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 reasons for this shortcoming are theoretically expounded and stated as follows.
According to the functional block diagram of the inverter to control the work of the motor, after the rectifier bridge of the inverter at the beginning and end of the three-phase power supply is rectified, it is sent to the inverter bridge (IGBT) through capacitive filtering, and then the output frequency and voltage of the inverter bridge are adjustable from the beginning to the end of the alternating current to control the work of the motor. The exchange electricity 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 infection of the stator winding change.
The three-phase stator windings of the motor change when an electric current flows through it, and according to electromagnetic theory, the electromotive force is generated in the motor housing. The size of this electromotive force depends on the size of the switching frequency of the IGBT and C*DV/DT (related to the switching speed of the IGBT); Because the high-performance control requires a higher switching frequency and the switching speed is required to be faster, 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 the 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 electrocuted by the human body after 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 assumed to be low, and after the control motor works, the voltage of the motor shell is lower, but its control function is poor and the dynamic response is slow.
Because the asynchronous motor works, there is 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 it is used to eliminate its electromotive force (that is, eliminate the 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 the switching frequency is much higher than the working rate, the motor shell will have a sense of leakage when the inverter controls the electric sensitive bullet.
Second, the solution to the problem of leakage
In order to avoid this problem, the inverter hardware intervenes in the electrical surge filter electricity at the time of assumption, and connects the grounding terminal of the inrush filter with the shell of the inverter, and at the same time, 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 earth) of the input power supply with the grounding terminal A of the inverter.
So that the current generated by the motor can be formed back through the grounding wire between the motor and the inverter, and the grounding wire between the inverter and the power supply, 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 a uniform 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 touches 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.
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