Case application analysis and treatment of VFD (Variable-frequency Drive) interference problems

0 Preface

In recent years, I often encountered VFD (Variable-frequency Drive) interference problems in my work, which caused equipment malfunctions and made the production line of the factory unable to run. Moreover, it is difficult to find the cause of this type of problem. After consulting relevant information, combined with Some experience in dealing with problems at work will specifically talk about the source of VFD (Variable-frequency Drive) interference, the way of propagation, and some solutions for different situations in which interference problems are encountered in practical applications.

1 VFD (Variable-frequency Drive) interference analysis

VFD (Variable-frequency Drive) interference problems are generally divided into VFD (Variable-frequency Drive) self-interference; electromagnetic waves generated by external equipment interfere with VFD (Variable-frequency Drive); VFD (Variable-frequency Drive) interferes with other weak current equipment 3 types of situations. VFD (Variable-frequency Drive) itself is a source of interference. As we all know, VFD (Variable-frequency Drive) is composed of two parts, the main circuit and the control circuit. The main circuit of VFD (Variable-frequency Drive) is mainly composed of rectifier circuit, inverter circuit , the control circuit, in which the rectifier circuit and the inverter circuit are composed of power electronic devices, and the power and electronic devices have nonlinear characteristics. When the VFD (Variable-frequency Drive) is running, it needs to perform fast switching actions, thus generating high-order harmonics. Wave, so VFD (Variable-frequency Drive) output waveform contains a large number of high-order harmonics in addition to the fundamental wave. Regardless of the type of interference, high-order harmonics are the main cause of VFD (Variable-frequency Drive) interference. VFD (Variable-frequency Drive) itself is a source of harmonic interference, so it will affect the equipment on the power supply side and output side. Compared with the main loop, the VFD (Variable-frequency Drive) control loop is a low-energy, weak-signal loop, which is very susceptible to interference from other devices. Therefore, when VFD (Variable-frequency Drive) is installed and used, anti-interference measures must be taken for the control circuit.

2 VFD (Variable-frequency Drive) interference case analysis and treatment

2.1 How to judge the interference problem of VFD (Variable-frequency Drive) The interference problem of VFD (Variable-frequency Drive) is mainly reflected in the operation of the motor. For example, the motor stops suddenly during operation, causing the motor to run fast and sometimes slow, the running speed is unstable, the motor cannot stop, the button does not work, etc. These are the manifestations of VFD (Variable-frequency Drive) being disturbed .

2.2 The general solution to the grounding interference problem of the third method is to ensure good grounding. The general requirements for the grounding terminal are: the grounding terminal should be grounded in the "third way" (separate grounding), the shorter the grounding wire, the better, and must The grounding is good; the control loop line uses shielded wires, and the far-end shielding layer of the shielded wires is suspended, and the near-end is grounded; according to product requirements, reasonable wiring, strong and weak currents are separated, and a certain distance is kept to avoid VFD (Variable-frequency Drive) power lines Parallel wiring with signal lines should be distributed; add anti-wireless interference filters, VFD (Variable-frequency Drive) input and output anti-interference filters or reactors; take shielding measures to prevent electromagnetic induction, and even VFD (Variable-frequency Drive) Frequency Drive) is shielded with a metal iron box; the carrier frequency is appropriately reduced; if the communication function is used, the RS485 communication line uses a twisted pair. Below, I will make a specific analysis of different interference situations in actual use.

2.3 The three-phase five-wire power supply has encountered such a situation that the VFD (Variable-frequency Drive) has been running, and the stop button does not work. After inspection, it is found that the ground wire of the VFD (Variable-frequency Drive) is only connected to the transformer. The neutral wire is connected, but the neutral wire of the transformer is not connected to the earth. After the neutral wire of the transformer is grounded, the VFD (Variable-frequency Drive) returns to normal. Nowadays, many small factories generally do not pay attention to the connection of the ground wire. When leaving the factory, according to the standards stipulated by the National Electrical Engineering Law, the ground wire and the neutral wire are strictly separated. The neutral wire in the power distribution cabinet has a special terminal, and the ground wire has a special grounding screw. Since the user has three phase wires and one neutral wire from the transformer, only the neutral wire is connected to the "N" terminal, and the ground wire is not connected to the neutral wire. Although the control wire uses a shielded wire, the shielding layer is also The grounding screw is connected, but it is not connected to the ground, and the shielding effect cannot be achieved, which causes the VFD (Variable-frequency Drive) to run out of control due to interference and the motor cannot stop. After connecting the neutral wire and the ground wire in the power distribution cabinet, it will return to normal, or you can directly connect the ground wire in the power distribution cabinet to the earth. Many users adopt the method of connecting the ground wire to the neutral wire, but there are disadvantages in this method, that is, if the neutral wire is disconnected and a certain action of the machine tool is started, the machine tool may be electrified and pose a threat to personal safety. This kind of interference belongs to the type of interference of VFD (Variable-frequency Drive) itself.

2.4 Cases of external devices interfering with VFD (Variable-frequency Drive) ① Phenomenon: The motor occasionally fails to stop. After inspection, the shielding layer is properly grounded, and lowering the carrier frequency does not work. VFD (Variable-frequency Drive) input side and output side plus magnetic ring filter does not work. ②Analysis: The power distribution cabinet where the VFD (Variable-frequency Drive) is installed is too close to the power distribution room. There is a large current flowing through the power distribution cabinet in the power distribution room, and there is a strong magnetic field around the current, which interferes with the VFD (Variable-frequency Drive). Frequency Drive) works normally, and it will return to normal after the power distribution cabinet is far away from the power distribution room. This is the interference of external equipment on VFD (Variable-frequency Drive).

2.5 VFD (Variable-frequency Drive) interference case to external equipment ① Phenomenon: After starting VFD (Variable-frequency Drive), the motor does not move. ②Analysis: The VFD (Variable-frequency Drive) is given the operating frequency by an external 4-20ma, and the 4-20MA DC signal is sent by the transmitter. Look at the display panel, and the frequency is displayed as 0.00. Measure the output terminal of the transmitter with an ammeter and find that there is no output. After connecting a 102 capacitor to the output terminal of the transmitter, restart it, and the device returns to normal, indicating that the signal source is disturbed. In engineering practice a simple signal line shunt capacitor is often effective as a practical solution to large problems.

This belongs to the interference of VFD (Variable-frequency Drive) on external equipment.

3 Conclusion

With the rapid development of industrial automation, the use of VFD (Variable-frequency Drive) is becoming more and more common, and VFD (Variable-frequency Drive) will encounter many interference problems. Many end users encounter this problem Sometimes I don't know how to solve it, I hope this article can provide them with some help.