We all know that the motor's three-phase stator winding through the current generates a rotating magnetic field, according to the principle of magnetic induction, the motor's shell will produce an induced electromotive force, the size of this electromotive force depends on the size of the switching frequency of the frequency converter IGBT, due to the high-performance control requirements of high switching frequency, the switching speed is very fast, then the DV/DT is large, at the same time, this induced electromotive force is large, the person touches the There is a feeling of electric shock.
Theoretically, the faster the switching speed of the 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 electrified after touching, on the contrary, the switching frequency of the IGBT is slow, the induced electromotive force is small, and the feeling of being touched by the human being is small, so the low-end inverter in the domestic market is designed to have a low switching frequency, and the feeling of controlling the motor is small, and the feeling of being electrified after touching is not big.
The switching frequency of the low-end domestic inverter design is low, after controlling the motor, the inductance is small, and the human touch has no feeling, but its control is poor, and the dynamic response is slow.
In order to avoid the occurrence of this problem, in the hardware design, it is added to the inductive power surge filter circuit, and the grounding end of the surge filter is connected to the shell of the inverter at the same time in the wiring of the inverter instructions, the grounding end of the motor is required to be connected to the grounding end of the inverter with the inverter B, and the ground of the input power supply (earth) is connected to the inverter with the grounding of the inverter A, so that the motor's induced electric power is connected to the grounding end of the inverter through the motor and the motor, and the grounding of the inverter with the power supply. Ground and inverter and power supply ground line to form a circuit, so that the motor ground inverter ground and power supply ground at the same potential, the potential difference between them is 0 volts voltage, so that people stand in the earth above the contact to the motor's shell, the equipment frame, inverter shell will not have to be the feeling of electricity.
But some factories for the convenience of wiring, high-voltage distribution room did not pull the ground into the workshop, and even the wrong idea that the earth is the ground, this idea is wrong, we may wish to think about, if the earth can be a local line, then why do we need to pull the daily life of the N line box ground? The N wire in the power station is also connected to the ground ah? We do not have to pull the N box ground wire is not to save a lot of wire? Why waste labor, material and time?
Program 1: motor, inverter, frame three wires connected together, so that they are at the same potential, and after the inverter internal surge absorption, discharge, so that the induction voltage is greatly reduced, so that people do not feel electrocuted, that is, there is no ground also does not matter, as long as a few of the ground is connected together on the good, so that the inverter internal surge filters only to play a role to the role.
Program 2: In general, after the processing of program 1, not to the phenomenon of electric people, but due to special reasons, the induction voltage is still relatively high, but also electric people, then in the premise of the program 1 and then in the input power supply side of the inverter to increase the inductive power surge filter.
And the inductive surge filter ground and motor ground, inverter ground connected together (so that the inductive surge filter once again on the motor of the inductive power absorption and discharge, and further reduce the induced voltage to achieve the prevention of leakage of electric current. Increase the inductive surge filter circuit principle and inverter internal surge filter circuit is the same, is due to the volume is too large, can not be designed to be installed in the internal circuit of the inverter, and therefore made into an external way.
We have had a large number of experiments to prove that, through the program two of this connection on-site rectification, in the application of the ground without power supply, can be generated by the motor operation of the induced voltage is reduced to less than 20V, to ensure that the site operator's safety, and will no longer be leakage of electricity to electrician's feeling. However, program two if connected to the power line ground, then there is no need to connect the external inductive surge filter can be.
In addition, if the scene is more than one inverter control motor operation, and is not convenient to install a number of inductive surge filters, is not necessarily required for each frequency converter with an inductive surge filter, you can also connect only one or two inductive surge filter, and the grounding end of the filter and the scene of several inverters, the grounding end of the motor on the scene, the equipment frame connected together, due to the inductive surge filters in each Frequency converter internal inductive surge filter circuit, but if the motor ground wire is not connected back to the ground terminal of the frequency converter to go, the inductive surge filter will not work, so the field application of motor grounding must be connected with the ground terminal of the frequency converter.
Of course, some equipment in some occasions the motor is not grounded will not have the feeling of leakage, which is said earlier in this paper, although the earth also belongs to the conductor, but the earth is, after all, the resistance, and according to the soil composition of different lands, the resistance is also different sizes of the principle is the same. But in accordance with the correct electrical safety norms, is required to motor good grounding, but the conditions do not allow (such as no power supply ground terminal), the motor ground, the cabinet shell and the inverter can always be connected to the ground.