminal of the electric motor, the casing of the motor and the frequency conversion The housing of the device and the ground (ie ground) of the power supply are not at the same potential. If in this case, the induced voltage generated by the operation of the motor is 100V, the motor is racked together with some part of the mechanical equipment, because the power supply's ground wire is not pulled in the power distribution room, and the human electrical equivalent model theory It can be equated to a resistance of about 2K ohms (if the human body sweats, the resistance value is even smaller when wet, and sometimes even only a few tens of ohms). When the human body stands on the ground and touches the metal of the device connected to the motor, the induction of the motor (For example, 100V), the human body can discharge electricity to the earth, and then the human body will have a current flowing through it and it will feel like being charged. Although, theoretically, the housing of the motor is connected to the rack of the mechanical equipment, and the rack of the equipment is installed on the ground, it stands to reason that the person standing on the earth touching the equipment rack should not be touched by induction electricity. However, do not forget that although the earth is also a conductor, the earth has resistance after all, and depending on the soil composition of different lands, the resistance varies. Otherwise, why does the National Power Supply Bureau require that the grounding resistance of each substation transformer's grounding line and each company's power distribution room be less than 4 ohms? Why is it that if the grounding resistance of a substation or a high-voltage power distribution room is not less than 4 ohms, approval is not given and electricity is not allowed? In fact, this is the truth. If people and equipment are separated, there will be induced voltage. When the human body touches the equipment, there will be current flowing through the human body, and there will be a feeling of being charged. It is only the size of the induction electricity that determines the size of the person being electrosensed.

However, in some factories, for the convenience of wiring, the ground wire inside the high-voltage power distribution room has not been pulled into the production hall at all, and even mistakenly believes that the earth is the ground wire. Why do you want to pull the ground wire? Isn't it superfluous? This kind of thinking is wrong. Everyone cannot think about it. If the earth can be used as a ground line, then why should all the wires in our daily life be drawn on the N line and the ground line? The N line inside the power station is actually connected to the ground wire. Can we not save large cables and wires without pulling the ground wire and N wire? Why do we need to do this kind of work that wastes manpower, wastes material resources, wastes time, and wastes money?

However, in reality, some factories do not pull the power ground wire, and the equipment cannot find the grounding point. However, in the use of the motor, there is a case of induced leakage. What to do if this happens? Here, we propose two scenarios as follows:

Solution 1: Connect the ground of the motor housing, the rack of the mechanical equipment and the ground of the inverter together (see Figure 3).

After the three ground wires of the motor, frequency converter, and frame are connected together, they are at the same potential and are absorbed and discharged by the inductive surge filter circuit inside the inverter, so that the induced voltage is greatly reduced. The induced voltage generated by the rotation of the motor is also greatly reduced with respect to the ground (i.e., ground) voltage of the power source, so as not to be electrically charged after being touched. In other words, there is no problem if there is no power ground wire, as long as the ground of the motor and the ground of the inverter are connected together with the chassis, so that the inductive surge filter inside the inverter will play a real role.

Option two: Under normal circumstances, the induced voltage generated by the rotation of the motor after the treatment of the first solution is already very small, and it will not be able to leak people, but due to some special reasons (such as: poor motor insulation, electrical cabinets When the appliance is not grounded, etc.), the induced voltage is still high, and there is a feeling of leakage of electricity, a proposal 2 is proposed.

The second option is to add an inductive surge filter to the input power supply of the inverter under the premise of the solution one.

The ground of the inductive surge filter is connected to the earth of the motor and the ground of the inverter (as shown by the red line in Fig. 4), so that the inductive surge filter absorbs the induced electricity of the motor again. And discharge, to further reduce 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 less than 20V, 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. Only one or two inductive surge filters are connected, and the ground of the filter is connected to the ground of several inverters in the site, the ground of the field motor, and the equipment rack, as shown in Figure 5:

Since each inverter has an inductive surge filter circuit inside, 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 the field application The ground end 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. However, 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.