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1. The purpose of using frequency conversion; Constant voltage control orConstant current controlWait.
2. The load type of the frequency converter; Such as vane pumps or positive displacement pumps, etc., pay special attention to the loadPerformance curve, the performance curve determines the way and method when applying.
3. The matching problem between the frequency converter and the load;
1) Voltage matching; The rated voltage and load of the frequency converterRated voltageMatch.
2) current matching; ordinarycentrifugal pump, the rated current of the frequency converter and the motorRated currentMatch. For special loads, such as deep water pumps, it is necessary to refer to the motor performance parameters to determine the inverter current and overload capacity with the maximum current.
3) Torque matching; This situation may occur at constant torque loadsReduction deviceIt can happen from time to time.
4. When using a frequency converter to drive a high-speed motor, due to the small reactance of the high-speed motor, the increase of high harmonics leads to the outputCurrent valueEnlarge. Therefore, the capacity of the frequency converter used for high-speed motors is slightly larger than that of ordinary motors.
5. If the frequency converter is to run with a long cable, measures should be taken to suppress the long cable to the groundCoupling capacitorsTherefore, in this case, the capacity of the inverter should be enlarged by one gear or the output of the inverter should be installed at the output of the inverterreactor。
6. For some special applications, such as high temperature and high altitude, it will cause the frequency converter at this timeSurrender, the capacity of the frequency converter should be enlarged by one stop.
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Control schematicdevise
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The design steps of the inverter control schematic are as follows:
1. First, confirm the installation environment of the frequency converter
1) Operating temperature. The frequency converter is inside, with high powerElectronic, is very susceptible to the influence of working temperature, the product is generally required to be 0~55°C, but in order to ensure the safety and reliability of work, there should be room for use, and it is best to control it below 40°C. In the control box, the frequency converter should generally be installed on the upper part of the box, and strictly abide by the installation requirements in the product manual, and absolutely do not allow itHeating elementsor heat-generating components are mounted close to the bottom of the drive.
2) Ambient temperature. When the temperature is too high and the temperature changes greatly, condensation is prone to occur inside the frequency converter, and its insulation performance will be greatly reduced, and may even cause short circuit accidents. If necessary, a desiccant and heater must be added to the box. In the water treatment room, the water vapor is generally heavier, and if the temperature changes greatly, this problem will be more prominent.
3) Corrosive gas. If the concentration of corrosive gas is high in the operating environment, it will not only corrode the leads and printing of componentsCircuit boardetc., and will also accelerate the aging of plastic devices and reduce insulation performance.
4) Vibration and shock. When the control cabinet equipped with the frequency converter is subjected to mechanical vibration and shock, it will cause poor electrical contact. Huai’an Thermal Power has such a problem. At this time, in addition to improving the mechanical strength of the control cabinet and staying away from vibration and shock sources, seismic rubber pads should also be used to fix the outside and inside of the control cabinetElectromagnetic switchesand other components that produce vibration. After the equipment has been running for a period of time, it should be inspected and maintained.
5）electromagnetic waveInterference.The frequency converter is in operation due to rectification andfrequency conversion, there are a lot of interference electromagnetic waves around, and these high-frequency electromagnetic waves have certain interference with nearby instruments and instruments. Therefore, the instrument and electronic system in the cabinet should be equipped with a metal shell to shield the interference of the frequency converter from the instrument. All components should be reliably grounded, in addition, the connection between each electrical component, instrument and meter should be selected with shielded control cables, and the shielding layer should be grounded. If electromagnetic interference is not handled well, it often makes the entire system unable to work, resulting inControl unitFailure or damage.
2. The distance between the frequency converter and the motor determines the cable and wiring method
1) The distance between the frequency converter and the motor should be as short as possible. This reduces the cableGround capacitors, reducing interference emission sources.
2) The control cable is shielded cable,Power cablesChoose shielded cables or shield all wire pipes from the frequency converter to the motor.
3) The motor cable should be independent of other cable routing, and its minimum distance is 500mm. At the same time, the motor cable should be avoided from running parallel to other cables over long distances, so as to reduce the electromagnetic interference caused by the rapid change of the output voltage of the frequency converter. IfControl cablesand power cables, they should be crossed at a 90-degree angle as much as possible. The analog signal lines related to the frequency converter are routed separately from the main return route, even in the control cabinet.
4) Related to frequency convertersAnalog signal lineIt is best to choose a shielded twisted pair, the power cable is shielded three-core cable (its specifications are larger than that of ordinary motor cables) or follow the user manual of the frequency converter.
3. Schematic diagram of frequency converter control
1）Main circuitThe role of the reactor is to prevent the high harmonics generated by the frequency converter from returning to the power grid through the input loop of the power supply and affecting other powered equipment.filterIt is installed at the output end of the frequency converter to reduce the high-order harmonics output of the frequency converter, and when the distance from the frequency converter to the motor is long, the filter should be installed. Although the inverter itself has various protection functions, but:Lack of phase protectionHowever, it is not perfect, the circuit breaker plays an overload in the main circuit, lacks equal protection, and can be selected according to the capacity of the frequency converter. It can be replaced by overload protection from the frequency converter itselfThermal relays。
2) Control loop: manual switching with power frequency conversion so that the power frequency can be manually cut to operate when the frequency conversion fails, because the output can not add voltage, and the fixed power frequency and frequency conversion must be interlocked.
4. Grounding of frequency converter
Proper grounding of frequency converters is an important means to improve system stability and noise suppression capabilities. The smaller the grounding resistance of the grounding terminal of the frequency converter, the better, and the cross-section of the grounding wire should not be less than 4mm and the length should not exceed 5m. The grounding of the frequency converter should be separated from the grounding point of the power equipment and cannot be shared with the ground. One end of the shield of the signal line is connected to the ground end of the frequency converter, and the other end is floating. The frequency converter and the control cabinet are electrically connected.
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Control cabinet design
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The frequency converter should be installed inside the control cabinet, and the control cabinet should pay attention to the following issues when designing
1. Heat dissipation problem
The heat generation of the frequency converter is generated by internal losses. In the loss of each part of the frequency converter, the main circuit is the main circuit, accounting for about 98%.Control circuitaccounted for 2%. In order to ensure the normal and reliable operation of the frequency converter, the frequency converter must be dissipated, we usually use fans for heat dissipation; The internal fan of the inverter can take away the heat dissipation inside the inverter box, and if the fan cannot work normally, the inverter should be stopped immediately; The high-power inverter also needs to add a fan to the control cabinet, the air duct of the control cabinet should be reasonably designed, and all air inlets should be equipped with dust screens, and the exhaust air should be smooth to avoid formation in the cabineteddy, forming a dust accumulation in a fixed position; Choose the matching fan according to the ventilation volume of the inverter manual, and pay attention to the shockproof problem when installing the fan.
2. Electromagnetic interference problem
1) The frequency converter is due to in the workrectifyingand frequency conversion, which generates a lot of interference electromagnetic waves around, theseHigh-frequency electromagnetic wavesThere is a certain amount of interference to nearby instruments and instruments, and it will be generatedHigh harmonicsThis high harmonic will enter the entire power supply network through the power supply loop, thus affecting other meters. If the power of the frequency converter is large, accounting for more than 25% of the entire system, anti-interference measures to control the power supply need to be considered.
2) When there is a high-frequency shock load in the system, such as:Welding machineWhen electroplating the power supply, the frequency converter itself will be protected due to interference, and the power quality of the whole system will be considered.
3. The following points need to be paid attention to for protection issues
1) Waterproof and anti-condensation: If the inverter is placed on site, it should be noted that there are no pipe flanges or other leakage points above the inverter cabinet, and there can be no splashing water near the inverter, in short, the protection level of the on-site cabinet should be above IP43.
2) Dustproof: All air inlets should be equipped with dust nets to block the entry of flocculent debris, and the dust nets should be designed as detachable to facilitate cleaning and maintenance. The grid of the dust net is determined according to the specific situation of the site, and the junction around the dust net and the control cabinet should be handled tightly.
3）Corrosion protectionSexual gas: This situation is more common in the chemical industry, and it can be used at this timeInverter cabinetPlaced in the control room.
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Wiring specifications
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The signal line and the power line must be routed separately: When using analog signals to remotely control the frequency converter, in order to reduce the interference of the analog from the frequency converter and other equipment, please connect the signal line controlling the frequency converter with the frequency converterStrong electrical circuit(main circuit and forward control loop) separate wiring. The distance should be more than 30cm. Even in the control cabinet, such wiring specifications must be maintained. The maximum length of the control return route between the signal and the frequency converter shall not exceed 50m.
The signal and power lines must be placed separately inside different metal pipes or metal hoses: the signal lines connecting the PLC to the frequency converter are highly vulnerable to frequency converters if they are not placed in metal pipesExternal devicesinterference; At the same time, because the frequency converter does not have a built-in reactor, the input and output power lines of the frequency converter will cause strong interference to the outside, so the metal pipe or metal hose where the signal line is placed must be extended to the control terminal of the frequency converter to ensure that the signal line is with the frequency converterPower lineCompletely separated. 1) The analog control signal line should use double-stranded shielded wire, and the wire specification is 0.75mm2. When wiring, it must be noted that the cable stripping should be as short as possible (about 5-7mm), and the shielding layer after stripping should be wrapped with insulating tape to prevent interference from the contact between the shield wire and other equipment. 2) In order to improve the simplicity and reliability of wiring, it is recommended to use crimping rod terminals on signal lines.
The operation of the frequency converter and the setting of related parameters: The frequency converter has many setting parameters, and each parameter has a certain selection range. Control mode: i.e. speed control,Rotation control, PID control, or other means. After adopting the control method, it is generally based on theControl precision, static or dynamic identification is required.Minimum operating frequency: That is, the minimum speed of the motor running, when the motor runs at low speed, its heat dissipation performance is very poor, and the motor runs at low speed for a long time, which will cause the motor to burn out. And at low speed, the current in the cable will also increase, which will also cause the cable to heat up.
Maximum operating frequency：The maximum frequency of the general frequency converter is up to 60Hz, and some even reach 400 Hz, and the high frequency will make the motor run at high speed. Carrier frequency: The higher the carrier frequency setting, the larger the higher harmonic component, which is closely related to the length of the cable, the heat generated by the motor, the heating of the cable and the heating of the frequency converter.Motor parameters: The frequency converter sets the power, current, voltage, speed, and maximum frequency of the motor in the parameters, which can be obtained directly from the motor nameplate. Frequency hopping: At a certain frequency point, resonance may occur, especially when the entire device is relatively high; In controlcompressorWhen the compressor is avoidedGasping point。
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Common failure analysis
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Overcurrent faults can be divided into acceleration, deceleration, and constant speed overcurrent. It may be caused by the acceleration and deceleration time of the frequency converter is too short, the load changes abruptly, the load distribution is uneven, and the output is short-circuited. In this case, it can usually be extended by extensionacceleration and deceleration time, reduce the sudden change of load, add energy-consuming brake elements, carry out load distribution design, and inspect the line. If the load inverter is disconnected or there is an overcurrent fault, the inverter is indicatedInverter circuitIt has been looped, and the frequency converter needs to be replaced.
2. Overload fault
Overload faults include frequency conversion overload andMotor overload。 It may be caused by too short acceleration time, too low grid voltage, excessive load, etc. Generally, it can be checked by extending the acceleration time, extending the braking time, and checkingGrid voltageWait. The load is too heavy and the selected motor and frequency converter cannot drag the load, which may also be due toMechanical lubricationIt’s not easy to cause. If the former, the high-power motor and frequency converter must be replaced; If the latter is the case, the production machinery should be overhauled
It means that there is a problem with the power input part of the frequency converter, which needs to be checked before it can be run.
