Causes and solutions of inverter leakage - Xinjie

1. The cause of the leakage problem

Some sites use frequency converters to control the motor, which will present the problem of leakage, and the leakage voltage ranges from tens of volts to 200 volts. In view of this problem, the reason for this problem is explained and stated as follows.

According to the functional block diagram of the inverter to control the work of the motor, 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 reversed to control the work of the motor. The alternating current with a difference of 120 degrees flows through the three-phase stator coil winding of the motor, which changes and makes the rotor of the motor automatically change under the influence of the stator winding change.

We all know that the three-phase stator winding of the motor changes after the current flows, and according to the principle of electromagnetism, the shell of the motor will generate electromotive force. The magnitude of this electromotive force depends on the switching frequency of the IGBT and C*DV/DT (related to the switching speed of the IGBT). Because high-performance control requires a higher switching frequency and a faster switching speed, the DV/DT is larger. If the electromotive force is large, then the person will feel the same as the electric shock when they come into contact with it. Theoretically, the higher the switching frequency of IGBT, the higher the invalid value (i.e., voltage) of the electromotive force of the motor shell, and the higher the control accuracy and dynamic response of the inverter to the motor, and the greater the feeling of being touched by electricity after human contact; Conversely, the lower the switching frequency of the IGBT, the lower the invalid value (voltage) of the kinetic potential of the motor housing, and the less the feeling of being electronically touched by the slave body. Therefore, the switching frequency of some domestic low-end inverter IGBT is imagined to be low, and after the control motor works, the voltage of the motor shell is low, but its control function is poor and the dynamic response is slow. The function and dynamic response of our inverter are better, so the switching frequency and switching speed of our inverter IGBT are higher, and the electromotive force will be relatively larger.

Because the asynchronous motor works, there is a voltage in the motor shell city (that is, the so-called leakage), so the motor manufacturer will install a grounding terminal in the junction box when the motor leaves the factory, so that the user can connect to the earth when using it to eliminate its electromotive force (i.e., eliminate leakage voltage), so as to deal with the feeling of being electronized when the human body is in contact with the motor. Of course, because the switching frequency of the power frequency is about 50HZ, which is very low, there will be no leakage in the general environment (unless the motor insulation is very poor). When the inverter is controlled, because its switching frequency is much higher than the number of cycles, the motor shell will have a sense of leakage when the inverter controls the electric flexible elastic.

Causes and solutions of inverter leakage - Xinjie

Causes and solutions of inverter leakage - Xinjie

Second, the treatment method of leakage problem

In order to prevent the occurrence of this problem, the inverter hardware intervenes in the electrical surge filter at the imaginary time, and connects the grounding terminal of the inverter with the shell of the inverter, and together in the wiring statement of the inverter, it is required to connect the grounding terminal of the motor with the grounding terminal B of the inverter, and connect the ground (i.e. the ground) of the input power supply with the grounding terminal A of the inverter.

So that the current generated by the motor can pass through the grounding wire between the motor and the inverter, and the grounding wire between the inverter and the power supply to form a return, so that the ground of the motor, the ground of the inverter and the ground of the power supply (i.e., the earth) are all in the same potential, and the potential difference between them is 0 volt voltage. If the human body stands on the ground (which is also the ground of the power supply) and comes into contact with the changed electric shell, the rack of mechanical equipment (the rack of general equipment is adjacent to the earth), and the shell of the inverter, it will not be touched by electricity, because the potential difference (voltage difference) between them is 0 volts, and the human body will not be able to feel whether there is electricity.

Causes and solutions of inverter leakage - Xinjie

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