1 Introduction
With the booming market for general-purpose frequency converters , excluding OEM imported converters, the annual consumption of general-purpose frequency converters in China exceeds 2.5 billion RMB. This has led to a surge in the workload of installing, commissioning, routine maintenance, and repairing frequency converters and their auxiliary equipment, causing significant direct and indirect losses to users. This article analyzes the causes of these problems based on the actual application experiences of numerous users, focusing on aspects such as application environment, electromagnetic interference and anti-interference, power grid quality, and motor insulation, and proposes some improvement suggestions.
2. Work environment issues
In practical applications of frequency converters, most domestic customers, except for a few with dedicated machine rooms, install them directly in industrial sites to reduce costs. These sites are typically dusty, hot, and, in southern regions, also humid. The cable industry also faces metal dust, while the ceramics and printing and dyeing industries encounter corrosive gases and dust. In coal mines and similar environments, explosion-proof requirements exist. Therefore, appropriate countermeasures must be developed based on the specific site conditions.
2.1 Basic Requirements for the Installation Design of Frequency Converters
(1) The frequency converter should be installed inside the control cabinet.
(2) The frequency converter is best installed in the middle of the control cabinet; the frequency converter should be installed vertically, and large components that may block the exhaust and intake should be avoided directly above and below it.
(3) The minimum distance between the upper and lower edges of the frequency converter and the top, bottom, partition, or large components that must be installed in the control cabinet should be greater than 300mm. As shown in Figure 1, the distance between h1 and h2 is as follows.
Figure 1. Basic requirements for installing frequency converters inside the cabinet.
(4) If a special user needs to remove the keyboard during use, the keyboard hole on the inverter panel must be sealed with tape or replaced with a fake panel to prevent a large amount of dust from entering the inverter.
(5) The frequency converter should be maintained regularly, and the internal dust should be cleaned in a timely manner.
(6) Other basic installation and usage requirements must be followed in accordance with the instructions in the user manual; if you have any questions, please contact the relevant manufacturer's technical support personnel in a timely manner.
2.2 Design Requirements for Dust Control Cabinets
When using frequency converters in dusty environments, especially those with metallic dust or fibrous materials, it is essential to take correct and reasonable protective measures. Proper dust prevention is crucial for ensuring the normal operation of the frequency converter. General requirements include: the control cabinet should be completely sealed and ventilated through specially designed air inlets and outlets; the top of the control cabinet should have a protective mesh and a protective top cover with an air outlet; the bottom of the control cabinet should have a base plate, air inlets, and cable entry holes, and be equipped with a dustproof mesh.
(1) The air duct of the control cabinet should be designed reasonably and the exhaust should be unobstructed to avoid the formation of eddies in the cabinet and the accumulation of dust in fixed positions.
(2) A protective cover shall be installed on the top air outlet of the control cabinet to prevent debris from falling directly in; the height of the protective cover shall be reasonable and shall not affect the exhaust. A protective net shall be installed on the side air outlet of the protective cover to prevent fibrous debris from falling directly in.
(3) If the top side exhaust method of the control cabinet is used, the air outlet must be equipped with a protective net.
(4) Ensure that the axial fan on top of the control cabinet rotates in the correct direction and exhausts air outwards. If the fan is installed on the outside of the control cabinet top, ensure there is sufficient height between the protective cover and the fan; if the fan is installed inside the control cabinet top, use anti-reverse springs for the screws to prevent the fan from falling off and damaging components and equipment inside the cabinet. It is recommended to install plastic or rubber vibration damping washers between the fan and the cabinet to greatly reduce noise caused by fan vibration.
(5) The front and rear doors and other joints of the control cabinet should be sealed with gaskets or sealant to prevent dust from entering.
(6) All air inlets and cable entry holes on the bottom and side panels of the control cabinet must be fitted with dust filters to prevent the entry of fibrous debris. The dust filters should be designed to be removable for easy cleaning and maintenance. The mesh size of the dust filters should be small enough to effectively block fine fibrous material (similar to the mesh size of a typical household mosquito and fly screen); or a suitable mesh size can be determined based on specific circumstances. The joint between the dust filters and the control cabinet must be tightly sealed.
(7) The control cabinet must be maintained regularly, and dust, lint and other debris inside and outside should be cleaned in a timely manner. The maintenance cycle can be determined according to the specific situation, but should be less than 2 to 3 months; for places with severe dust, it is recommended that the maintenance cycle be about 1 month.
Figure 2 Installation requirements for dust control cabinet
2.3 Design requirements for control cabinets to prevent moisture and mold growth
Most inverter manufacturers fail to provide special moisture- and mildew-proofing treatments for their internal printed circuit boards and metal structural components. If the inverter is exposed to such conditions for extended periods, the metal components are prone to corrosion. This corrosion process is exacerbated by high-temperature operation on the conductive copper busbars. Corrosion will also damage the delicate copper wires on the microcomputer control board and drive power supply board. Therefore, for inverters used in humid environments or those containing corrosive gases, basic requirements must be placed on their internal design. For example, printed circuit boards must be coated with conformal coating, and structural components must undergo nickel-chromium plating. In addition, other proactive, effective, and reasonable measures must be taken to prevent moisture and corrosive gases.
(1) The control cabinet can be installed in a separate, enclosed air-conditioned machine room. This method is suitable for situations where there are many control devices and the cost of building the machine room is lower than that of the cabinet being enclosed separately. In this case, the control cabinet can be designed with dustproof or general environmental protection as described above.
(2) Use an independent air inlet. A separate air inlet can be located at the bottom of the control cabinet and connected to the clean external environment through an independent sealed trench. This method requires the installation of a dust screen at the air inlet. If the trench is more than 5m long, a blower can be considered.
(3) A moisture-absorbing desiccant or an active material that adsorbs toxic gases can be installed in the sealed control cabinet and replaced in the near future.
3. Interference problem
3.1 Interference of the frequency converter to the microcomputer control board
In the control systems of injection molding machines, elevators, and similar equipment, microcomputers or PLCs are often used for control. During system design or modification, it is crucial to pay attention to the interference of the frequency converter on the microcomputer control board. Because user-designed microcomputer control boards are generally of poor manufacturing quality and do not meet EMC international standards, the conducted and radiated interference generated after using a frequency converter often leads to abnormal operation of the control system. Therefore, necessary measures must be taken.
(1) Good grounding. The grounding wire of the high-voltage control system such as motor must be reliably grounded through the grounding busbar. The shielding ground of the microcomputer control board should preferably be grounded separately. For some severe interference situations, it is recommended to connect the shielding layer of the sensor and I/O interface to the control ground of the control board [3].
(2) Adding an EMI filter, common-mode inductor, and high-frequency magnetic ring to the input power supply of the microcomputer control board is cost-effective. As shown in Figure 3, it can effectively suppress conducted interference. In addition, in situations with severe radiated interference, such as when there are GSM or PHS base stations nearby, a metal mesh shield can be added to the microcomputer control board for shielding.
Figure 3. Power supply anti-interference measures for the microcomputer control board
(3) Adding an EMI filter to the inverter input can effectively suppress conducted interference from the inverter to the power grid. Adding input AC and DC reactors l1 and l2 can improve the power factor and reduce harmonic pollution, resulting in a good overall effect. In some cases where the distance between the motor and the inverter exceeds 100m, an AC output reactor l3 needs to be added to the inverter side to solve the leakage current protection caused by the distributed parameters of the output conductor to ground and reduce external radiated interference. An effective method is to use steel pipes or shielded cables and reliably connect the steel pipe shell or cable shielding layer to the ground. Please note that if steel pipes or shielded cables are used without adding an AC output reactor l3, the distributed capacitance of the output to ground will increase, which can easily lead to overcurrent. As shown in Figure 4, in practice, only one or more of these methods are usually used.
Figure 4. Measures to reduce the interference of frequency converters to external control equipment
(4) Electrically shield and isolate analog sensor inputs and analog control signals. In the design of control systems composed of frequency converters, it is recommended to avoid analog control as much as possible, especially when the control distance is greater than 1m and the system is installed across control cabinets. This is because frequency converters generally have multi-speed settings and switching frequency input/output, which can meet the requirements. If analog control must be used, it is recommended to use shielded cables and ground at a remote point on either the sensor side or the frequency converter side. If interference is still severe, DC/DC isolation measures are required. Standard DC/DC modules can be used, or a V/F converter with optocoupler isolation and frequency setting input can be employed.
3.2 Inverter's own anti-interference problem
When there are high-frequency impact loads such as welding machines, electroplating power supplies, electrolytic power supplies, or in applications using slip ring power supplies near the inverter's power supply system, the inverter itself is prone to triggering its protection due to interference. Users are advised to take the following measures:
(1) Add an inductor and a capacitor to the input side of the frequency converter to form an lc filter network.
(2) The power supply of the frequency converter is directly supplied from the transformer side.
(3) Where conditions permit, a separate transformer may be used.
(4) When using external switch control terminals, it is recommended to use shielded cables when the connection lines are long. When both the control lines and the main circuit power supply are buried in the trench, in addition to the control lines which must be shielded cables, the main circuit lines must be shielded with steel pipes to reduce mutual interference and prevent the inverter from malfunctioning.
(5) When using external analog control terminals, if the connection line is within 1m, a shielded cable can be used for connection and the inverter side can be grounded at one point; if the line is long and there is severe interference on site, it is recommended to install a DC/DC isolation module on the inverter side or use a frequency command given mode after V/F conversion for control.
(6) When using external communication control terminals, it is recommended to use shielded twisted-pair cables and ground the shielding layer on the inverter side (PE). If interference is severe, it is recommended to connect the shielding layer to the control power ground (GND). For RS232 communication, the control line should not exceed 15m as much as possible. If it needs to be extended, the communication baud rate must be reduced accordingly. At around 100m, the baud rate for normal communication is less than 600bps. For RS485 communication, the terminal matching resistor must also be considered. For high-speed control systems using fieldbus, dedicated cables must be used for communication, and multi-point grounding must be employed to improve reliability.
4. Power Grid Quality Issues
In high-frequency impact loads such as welding machines, electroplating power supplies, and electrolytic power supplies, voltage flicker frequently occurs. In a workshop with hundreds of frequency converters and other capacitive rectifier loads operating, the harmonics in the power grid are very high, causing serious pollution to the grid quality and considerable damage to the equipment itself. This can range from preventing continuous normal operation to damaging the equipment's input circuit. The following measures can be taken:
Figure 5. DC common bus power supply method with centralized rectification.
(1) In high-frequency impact loads such as welding machines, electroplating power supplies, and electrolytic power supplies, it is recommended that users add reactive power compensation devices to improve the power factor and quality of the power grid.
(2) In workshops where frequency converters are concentrated, it is recommended to adopt centralized rectification and DC common bus power supply. It is recommended that users adopt the 12-pulse rectification mode. As shown in Figure 5, the advantages are low harmonics and energy saving, and it is particularly suitable for occasions with frequent starting and braking, and simultaneous electric and generator operation.
(3) Adding a passive lc filter to the input side of the frequency converter reduces input harmonics, improves the power factor, has low cost, high reliability, and good effect.
(4) Adding an active PFC device to the input side of the frequency converter has the best effect, but the cost is higher.
5. Problems with motor leakage current, shaft voltage, and bearing current.
The motor model of the inverter-driven induction motor is shown in Figure 6. In the figure, csf is the equivalent capacitance between the stator and the housing, csr is the equivalent capacitance between the stator and the rotor, crf is the equivalent capacitance between the rotor and the housing, rb is the resistance of the bearing to the shaft, and cb and zb are the capacitance and nonlinear impedance of the bearing oil film.
Under high-frequency PWM pulse input, the voltage coupling effect of the distributed capacitor in the motor forms a common-mode circuit in the system, which causes problems with ground leakage current, shaft voltage and bearing current.
Figure 6. Motor model of an induction motor driven by a frequency converter.
Leakage current is mainly generated between the three-phase power supply voltage of the PWM and its instantaneous unbalanced voltage and ground through CSF. Its magnitude is related to the PWM's DV/DT and the switching frequency, directly causing the tripping of leakage protection devices. Furthermore, for older motors, due to poor insulation materials and long-term aging, some suffer insulation damage after frequency conversion. Therefore, it is recommended to conduct insulation testing before conversion. The insulation requirements for new frequency conversion motors should be one level higher than those for standard motors.
Bearing current exists primarily in three forms: dv/dt current, EDM (electric discharge machining) current, and loop current. The magnitude of the shaft voltage depends not only on the coupling capacitance parameters of various parts within the motor but also on the rise time and amplitude of the pulse voltage. The dv/dt current is mainly related to the rise time (tr) of the PWM; the smaller the tr, the larger the amplitude of the dv/dt current. Higher inverter carrier frequencies result in a greater proportion of dv/dt current in the bearing current. The occurrence of EDM current is somewhat random. It only occurs when the bearing lubricating oil layer is broken down or internal contact occurs within the bearing. The charge (1/2crf × urf) stored in the electronic rotor-to-ground capacitance (crf) then discharges to ground through the bearing's equivalent circuit (rb, cb, zb), causing a decrease in bearing surface finish, reducing service life, and potentially leading to direct damage. The degree of damage depends primarily on the shaft voltage and the size of the electronic rotor-to-ground capacitance (crf).
Loop currents occur in the circuits between the power grid transformer ground wire, the inverter ground wire, the motor ground wire, and the motor load and the earth ground wire (such as in water pump loads). Loop currents mainly cause conducted interference and grounding interference, with little impact on the inverter and motor. The method to avoid or reduce loop currents is to minimize the impedance of the grounding circuit. Since the inverter ground wire (PE inverter) is generally connected to the motor ground wire (PE motor 1) at the same point, the diameter of the motor grounding cable must be increased as much as possible to reduce the resistance between them. Simultaneously, the ground wire between the inverter and the power supply should use a copper grounding busbar or a dedicated grounding cable to ensure good grounding. For loads like submersible deep well pumps, the grounding impedance of ze motor 2 may be less than the sum of the ze transformer and ze inverter, easily forming grounding loop currents. It is recommended to disconnect the ze inverter for better anti-interference performance.
Adding a sinusoidal filter consisting of an inductor and an RC circuit in series at the inverter output is an effective way to suppress shaft voltage and bearing current. Several manufacturers currently offer standard filters.
6. Conclusion
This article starts with the problems encountered in the actual application system of frequency converters, and proposes some solutions and improvement suggestions in a targeted manner from aspects such as application environment, electromagnetic compatibility, power grid quality, and motor insulation. It has certain reference value for the application of frequency converters in practical engineering.
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