Inverter control motor leakage causes and measures

The cause of the leakage problem

We all know that the three-phase stator winding of the motor flows through the current to produce a rotating magnetic field, according to the principle of magnetoelectric induction, the shell of the motor will produce induced electromotive force, the size of this electromotive force depends on the size of the switching frequency of the inverter IGBT, due to the high performance control requirements of the high switching frequency, the switching speed is very fast, then the DV/DT is large, and the induction electromotive force is large, and the person touches the feeling of electric shock. Theoretically, the faster the switching speed of IGBT, the higher the induced electromotive force on the motor shell, 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, on the contrary, the switching frequency of the IGBT is slow, the induction electricity is small, and the feeling of human touch is small, so the switching frequency of the domestic low-end inverter design is low, and the induction electricity is small after controlling the motor, and the person does not feel anything when he touches, but its control is poor, and the dynamic response is slower.

Solutions to leakage problems

In order to avoid the occurrence of this problem, when the hardware is designed, the induction surge filter circuit is added, and the grounding end of the surge filter is connected to the shell of the inverter, and at the same time, in the wiring description of the inverter, the grounding terminal of the motor is required to be connected with the grounding B of the inverter, and the ground (earth) of the input power supply is connected to the grounding A of the inverter, so that the induction electricity of the motor forms a loop through the grounding of the motor and the inverter and the grounding wire of the inverter and the power supply. The ground of the ground inverter of the motor and the ground of the power supply are at the same potential, and the potential difference between them is 0 volt voltage, so that people standing on the ground and touching the shell of the motor, the rack of the equipment, and the shell of the inverter will not have the feeling of being electrified.

However, in some factories for the convenience of wiring, the high-voltage power distribution room does not pull the ground wire into the workshop, and even mistakenly thinks that the earth is the ground wire, this idea is wrong, you may wish to think about it, if the earth can be wired, then why should we pull the N wire box ground wire in our daily life? The N line in the power station is also connected to the ground wire. Doesn't we have to pull the N wire box ground wire to save 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, and the motor has induction leakage in use.

Scheme 1: After the three wires of the motor, inverter and frame are connected together, they are at the same potential, and the induced voltage is greatly reduced by the surge absorption and discharge inside the inverter, so that the induced voltage is greatly reduced, so that people will not feel electrocuted, that is to say, there is no ground wire and it doesn't matter, as long as a few ground are connected together, so that the surge filter inside the inverter can play a role.

Scheme 2: Under normal circumstances, after the treatment of scheme 1, there will be no phenomenon of electric person, but due to special reasons, the induced voltage is still relatively high, and it can also be electrified, so under the premise of scheme 1, an inductive surge filter is added to the input power supply end of the inverter.

And the ground of the induction surge filter is connected with the ground of the motor and the ground of the inverter together (as shown by the red line in Figure 4), so that the induction surge filter absorbs and discharges the induced electricity of the motor again, further reduces the induced voltage, and achieves the current goal of preventing leakage. The circuit principle of the increased inductive surge filter is the same as that of the inverter, but it is too large to be designed and installed in the internal circuit of the inverter, so it is made 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 of the ground wire without the power supply, so as to ensure the safety of the on-site operator and no longer have the feeling of being leaked. However, if the ground wire of the power cord is connected to the second solution, then there is no need to connect an external induced surge filter.

In addition, if there are multiple inverters in the field to control the operation of the motor, and it is inconvenient to install a plurality of inductive surge filters, it is not necessarily required that each inverter is equipped with an inductive surge filter, or only one or two inductive surge filters can be connected, and the grounding end of the filter is connected with the grounding terminal of several inverters in the field, the grounding terminal of the field motor, and the equipment rack together, as shown in Figure 5: because there is an inductive surge filter circuit inside each inverter, However, if the grounding wire of the motor is not connected back to the grounding terminal of the inverter, the induction surge filter will not work, so the grounding terminal of the motor in the field application must be connected to the grounding terminal of the inverter.

Of course, some equipment in some occasions the motor is not grounded and will not have the feeling of leakage, which is the same as the previous article said that although the earth is also a conductor, but the earth has a resistance value after all, and according to the soil composition of different land, the resistance value is also different The principle is the same. However, according to the correct electrical safety specifications, the motor is required to be well grounded, but the conditions do not allow it (such as no power grounding terminal), the ground of the motor, the electrical cabinet shell and the ground of the inverter can always be connected together.
Inverter control motor leakage causes and measures

Inverter control motor leakage causes and measures

Inverter control motor leakage causes and measures

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