Selection
1) The purpose of using frequency converters: constant voltage control or constant current control, etc.
2) The load type of the frequency converter ; such as vane pump or positive displacement pump, etc. Pay special attention to the performance curve of the load, as the performance curve determines the application method.
3) Matching issue between the frequency converter and the load;
I. Voltage matching; the rated voltage of the frequency converter matches the rated voltage of the load.
II. Current Matching: For ordinary centrifugal pumps, the rated current of the frequency converter matches the rated current of the motor. For special loads such as deep-water pumps, it is necessary to refer to the motor performance parameters to determine the frequency converter current and overload capacity based on the maximum current.
III. Torque matching; This situation may occur under constant torque loads or with a speed reduction device.
4) When using a frequency converter to drive a high-speed motor , the output current increases due to the low reactance of the high-speed motor and the increase in higher harmonics. Therefore, the capacity of the frequency converter used for high-speed motors should be slightly larger than that used for ordinary motors.
5) If the frequency converter is to be operated with a long cable, measures should be taken to suppress the influence of the coupling capacitance between the long cable and the ground to avoid insufficient output of the frequency converter. Therefore, in this case, the capacity of the frequency converter should be increased by one level or an output reactor should be installed at the output end of the frequency converter.
6) For some special applications, such as high temperature and high altitude, the inverter capacity will be reduced, and the inverter capacity will need to be increased by one level.
Control Schematic Design
1) First, confirm the installation environment of the frequency converter;
I. Operating Temperature. The inverter contains high-power electronic components that are highly susceptible to temperature fluctuations. The product generally requires a temperature range of 0–55℃, but to ensure safe and reliable operation, a margin of safety should be considered, ideally keeping it below 40℃. In the control box, the inverter should generally be installed at the top of the enclosure, strictly adhering to the installation requirements in the product manual. It is absolutely forbidden to install heat-generating or easily heated components close to the bottom of the inverter.
II. Ambient Temperature. When the temperature is too high or fluctuates significantly, condensation can easily occur inside the frequency converter, greatly reducing its insulation performance and potentially causing a short circuit. If necessary, desiccant and heaters must be added to the enclosure. In water treatment rooms, where moisture levels are generally high, this problem becomes more pronounced with large temperature variations.
III. Corrosive gases. If the concentration of corrosive gases in the operating environment is high, it will not only corrode the leads of components and printed circuit boards, but also accelerate the aging of plastic components and reduce their insulation performance.
IV. Vibration and Shock. Mechanical vibration and shock can cause poor electrical contact in control cabinets equipped with frequency converters. This problem occurred at Huaian Thermal Power Plant. In addition to increasing the mechanical strength of the control cabinet and keeping it away from vibration and shock sources, anti-vibration rubber pads should be used to secure vibrating components such as electromagnetic switches both inside and outside the control cabinet. The equipment should be inspected and maintained after a period of operation.
V. Electromagnetic Interference. During operation, frequency converters generate numerous interfering electromagnetic waves due to rectification and frequency conversion. These high-frequency electromagnetic waves can interfere with nearby instruments and meters. Therefore, instruments and electronic systems within the cabinet should be housed in metal casings to shield them from the frequency converter's interference. All components should be reliably grounded. Furthermore, shielded control cables should be used for connections between electrical components, instruments, and meters, and the shielding layer should be grounded. Failure to properly manage electromagnetic interference can often render the entire system inoperable, leading to control unit malfunction or damage.
2) Determine the cable and wiring method based on the distance between the frequency converter and the motor;
I. The distance between the frequency converter and the motor should be as short as possible. This reduces the cable's capacitance to ground and decreases sources of interference.
II. Shielded cables should be used for control cables and shielded cables for power cables, or the entire cable from the frequency converter to the motor should be shielded using conduit.
III. Motor cables should be routed independently of other cables, with a minimum distance of 500mm. Long-distance parallel routing of motor cables with other cables should also be avoided to reduce electromagnetic interference caused by rapid changes in the inverter's output voltage. If control cables and power cables cross, they should cross at a 90-degree angle whenever possible. Analog signal lines related to the inverter should be routed separately from the main circuit line, even within the control cabinet.
IV. Shielded twisted-pair cables are preferred for analog signal lines related to the frequency converter, and shielded three-core cables (with a larger specification than ordinary motor cables) should be used for power cables, or follow the frequency converter's user manual.
3) Variable frequency drive control schematic diagram;
I. Main Circuit: The reactor's function is to prevent high-order harmonics generated by the frequency converter from returning to the power grid through the power input circuit and affecting other electrical equipment. Whether a reactor is needed depends on the frequency converter's capacity. A filter is installed at the frequency converter's output to reduce high-order harmonics. A filter should be installed when the distance between the frequency converter and the motor is long. Although the frequency converter itself has various protection functions, phase loss protection is not perfect. Circuit breakers in the main circuit provide overload and phase loss protection; their selection should be based on the frequency converter's capacity. The frequency converter's built-in overload protection can replace the thermal relay.
II. Control circuit: It has a manual switching function between power frequency and frequency conversion, so that the power frequency can be manually switched to operation when the frequency converter fails. Since the output terminal cannot be energized, the power frequency and frequency converter must be interlocked.
4) Grounding of the frequency converter;
Proper grounding of the frequency converter is crucial for improving system stability and noise suppression. The lower the grounding resistance of the frequency converter's grounding terminal, the better. The cross-sectional area of the grounding wire should be no less than 4mm², and its length should not exceed 5m. The frequency converter's grounding should be separate from the grounding point of the power equipment; they cannot share a common ground. One end of the signal cable's shielding layer should be connected to the frequency converter's grounding terminal, while the other end should remain floating. The frequency converter and the control cabinet should be electrically connected.
Control cabinet design
The frequency converter should be installed inside the control cabinet. The following issues should be considered when designing the control cabinet:
1) Heat dissipation problem:
The heat generated by a frequency converter is due to internal losses. Among the various losses in a frequency converter, the main circuit accounts for approximately 98%, while the control circuit accounts for 2%. To ensure the normal and reliable operation of the frequency converter, heat dissipation is essential, typically achieved using a fan. The internal fan of the frequency converter removes heat from the internal enclosure. If the fan malfunctions, the frequency converter should be stopped immediately. High-power frequency converters also require additional fans on the control cabinet. The air duct of the control cabinet must be designed reasonably, with dust filters installed at all air inlets, ensuring unobstructed exhaust and preventing the formation of eddies and dust accumulation in fixed locations within the cabinet. The appropriate fan should be selected based on the ventilation requirements specified in the frequency converter's instruction manual, and vibration prevention should be considered during fan installation.
2) Electromagnetic interference issues:
I. During operation, frequency converters generate a lot of interfering electromagnetic waves due to rectification and frequency conversion. These high-frequency electromagnetic waves can interfere with nearby instruments and meters, and can also generate higher harmonics. These higher harmonics can enter the entire power supply network through the power supply circuit, thus affecting other instruments. If the power of the frequency converter is very large, accounting for more than 25% of the entire system, anti-interference measures for the control power supply need to be considered.
II. When there are high-frequency impact loads in the system, such as welding machines or electroplating power supplies, the frequency converter itself may trigger protection due to interference. In this case, the power quality of the entire system should be considered.
3) The following points should be noted regarding protection:
I. Waterproof and anti-condensation: If the frequency converter is placed on site, it is necessary to ensure that there are no pipe flanges or other leaks above the frequency converter cabinet, and that there is no water splashing near the frequency converter. In short, the protection level of the on-site cabinet should be above IP43.
II. Dust Control: All air inlets must be equipped with dust screens to prevent fibrous debris from entering. The dust screens should be designed to be removable for easy cleaning and maintenance. The mesh size of the dust screens should be determined based on the specific site conditions, and the joints between the dust screens and the control cabinet must be tightly sealed.
III. Resistance to corrosive gases: This is more common in the chemical industry, in which case the frequency converter cabinet can be placed in the control room.
Wiring specifications
Signal lines and power lines must be routed separately: When using analog signals to remotely control the frequency converter, to reduce interference from the frequency converter and other equipment, please route the signal lines controlling the frequency converter separately from the high-voltage circuits (main circuit and sequential control circuit). The distance should be at least 30cm. Even inside the control cabinet, this wiring standard must be maintained. The control loop between the signal line and the frequency converter must not exceed 50m.
Signal lines and power lines must be placed in separate metal conduits or flexible metal hoses: If the signal lines connecting the PLC and the frequency converter are not placed in metal conduits, they are easily interfered with by the frequency converter and external equipment; at the same time, since the frequency converter does not have a built-in reactor, the input and output power lines of the frequency converter will generate strong interference to the outside. Therefore, the metal conduit or flexible metal hose containing the signal lines must extend to the control terminals of the frequency converter to ensure that the signal lines and power lines are completely separated.
1) Analog control signal lines should use double-strand shielded cable with a wire specification of 0.75mm². When wiring, be sure to strip the cable as short as possible (about 5-7mm), and wrap the stripped shielding layer with insulating tape to prevent the shielding cable from contacting other equipment and introducing interference.
2) To improve the ease and reliability of wiring, it is recommended to use wire clamp terminals on signal lines.
Parameter settings
Inverters have many setting parameters, each with a certain selection range. During use, it is common to encounter situations where the inverter cannot work properly due to improper setting of individual parameters.
Control method: This includes speed control, torque control, PID control, or other methods. After adopting a control method, static or dynamic identification is generally required depending on the control accuracy.
Minimum operating frequency: This refers to the minimum speed at which the motor can operate. When a motor runs at low speeds, its heat dissipation performance is very poor, and prolonged operation at low speeds can lead to motor burnout. Furthermore, at low speeds, the current in the cables also increases, causing the cables to overheat.
Maximum operating frequency: The maximum frequency of a typical frequency converter is 60Hz, and some even reach 400Hz. High frequency will cause the motor to run at high speed. For ordinary motors, the bearings cannot run at speeds exceeding the rated speed for a long time. Can the motor rotor withstand such centrifugal force?
Carrier frequency: The higher the carrier frequency is set, the greater the higher harmonic components will be. This is closely related to factors such as cable length, motor heating, cable heating, and inverter heating.
Motor parameters: The inverter sets the motor's power, current, voltage, speed, and maximum frequency in the parameters. These parameters can be obtained directly from the motor nameplate.
Frequency hopping: Resonance may occur at a certain frequency point, especially when the entire device is relatively high; when controlling the compressor, the compressor surge point should be avoided.
Common Fault Analysis
1) Overcurrent fault: Overcurrent faults can be categorized into acceleration, deceleration, and constant speed overcurrent. They may be caused by factors such as excessively short acceleration/deceleration times of the inverter, sudden load changes, uneven load distribution, or output short circuits. Remedies generally include extending acceleration/deceleration times, reducing sudden load changes, adding energy-consuming braking components, improving load distribution design, and inspecting the circuit. If the inverter still experiences an overcurrent fault after disconnecting the load, it indicates a loop in the inverter circuit, requiring replacement of the inverter.
2) Overload Faults: Overload faults include inverter overload and motor overload. These may be caused by factors such as excessively short acceleration time, low grid voltage, or excessive load. They can generally be resolved by extending the acceleration time, extending the braking time, and checking the grid voltage. Excessive load may be caused by the selected motor and inverter being unable to drive the load, or it may be due to poor mechanical lubrication. In the former case, a higher-power motor and inverter must be replaced; in the latter case, the production machinery must be repaired.
3) Undervoltage: This indicates a problem with the power input section of the frequency converter. It needs to be checked before operation.
