Some Suggestions on Prolonging the Service Life of VFD (Variable-frequency Drive)

With the growing prosperity of the general VFD (Variable-frequency Drive) market, its annual usage is increasing day by day. However, there are still some unsatisfactory aspects of VFD (Variable-frequency Drive) under different operating conditions, which lead to its use The shortened service life and the corresponding increase in the installation, commissioning, daily maintenance and repair workload of ancillary equipment have caused significant direct and indirect economic losses to users. Now it analyzes the application environment of VFD (Variable-frequency Drive), electromagnetic interference and anti-interference, power grid quality, etc., and puts forward the problems that should be paid attention to when using VFD (Variable-frequency Drive) and the corresponding improvement suggestions. I believe this It will have a significant effect on improving the service life of VFD (Variable-frequency Drive).

1 The influence of electromagnetic interference on VFD (Variable-frequency Drive)

In modern industrial control systems, microcomputer or PLC control technology is mostly used. During system design or transformation, attention must be paid to the interference of VFD (Variable-frequency Drive) on the microcomputer control board. VFD (Variable-frequency Drive) is affected by external interference sources as shown in Figure 1. Since the microcomputer control board designed by the user is of poor technology and does not meet the international EMC standard, after using VFD (Variable-frequency Drive), the generated conduction and radiation interference often lead to abnormal operation of the control system, so the following necessary measures need to be taken.

1) Good grounding. The grounding wire of the motor and other strong current control systems must be reliably grounded through the grounding busbar, and the shielding ground of the microcomputer control board should be grounded separately. For some occasions with severe interference, it is recommended to connect the sensor and I/0 interface shielding layer to the control ground of the control board.

2) Add EMI filter, common mode inductor, high frequency magnetic ring, etc. to the input power of the microcomputer control board, which can effectively suppress the conduction interference. In addition, in the case of serious radiation interference, such as GSM or PHS base stations around, a metal mesh shield can be added to the microcomputer control board for shielding treatment.

3) Adding an EMI filter to the input end of the VFD (Variable-frequency Drive) can effectively suppress the conduction interference of the VFD (Variable-frequency Drive) to the power grid, and adding input AC and DC reactors can improve the power factor and reduce Harmonic pollution, good overall effect. In some occasions where the distance between the motor and the VFD (Variable-frequency Drive) exceeds 100 m, it is necessary to add an AC output reactor on the VFD (Variable-frequency Drive) side to solve the leakage current protection caused by the distribution parameters of the output wires to the ground. and reduce external radiation interference. An effective method is to use steel pipes for threading or shielding cables, and to reliably connect the steel pipe casing or cable shielding layer to the ground. It is worth noting that when no AC output reactor is added, if the method of threading steel pipes or shielding cables is used, the distributed capacitance of the output to ground will be increased, and overcurrent will easily occur. Of course, one or more methods are generally adopted in practical applications.

4) Electrically shield and isolate the analog sensor detection input and the analog control signal. In the design process of the control system composed of VFD (Variable-frequency Drive), it is recommended not to use analog control as much as possible, especially when the control distance is greater than 1m and installed across the control cabinet. Because VFD (Variable-frequency Drive) generally has multi-stage speed settings and switching frequency input and output, it can meet the requirements. If it is necessary to use analog control, it is recommended to use shielded cables, and realize a remote point grounding on the sensor side or VFD (Variable-frequency Drive) side. If the interference is still serious, DC/DC isolation measures need to be implemented. A standard DC/DC module can be used, or the optical isolation of v/f conversion can be used, and then the method of frequency setting input can be used.

2 Influence of working environment

In the practical application of VFD (Variable-frequency Drive), most of the domestic customers in China install VFD (Variable-frequency Drive) directly on the industrial site in order to reduce costs, except for a few with dedicated machine rooms. The work site generally has problems of large dust, high temperature, and high humidity, as well as metal dust and corrosive gases in the aluminum industry. Therefore, corresponding countermeasures must be made according to the site conditions, as shown in Figure 2.

1) VFD (Variable-frequency Drive) should be installed inside the control cabinet.

2) VFD (Variable-frequency Drive) is best installed in the middle of the control cabinet; VFD (Variable-frequency Drive) should be installed vertically, and large components that may block the exhaust and air intake should be avoided directly above and below.

3) The minimum distance between the upper and lower edges of the VFD (Variable-frequency Drive) and the top, bottom, or partition of the control cabinet, or large components that must be installed, should be greater than 300 mm.

4) If a special user needs to remove the keyboard during use, the keyboard hole of the VFD (Variable-frequency Drive) panel must be strictly sealed with tape or replaced with a dummy panel to prevent a large amount of dust from entering the VFD (Variable-frequency Drive) .

5) When VFD (Variable-frequency Drive) is used in a place with a lot of dust, especially a place with a lot of metal dust and flocs, the overall control cabinet is required to be sealed as a whole, and the air inlet and outlet are specially designed for ventilation; the top of the control cabinet should have The protective net and the air outlet of the protective top cover; the bottom of the control cabinet should have a bottom plate, an air inlet, and a wire inlet hole, and a dust-proof net should be installed.

6) Most VFD (Variable-frequency Drive) manufacturers' internal printed boards and metal structural parts have not been specially treated against moisture and mildew. If the VFD (Variable-frequency Drive) is in a harsh working environment for a long time, the metal structural parts will Rust occurs. The conductive copper bar will intensify the corrosion process under the condition of high temperature operation, and the corrosion will cause damage to the small copper wires on the microcomputer control board and the driving power board. Therefore, for applications in wet and corrosive gas applications, there must be basic requirements for the internal design of the VFD (Variable-frequency Drive) used. For example, printed circuit boards must be treated with three-proof paint, and structural parts must be Nickel-chrome plating and other treatment processes. In addition, it is necessary to take other active, effective and reasonable measures to prevent moisture and corrosive gases.

3 Impact of grid quality on VFD (Variable-frequency Drive)

In shock loads such as electric welding machines, electric arc furnaces, rolling mills, etc., the voltage often flickers; in a workshop, when multiple capacitive rectification loads such as VFD (Variable-frequency Drive) are working, the harmonics generated by them It has serious pollution to the quality of the power grid, and also has a considerable destructive effect on the equipment itself, ranging from being unable to continue normal operation, to causing damage to the input circuit of the equipment. The following measures can be taken.

1) In the occasions of impact loads such as electric welding machines, electric arc furnaces, rolling mills, etc., it is recommended that users add reactive power compensation devices to improve power factor and quality of the power grid.

2) In workshops where VFD (Variable-frequency Drive) is relatively concentrated, it is recommended to adopt centralized rectification and DC common bus power supply. It is recommended that users adopt 12-pulse rectification mode. The advantages are small harmonics and energy saving, especially suitable for frequent starting and braking, where the motor is in both electric operation and power generation operation.

3) A passive LC filter is installed on the input side of the VFD (Variable-frequency Drive) to reduce input harmonics, improve power factor, high reliability and good effect.

4) An active PFC device is installed on the input side of the VFD (Variable-frequency Drive), the effect is the best, but the cost is relatively high.

4 Epilogue

Starting from the problems in the actual application system of VFD (Variable-frequency Drive), from the aspects of external factors interference, use environment, power grid quality, etc., it is pointed out that in practical application, due to adverse factors, VFD (Variable-frequency Drive) Drive), and then summarize some solutions to this problem and suggestions for improvement, for