Why frequency converter leakage, how to solve?

First, leakage causes the problem

Some on-site use inverter control motor, there will be leakage problems, leakage voltage tens of volts to two hundred volts voltage range. In response to this problem, here in particular the cause of this failure to carry out theoretical analysis and description are as follows.

According to the functional block diagram of the inverter controlling the operation of the motor (Figure 1), the three-phase power is rectified by the rectifier bridge of the inverter and then sent to the inverter bridge (IGBT) through the capacitor filter and then output through the inverter bridge with the adjustable voltage of three Alternating current to control the operation of the motor. Three-phase difference of 120 degrees of AC power in the three-phase stator windings of the motor coil flow, resulting in rotating magnetic field, so that the motor rotor in the stator winding rotating magnetic field automatically rotate up.

We all know that the three-phase stator windings of the motor produce a rotating magnetic field when they flow through the current. According to the principle of electromagnetic induction, the motor’s case generates induced electromotive force. The magnitude of this induced electromotive force depends on the magnitude of the switching frequency of the inverter IGBT and C * DV / DT (related to the switching speed of the IGBT); the high switching frequency requires a higher switching speed than the control of high performance Fast, then DV / DT is too large. If this induction electromotive force is bigger, then the human touch will feel the same electric shock. In theory, the higher the switching frequency of the IGBT, the higher the effective value of the induced electromotive force (ie, the induced voltage) of the motor case, and the higher the control accuracy and dynamic response of the inverter to the motor. On the other hand, the lower the switching frequency of the IGBT, the lower the effective value of the induced voltage (induced voltage) of the motor case and the less the feeling of being electrified after the body touches it. Therefore, some domestic low-end inverter IGBT switching frequency design is low, the motor control motor operation, the induction motor housing voltage is low, but its control performance is poor, the dynamic response is slow. Our inverter performance and dynamic response are good, so our inverter IGBT switching frequency and switching speed are higher, the relative will be larger induced electromotive force.

As the asynchronous motor running, the motor casing will have induced voltage (the so-called leakage), so the motor factory in the motor when the factory, in the junction box inside the ground terminal, user-friendly connection to the earth to eliminate The induction electromotive force (ie, eliminate induced leakage voltage), in order to solve the human body contact with the motor when the feeling of electricity. Of course, because the frequency of running the motor, the frequency switching frequency is about 50HZ, very low, so under normal circumstances almost no leakage sense (unless the motor insulation is poor). The frequency converter control, because of its switching frequency is much higher than the frequency of the frequency, so when the inverter controls the motor rotation, the motor shell will have the feeling of leakage.

Second, leakage solutions to the problem

In order to avoid this problem, the inverter hardware in the design of the time, it joined the inductive surge filter circuit (equivalent circuit shown in Figure 1), and the induction surge filter ground and inverter , Meanwhile, in the wiring instruction of the inverter, it is required to connect the grounding end of the motor with the grounding terminal B of the inverter and connect the ground of the input power (ie the earth) with the grounding terminal A of the inverter.

So that the induced current generated by the motor running through the motor and inverter ground wire, ground wire between the inverter and power to form a loop, so that the motor ground, inverter ground and power (that is, the ground) are equal Potential, the potential difference between them is 0 volts. In this way, the human body stands on the ground (also the power of the ground) contact with rotating electric shell, mechanical equipment rack (general equipment rack is connected with the earth), the inverter shell will not have The feeling of electricity, because the potential difference between them (voltage difference) is 0 volts, the human body can not feel it out of electricity.

When the ground wire of the motor is not connected with the ground terminal of the inverter, and the ground wire of the power source is not connected with the ground of the inverter, the shell of the mechanical equipment or the grounding terminal of the electric motor, the shell of the motor, Shell, the power of the ground (that is, the earth) is not in the same potential. In this case, the induced voltage generated by the motor running is 100V, and the motor is also racked with a part of the mechanical equipment because the ground of the power supply is not pulled in the distribution room, and the human body’s electrical equivalent model theory Can be equivalent to a resistance of about 2K ohms (if the human body sweats, damp, when the resistance is smaller, and sometimes even only a few dozen ohms), the human body on the ground touch the motor connected to the metal device, the induction motor (Such as 100V) will be able to discharge the human body to the earth, then the human body there will be a current flow, there will be the feeling of being electrified. Although, in theory, the motor housing and the mechanical equipment rack is connected, and the equipment rack is installed on the ground, it stands to reason that people standing on the ground touch the equipment rack should not be induced electric shock to However, do not forget that although the earth is also a conductor, the earth is, after all, a resistance, and the resistance varies depending on the soil composition of the land. Otherwise, why the national power supply bureau will ask each substation transformer grounding wire, each company’s distribution room grounding resistance requirements of less than 4 ohms? Why if the substation or high-voltage power distribution room grounding resistance is not less than 4 ohms not to approve, do not allow electricity? In fact, this is the truth. There will be a distance between the man and the device induction voltage, the human body will touch the device current flow through the body, there will be the feeling of being electric, but the size of the induction of electricity, people feel the power of the size is not the same.

However, some factories in order to facilitate wiring within the high-voltage power distribution room inside the ground simply did not pull inside the production workshop, and even mistakenly think that the earth is the ground, why pull the ground? Is not it any more? This idea is wrong, we do not think about it, if the earth can be used as a ground, then why do we have to pull all the wires in our daily life N and ground it? In fact, the N-line power station is also connected with the ground you? We do not have to pull the ground and N lines can not save a lot of cables, wires yet? Why is it necessary to do this kind of waste of manpower, waste of material resources, a waste of time, but also a waste of money jobs?

However, in reality, some factories did not pull the power ground wire, and the equipment could not find the ground point. However, the motor did not have a sense of leakage during operation. What should we do if this happens? Here, we propose two options as follows:

Option One: The motor shell ground, mechanical equipment rack and drive ground connected together (Figure 3),

Motor, inverter, rack three ground connected together, so that they are at the same potential, and through the inverter’s internal induction surge filter circuit to absorb, release, so that the induced voltage is greatly reduced, so that The voltage induced by the rotation of the motor is also greatly reduced with respect to the ground of the power source (ie, the ground), so that there is no feeling of being electrocuted after touching. In other words, there is no power ground connection does not matter, as long as the motor ground, inverter and chassis connected together just fine, so that the inverter inductive surge filter will play a real role.

Motor, inverter, rack three ground connected together, so that they are at the same potential, and through the inverter’s internal induction surge filter circuit to absorb, release, so that the induced voltage is greatly reduced, so that The voltage induced by the rotation of the motor is also greatly reduced with respect to the ground of the power source (ie, the ground), so that there is no feeling of being electrocuted after touching. In other words, there is no power ground connection does not matter, as long as the motor ground, inverter and chassis connected together just fine, so that the inverter inductive surge filter will play a real role.

Scenario 2: Under normal circumstances, through the treatment of a program, the induced voltage generated by the motor rotation is already small, will not have to leakage of people, but due to some special reasons (such as: motor insulation is not good, the electrical cabinet In the electrical equipment when all did not ground, etc.), the induced voltage is still high, there will be the feeling of leakage when the proposed program II.

Option two is in the premise of a program, and then add an inductive surge filter in the inverter input power supply side.

And the induction surge filter and the motor ground, the inverter connected to the ground (as shown in the red line in Figure 4), so that induction surge filter once again on the induction motor to absorb And bleed, to further reduce the induced voltage, to prevent leakage of current people. The circuit principle of the added inductive surge filter is the same as that of the surge filter in the inverter. Because of the large volume, it can not be designed and installed in the internal circuit of the inverter, so the external way is made.

And the induction surge filter and the motor ground, the inverter connected to the ground (as shown in the red line in Figure 4), so that induction surge filter once again on the induction motor to absorb And bleed, to further reduce the induced voltage, to prevent leakage of current people. The circuit principle of the added inductive surge filter is the same as that of the surge filter in the inverter. Because of the large volume, it can not be designed and installed in the internal circuit of the inverter, so the external way is made.

We have a large number of experiments have proved that the solution through the scene of the second connection of the on-site rectification in the absence of ground applications connected to the power supply, the induction motor voltage can be reduced to 20V below to ensure that field operators Safety, no longer have the feeling of leakage of electricity. However, if there is a ground connection of the power line in the second scheme, then no external inductive surge filter is needed.

In addition, if there are multiple inverters controlling the operation of the motor at the scene, and it is not convenient to install multiple inductive surge filters, it is not always required that each inverter be equipped with an inductive surge filter. Only one or two inductive surge filter, and the filter ground and the scene a few sets of inverter ground, field motor ground, equipment rack connected together, as shown in Figure 5:

Since each inverter has an inductive surge filter circuit inside it, the inductive surge filter will not work if the motor’s ground wire is not connected back to the inverter’s ground terminal, so field applications The ground terminal of the motor must be connected to the ground terminal of the inverter. Of course, some of the equipment in some occasions, the motor does not have a ground wire will not have the feeling of leakage, which, as mentioned earlier in this article “Although the earth is also a conductor, but the earth after all, there is resistance, and according to different soil components , The resistance is not the same size “principle is the same. However, in accordance with the correct electrical safety regulations, the motor is required to be well grounded. However, the motor ground, the cabinet shell and the inverter can always be connected together if the conditions are not allowed (if there is no power ground).

Since each inverter has an inductive surge filter circuit inside it, the inductive surge filter will not work if the motor’s ground wire is not connected back to the inverter’s ground terminal, so field applications The ground terminal of the motor must be connected to the ground terminal of the inverter. Of course, some of the equipment in some occasions, the motor does not have a ground wire will not have the feeling of leakage, which, as mentioned earlier in this article “Although the earth is also a conductor, but the earth after all, there is resistance, and according to different soil components , The resistance is not the same size “principle is the same. However, in accordance with the correct electrical safety regulations, the motor is required to be well grounded. However, the motor ground, the cabinet shell and the inverter can always be connected together if the conditions are not allowed (if there is no power ground).