With the increasing prosperity of the frequency converter market, the workload of installation, commissioning, daily maintenance and repair of frequency converters (hereinafter referred to as -VFD) and its ancillary equipment has increased dramatically. In view of the causes of the above problems, the analysis is carried out from the aspects of application environment, electromagnetic interference and anti-interference, power grid quality, and motor insulation.
1. Work environment issues
In the actual application of VFD, domestic customers, except for a few dedicated machine rooms, mostly install VFD directly on the industrial site in order to reduce costs. Generally, the construction site is dusty, the temperature is high, and there is also a problem of high humidity in the south. The cable industry has metal dust, ceramics, printing and dyeing industries have corrosive gases and dust, coal mines and other occasions have explosion-proof requirements, and so on. Therefore, it is necessary to formulate corresponding countermeasures according to the situation on the spot.
2. Basic requirements for VFD installation design
(1) VFD should be installed in the control cabinet.
(2) The VFD is best installed in the middle of the control cabinet; the VFD It should be installed vertically, and the top and bottom should avoid large parts that may block the exhaust and air intake.
(3) The minimum distance between the upper and lower edges of the VFD control cabinet or partition, or the large parts that must be installed, should be greater than 300mm. Basic Requirements for Installing a VFD in a Cabinet
(4) If a special user needs to disassemble the keyboard during use, the keyboard hole of the VFD panel must be tightly sealed with adhesive tape or replaced with a fake panel to prevent a large amount of dust from entering the VFD.
(5) VFD should be maintained regularly, and the internal dust should be cleaned in time.
(6) Other basic installation and use requirements must comply with the relevant instructions in the user manual; if you have any questions, please contact the technical support personnel of the corresponding manufacturer in time.
3. Design requirements for dust-proof control cabinets
When using VFD in a dusty place, especially a place with a lot of metal dust and flocs, correct and reasonable protective measures must be taken. Proper dust prevention measures are very important to ensure the normal operation of VFD. The overall control cabinet is required to be sealed and ventilated through specially designed air inlets and outlets; the top of the control cabinet should have a protective net and the air outlet of the protective roof; the bottom of the control cabinet should have a bottom plate, air inlet and wire inlet, and install a dust filter.
(1)The air duct design of the control cabinet should be reasonable, and the exhaust should be unobstructed to avoid the formation of eddy currents in the cabinet and dust accumulation in fixed positions.
(2)A protective cover should be installed above the air outlet on the top of the control cabinet to prevent debris from falling directly; the height of the top cover should be reasonable so as not to affect the exhaust. The air outlet on the side of the top cover should be installed with a protective net to prevent flocculent debris from falling directly.
(3)If the top of the control cabinet is used for exhaust, the air outlet must be installed with a protective net.
(4)Make sure that the axial flow fan on the top of the control cabinet rotates in the correct direction and exhausts air outward. If the fan is installed outside the top of the control cabinet, it is necessary to ensure that there is a sufficient height between the protective top cover and the fan; if the fan is installed inside the top of the control cabinet, the screws required for installation must use irreversible springs to prevent the fan from falling off and damage the components and equipment in the cabinet. It is recommended to install a plastic or rubber shock-absorbing gasket between the fan and the cabinet, which can greatly reduce the noise generated by the fan vibration.
(5)The front and rear doors of the control cabinet and other joints should be sealed with gaskets or sealants to prevent dust from entering.
(6)All air inlets and wire inlets on the bottom and side panels of the control cabinet must be equipped with dust-proof nets. Prevent flocculent impurities from entering. The dust filter should be designed to be detachable for easy cleaning and maintenance. The mesh of the dust-proof net should be small, which can effectively block tiny flocs (similar to the mesh of general household mosquito and fly nets); or determine the appropriate mesh size according to the specific situation. The connection between the dust filter and the control cabinet should be handled tightly.
(7) The control cabinet must be maintained regularly, and the internal and external dust, fluff and other sundries must be cleaned up in time. The maintenance period can be determined according to the specific situation, but it should be less than 2 to 3 months; for places with severe dust, the recommended maintenance period is about 1 month. Installation requirements for dust-proof control cabinet 4. The design of the moisture-proof and mildew-proof control cabinet requires that the internal printed boards and metal structures of most VFD manufacturers have not undergone special treatment for moisture-proof and mildew-proof. If the VFD is in this state for a long time, the metal structure is easy to rust, and the conductive copper busbar runs at high temperature, and the corrosion process is intensified. Corrosion can cause damage to the tiny copper wires on the microcomputer control board and the drive power board. Therefore, for occasions that are humid and contain corrosive gases, there must be basic requirements for using VFDs for internal design. For example, printed circuit boards must be sprayed with conformal paint, and structural parts must be nickel-chrome-plated.
In addition, other positive, effective and reasonable measures should be taken to prevent moisture and corrosive gases.
- (1) The control cabinet can be installed in a separate, closed air-conditioning machine room. This method is suitable for the situation where there are many control devices and the cost of building a computer room is lower than that of a separate closed cabinet. At this time, the control cabinet can adopt the above-mentioned dustproof or general environmental protection design.
- (2) Independent air inlet is adopted. A separate air inlet can be set at the bottom of the control cabinet, which is connected to the external clean environment through an independent closed trench. This method requires the installation of a dust filter at the air intake. If the trench exceeds 5m, consider installing a blower.
- (3) Hygroscopic desiccants or active substances that absorb toxic gases can be installed in the closed control cabinet, which should be replaced in the near future.
4. Interference problem
VFD interferes with the microcomputer control panel. In the control systems of injection molding machines and elevators, microcomputers or PLCs are mostly used for control. In the process of system design or modification, attention must be paid to the interference of VFD on the microcomputer control panel. Because the microcomputer control board designed by the user is generally of poor technical level and does not meet the international EMC standard, the conduction and radiation interference generated after adopting VFD often leads to abnormal operation of the control system, so necessary measures need to be taken.
(1)Good grounding. The grounding wire of the motor control system 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 shielding layer of the sensor and I/O interface to the control ground of the control board.
(2) Add EMI filter, common mode choke coil, high frequency magnetic ring, etc. Input power to the microcomputer control board, and low cost. Can effectively suppress conducted interference. In addition, when the radiation interference is serious, such as the surrounding GSM or PHS stations, the microcomputer control board can be shielded by adding metal mesh shielding. Anti-interference measures for the power supply of the microcomputer control board.
(3) Adding an EMI filter to the VFD can effectively suppress the conduction interference of the VFD to the power grid, and adding input AC and DC reactors L1 and L2 can improve the power factor and reduce harmonic pollution, and the overall effect is good. When the distance between some motors and the VFD exceeds 100m, it is necessary to add an AC output reactor L3 on the VFD side to protect the leakage current caused by the distribution parameters of the output line to the ground and reduce external radiation interference. An effective method is to use steel pipes to thread or shield cables, and to reliably connect the steel pipe shell or cable shielding layer to the ground. Please note that if the AC output reactor L3 is not added, if steel pipe threading or shielded cable is used, the distributed capacitance of the output to ground will increase, which is prone to overcurrent. Of course, in practice, usually only one or a few methods are used.
(4) Measures to reduce VFD interference to external control equipment Electrical shielding and isolation of analog sensor detection input and analog control signals. In the design process of the control system composed of VFD, it is recommended not to use analog control as much as possible, especially when the control distance is greater than 1M and the control cabinet is installed across. Because VFD generally has multi-speed settings and switching frequency input and output, it can meet the requirements. If analog control is required, it is recommended to use shielded cable and implement remote point grounding on the sensor side or VFD side. If the interference is still serious, DC/DC isolation measures should be implemented. Standard DC/DC modules can be used, as well as V/F conversion, optocoupler isolation and frequency setting input.
With the increasing prosperity of the frequency converter market, the workload of installation, commissioning, daily maintenance and repair of frequency converters (hereinafter referred to as -VFD) and its ancillary equipment has increased dramatically. In view of the causes of the above problems, the analysis is carried out from the aspects of application environment, electromagnetic interference and anti-interference, power grid quality, and motor insulation.
1. Work environment issues
In the actual application of VFD, domestic customers, except for a few dedicated machine rooms, mostly install VFD directly on the industrial site in order to reduce costs. Generally, the construction site is dusty, the temperature is high, and there is also a problem of high humidity in the south. The cable industry has metal dust, ceramics, printing and dyeing industries have corrosive gases and dust, coal mines and other occasions have explosion-proof requirements, and so on. Therefore, it is necessary to formulate corresponding countermeasures according to the situation on the spot.
2. Basic requirements for VFD installation design
(1) VFD should be installed in the control cabinet.
(2) The VFD is best installed in the middle of the control cabinet; the VFD It should be installed vertically, and the top and bottom should avoid large parts that may block the exhaust and air intake.
(3) The minimum distance between the upper and lower edges of the VFD control cabinet or partition, or the large parts that must be installed, should be greater than 300mm. Basic Requirements for Installing a VFD in a Cabinet
(4) If a special user needs to disassemble the keyboard during use, the keyboard hole of the VFD panel must be tightly sealed with adhesive tape or replaced with a fake panel to prevent a large amount of dust from entering the VFD.
(5) VFD should be maintained regularly, and the internal dust should be cleaned in time.
(6) Other basic installation and use requirements must comply with the relevant instructions in the user manual; if you have any questions, please contact the technical support personnel of the corresponding manufacturer in time.
3. Design requirements for dust-proof control cabinets
When using VFD in a dusty place, especially a place with a lot of metal dust and flocs, correct and reasonable protective measures must be taken. Proper dust prevention measures are very important to ensure the normal operation of VFD. The overall control cabinet is required to be sealed and ventilated through specially designed air inlets and outlets; the top of the control cabinet should have a protective net and the air outlet of the protective roof; the bottom of the control cabinet should have a bottom plate, air inlet and wire inlet, and install a dust filter.
(1)The air duct design of the control cabinet should be reasonable, and the exhaust should be unobstructed to avoid the formation of eddy currents in the cabinet and dust accumulation in fixed positions.
(2)A protective cover should be installed above the air outlet on the top of the control cabinet to prevent debris from falling directly; the height of the top cover should be reasonable so as not to affect the exhaust. The air outlet on the side of the top cover should be installed with a protective net to prevent flocculent debris from falling directly.
(3)If the top of the control cabinet is used for exhaust, the air outlet must be installed with a protective net.
(4)Make sure that the axial flow fan on the top of the control cabinet rotates in the correct direction and exhausts air outward. If the fan is installed outside the top of the control cabinet, it is necessary to ensure that there is a sufficient height between the protective top cover and the fan; if the fan is installed inside the top of the control cabinet, the screws required for installation must use irreversible springs to prevent the fan from falling off and damage the components and equipment in the cabinet. It is recommended to install a plastic or rubber shock-absorbing gasket between the fan and the cabinet, which can greatly reduce the noise generated by the fan vibration.
(5)The front and rear doors of the control cabinet and other joints should be sealed with gaskets or sealants to prevent dust from entering.
(6)All air inlets and wire inlets on the bottom and side panels of the control cabinet must be equipped with dust-proof nets. Prevent flocculent impurities from entering. The dust filter should be designed to be detachable for easy cleaning and maintenance. The mesh of the dust-proof net should be small, which can effectively block tiny flocs (similar to the mesh of general household mosquito and fly nets); or determine the appropriate mesh size according to the specific situation. The connection between the dust filter and the control cabinet should be handled tightly.
(7) The control cabinet must be maintained regularly, and the internal and external dust, fluff and other sundries must be cleaned up in time. The maintenance period can be determined according to the specific situation, but it should be less than 2 to 3 months; for places with severe dust, the recommended maintenance period is about 1 month. Installation requirements for dust-proof control cabinet 4. The design of the moisture-proof and mildew-proof control cabinet requires that the internal printed boards and metal structures of most VFD manufacturers have not undergone special treatment for moisture-proof and mildew-proof. If the VFD is in this state for a long time, the metal structure is easy to rust, and the conductive copper busbar runs at high temperature, and the corrosion process is intensified. Corrosion can cause damage to the tiny copper wires on the microcomputer control board and the drive power board. Therefore, for occasions that are humid and contain corrosive gases, there must be basic requirements for using VFDs for internal design. For example, printed circuit boards must be sprayed with conformal paint, and structural parts must be nickel-chrome-plated.
In addition, other positive, effective and reasonable measures should be taken to prevent moisture and corrosive gases.
- (1) The control cabinet can be installed in a separate, closed air-conditioning machine room. This method is suitable for the situation where there are many control devices and the cost of building a computer room is lower than that of a separate closed cabinet. At this time, the control cabinet can adopt the above-mentioned dustproof or general environmental protection design.
- (2) Independent air inlet is adopted. A separate air inlet can be set at the bottom of the control cabinet, which is connected to the external clean environment through an independent closed trench. This method requires the installation of a dust filter at the air intake. If the trench exceeds 5m, consider installing a blower.
- (3) Hygroscopic desiccants or active substances that absorb toxic gases can be installed in the closed control cabinet, which should be replaced in the near future.
4. Interference problem
VFD interferes with the microcomputer control panel. In the control systems of injection molding machines and elevators, microcomputers or PLCs are mostly used for control. In the process of system design or modification, attention must be paid to the interference of VFD on the microcomputer control panel. Because the microcomputer control board designed by the user is generally of poor technical level and does not meet the international EMC standard, the conduction and radiation interference generated after adopting VFD often leads to abnormal operation of the control system, so necessary measures need to be taken.
(1)Good grounding. The grounding wire of the motor control system 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 shielding layer of the sensor and I/O interface to the control ground of the control board.
(2) Add EMI filter, common mode choke coil, high frequency magnetic ring, etc. Input power to the microcomputer control board, and low cost. Can effectively suppress conducted interference. In addition, when the radiation interference is serious, such as the surrounding GSM or PHS stations, the microcomputer control board can be shielded by adding metal mesh shielding. Anti-interference measures for the power supply of the microcomputer control board.
(3) Adding an EMI filter to the VFD can effectively suppress the conduction interference of the VFD to the power grid, and adding input AC and DC reactors L1 and L2 can improve the power factor and reduce harmonic pollution, and the overall effect is good. When the distance between some motors and the VFD exceeds 100m, it is necessary to add an AC output reactor L3 on the VFD side to protect the leakage current caused by the distribution parameters of the output line to the ground and reduce external radiation interference. An effective method is to use steel pipes to thread or shield cables, and to reliably connect the steel pipe shell or cable shielding layer to the ground. Please note that if the AC output reactor L3 is not added, if steel pipe threading or shielded cable is used, the distributed capacitance of the output to ground will increase, which is prone to overcurrent. Of course, in practice, usually only one or a few methods are used.
(4) Measures to reduce VFD interference to external control equipment Electrical shielding and isolation of analog sensor detection input and analog control signals. In the design process of the control system composed of VFD, it is recommended not to use analog control as much as possible, especially when the control distance is greater than 1M and the control cabinet is installed across. Because VFD generally has multi-speed settings and switching frequency input and output, it can meet the requirements. If analog control is required, it is recommended to use shielded cable and implement remote point grounding on the sensor side or VFD side. If the interference is still serious, DC/DC isolation measures should be implemented. Standard DC/DC modules can be used, as well as V/F conversion, optocoupler isolation and frequency setting input.
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