Inverter control motor has leakage solution problem - Database & Sql Blog Articles

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1. Causes of leakage problems

In some places, using the inverter to control the motor will cause leakage problems. The leakage voltage ranges from tens of volts to 200 volts. The theoretical analysis and explanation of the cause of this fault is as follows.

figure 1

According to the function diagram of the inverter control motor operation (Fig. 1), the three-phase power is supplied to the inverter bridge (IGBT) through the rectification and filtering capacitor, and then the inverter output frequency and voltage adjustable three-phase electric power are used to control the operation of the motor. .

We all know that the three-phase stator winding of the motor flows through a current to generate a rotating magnetic field. According to the principle of magnetoelectric induction, the outer casing of the motor generates an induced electromotive force. The magnitude of this electromotive force depends on the switching frequency of the inverter IGBT. The performance control requires a high switching frequency, and its switching speed is fast, then the DV/DT is too large, and the induced electromotive force is too large, and there is a feeling of electric shock when the person touches. Theoretically, 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 inverter to the motor, the higher the feeling of being charged after the person touches, and vice versa, the switching frequency of the IGBT. Slow, the induction power is small, the feeling of human touch is small, so the low-end inverter design of the domestic switching frequency is low, the induction motor after the motor is small, the human touch feels no sense, but its control is poor, dynamic The response is slow. Our inverter's performance and dynamic response are better, so our IGBT's switching frequency and speed are higher, and the induced electromotive force will be larger. Because the frequency of the motor running motor is only 50HZ, there is no leakage feeling in one case, and the switching frequency is high when the inverter is controlled, and the motor casing will have a leakage feeling.

2, the solution to the leakage problem

In order to avoid this problem, in the hardware design, the inductive electric surge filter circuit (the equivalent circuit is shown in Figure 1) is added, and the grounding end of the surge filter is connected to the outer casing of the inverter. At the same time, in the wiring description of the inverter, it is required to connect the grounding end of the motor to the grounding B of the inverter, and connect the ground (earth) of the input power source to the grounding A of the inverter, so that the induction current of the motor passes through the motor and The grounding of the inverter and the grounding line of the inverter and the power supply form a loop, so that the ground of the grounding of the motor and the ground of the power supply are at the same potential, and the potential difference between them is 0 volt, so that the person stands on the ground. The outer casing of the motor, the frame of the equipment, and the outer casing of the inverter will not be electrically felt.

However, in some factories, for the convenience of wiring, the high-voltage power distribution room did not pull the ground wire into the workshop, and even mistakenly thought that the earth is the ground wire. This idea is wrong. You may wish to think about it if the earth can be localized. What is in our daily life?

figure 2

Will you pull the N-line box ground? The N line in the power station is also connected to the ground line? We don't have to pull the N-line box ground wire to save a lot of wires? Why waste manpower, material resources and time? However, in reality, many factories do not pull the power ground, the equipment can not find the grounding point, and the motor is in use in the case of induced leakage, in this case, we offer two options:

Option 1: Connect the grounding end of the motor, the frame of the rack, and the grounding end of the inverter (Figure 3).

image 3

After the three wires of the motor, the inverter and the frame are connected together, they are at the same potential, and are absorbed and discharged by the internal surge of the inverter, so that the induced voltage is greatly reduced, so that the feeling of electric shock is not caused. That is to say, there is no relationship with the ground wire, as long as several grounds are connected together, so that the surge filter inside the inverter works.

Option 2: Under normal circumstances, after the treatment of the scheme 1, there will be no electrician phenomenon, but for special reasons, the induced voltage is still relatively high, and it can be electric, then the inverter is in the premise of the scheme 1. An inductive surge filter is added to the input power supply.

Figure 4

Connect the ground of the inductive surge filter to the ground of the motor and the ground of the inverter (as shown by the red line in Figure 4). Let the inductive surge filter absorb the induced current of the motor again. Relief, further reduce the induced voltage, to prevent the current leakage of electricity. The circuit principle of the increased inductive surge filter is the same as the surge filter circuit inside the inverter. It is too large to be designed and installed in the internal circuit of the inverter, so it is made externally.
We have had a lot of experiments to prove that the on-site rectification of the connection method of the second scheme can reduce the induced voltage generated by the motor operation to below 20V in the application of the ground wire without the power supply, ensuring the on-site operator. The safety will no longer be felt by people who are leaking electricity. However, if the ground wire of the power cord is connected in the second scheme, then no external inductive surge filter is needed.
In addition, if there are multiple inverters in the field to control the motor operation, and it is not convenient to install multiple inductive surge filters, it is not necessary to require each inverter to be equipped with an inductive surge filter. Connect only one or two inductive electric surge filters, and connect the grounding end of the filter to the grounding end of several inverters in the field, the grounding end 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 the field application The ground terminal of the motor must be connected to the ground terminal of the inverter. Of course, some equipments do not have ground leakage when the motor is not grounded. In some cases, the earth is also a conductor, but the earth is resistant, and according to the soil composition of different land. The resistance is also different in size. The principle is the same. However, according to the correct electrical safety regulations, the motor is required to be well grounded, but the conditions are not allowed (if there is no power supply ground), the ground of the motor, the cabinet of the electric cabinet and the ground of the inverter can always be connected together.

Figure 5