1, the cause of leakage problems
In some cases the use of inverter control motor leakage problems occur, the leakage voltage ranging from tens of volts to 200 volts, here for the reasons for the failure of the theoretical analysis and description are as follows.
According to the inverter control motor running function diagram (Figure 1) Three-phase power is rectified by the filter capacitor power supply to the inverter bridge (IGBT), and then by the inverter output frequency and voltage adjustable three-phase inverter to control the operation of the motor .
We all know that three-phase stator windings of the motor flow through the current to produce a rotating magnetic field, according to the principle of magnetic induction, the motor shell will produce induced electromotive force, the size of the emf depends on the size of the inverter IGBT switching frequency, due to high Performance control requires high switching frequency, the switching speed is very fast, the DV / DT is too large, while the induced electromotive force is too large, people feel there is a shock on the touch. In theory, the faster the switching speed of the IGBT, the higher the induced electromotive force on the motor housing, and the higher the control accuracy and response of the inverter to the motor, the higher the feeling of being touched by the human after being touched. On the contrary, the IGBT switching frequency Slow, the sense of electricity is small, the feeling of human touch is small, so the design of low-end domestic inverter switching frequency is low, the induction motor control small, people feel nothing feeling, but its control is poor, dynamic Slow response. Our frequency converter performance and dynamic response are good, so I Division’s IGBT switching frequency and speed are higher, the corresponding induced electromotive force will be larger. Because the frequency of running the motor is the frequency of only 50HZ, so the situation will not have the feeling of leakage, while the inverter control, the switching frequency is high, there will be leakage in the motor shell feel.
2, leakage problem solution
In order to avoid this problem, in the hardware design, it joined the inductive surge filter circuit (the equivalent circuit shown in Figure 1), and the surge filter grounding connected to the shell of the inverter At the same time, in the inverter’s wiring instructions, it is required to connect the grounding end of the motor with the grounding B of the inverter and connect the ground (ground) of the input power source with the grounding A of the inverter so that the induced electricity of the motor passes through the motor The grounding of the inverter and the ground wire of the inverter and the power supply form a loop so that the ground of the inverter of the motor and the ground of the power supply are at the same potential, and the potential difference between them is 0V, so that the person stands on the ground Contact with the motor housing, equipment rack, inverter shell will not have the feeling of electricity.
However, some factories in order to facilitate wiring, high-voltage power distribution room did not pull the ground into the shop, and even the wrong that the earth is the ground, this idea is wrong, we might think, if the earth can be local lines, Why in our daily life why N cable box ground it? The N line in the power station is also connected to the ground line. We do not have to pull the N-box cable is not the province a lot of wires it? Why waste manpower, material resources, time? However, in reality, many factories do not pull the power ground wire, and the equipment can not find the ground point. However, the motor has the situation of sensing leakage in use. In this case, we provide two solutions:
Scenario 1: The motor ground terminal, chassis shell, inverter grounding terminal connected together
Motor, inverter, rack three wires together, so that they are in the same potential, and after the surge in the inverter to absorb, release, so that the induced voltage is greatly reduced, so that people do not feel electric shock , That is to say there is no ground does not matter, as long as the few connected together just fine, so that the surge filter within the inverter to play a role.
Scenario 2: Under normal circumstances after the treatment of the program 1, will not have the phenomenon of electrical people, but due to special reasons, the induced voltage is still relatively high, you can also power people, then in the program under the premise of 1 in the inverter Add an inductive surge filter to the input supply.
And the induction surge filter and the motor ground, the inverter ground (together with the red line in Figure 4) so that inductive surge filter once again on the induction motor to absorb and Leakage, to further reduce the induced voltage, to prevent leakage of current people. The circuit principle of the added inductive surge filter is the same as that of the surge filter in the inverter. Because of the large volume, it can not be designed and installed in the internal circuit of the inverter, so the external way is made.
We have a large number of experiments have proved that the solution through the scene of the second connection of the on-site rectification in the absence of ground applications connected to the power supply, the induction motor voltage can be reduced to 20V or less to ensure on-site operators Safety, no longer have the feeling of leakage of electricity. However, if there is a ground connection of the power line in the second scheme, then no external inductive surge filter is needed.
In addition, if there are multiple inverters controlling the operation of the motor at the scene, and it is not convenient to install multiple inductive surge filters, it is not always required that each inverter be equipped with an inductive surge filter. Only one or two inductive surge filter, and the filter ground and the scene a few sets of inverter ground, field motor ground, equipment rack connected together, as shown in Figure 5: Due to Each inverter has an inductive surge filter circuit, but if the motor’s ground wire is not connected back to the drive’s ground terminal, the inductive surge filter will not work, so in field applications The ground terminal of the motor must be connected to the ground terminal of the inverter. Of course, some of the equipment in some occasions, the motor does not have a ground wire will not have the feeling of leakage, which, as mentioned earlier in this article “Although the earth is also a conductor, but the earth after all, there is resistance, and according to different soil components , The resistance is not the same size “principle is the same. However, in accordance with the correct electrical safety regulations, the motor is required to be well grounded. However, the motor ground, cabinet enclosure and inverter may always be connected if the conditions are not allowed (if there is no power supply ground).