Introduction: Frequency converter is a power control device that uses the switching function of power semiconductor devices to convert power frequency power supply to another frequency. The frequency converter we use now mainly adopts AC -DC- AC mode (VVVF frequency conversion or vector control frequency conversion). Firstly, the power frequency AC power supply is converted into DC power supply through rectifier, and then the DC power supply is converted into AC power supply with frequency and voltage control to supply power to the motor. Inverter circuit generally consists of four parts: rectifier, intermediate DC link, inverter and control. The rectifier part is a three-phase bridge controlled rectifier, and the inverter part is an IGBT three-phase bridge inverter, which outputs PWM waveform, filters the intermediate DC link, stores DC energy and buffers reactive power.
Frequency converter is a control device that uses the switching function of power semiconductor devices to convert power frequency power supply into electric energy of another frequency. The frequency converter we use now mainly adopts AC -DC- AC mode (VVVF frequency conversion or vector control frequency conversion). Firstly, the power frequency AC power supply is converted into DC power supply through rectifier, and then the DC power supply is converted into AC power supply with frequency and voltage control to supply power to the motor. Inverter circuit generally consists of four parts: rectifier, intermediate DC link, inverter and control. The rectifier part is a three-phase bridge controlled rectifier, and the inverter part is an IGBT three-phase bridge inverter, which outputs PWM waveform, filters the intermediate DC link, stores DC energy and buffers reactive power.
Frequency converter selection:
When selecting a frequency converter, the following points should be determined:
1) adopts frequency conversion; Constant voltage control or constant current control, etc.
2) Load type of frequency converter; Such as vane pump or positive displacement pump, special attention should be paid to the load performance curve, which determines the application method.
3) Matching between inverter and load;
First, voltage matching; The rated voltage of the frequency converter is consistent with the rated voltage of the load.
Two. Current matching; For ordinary centrifugal pumps, the rated current of frequency converter is consistent with the rated current of motor. For special loads such as deep water well pumps, it is necessary to refer to the motor performance parameters to determine the inverter current and overload capacity with the maximum current.
Three. Torque matching; This happens from time to time in constant torque load or deceleration device.
4) When the inverter is used to drive the high-speed motor, the high-order harmonics increase and the output current value increases due to the small reactance of the high-speed motor. Therefore, for the selection of high-speed motor inverter, its capacity is slightly larger than that of ordinary motor.
5) If the frequency converter needs a long cable to run, measures should be taken to suppress the influence of the long cable on the coupling capacitance to prevent the frequency converter from exerting enough power. Therefore, in this case, the capacity of the inverter should be expanded or an output reactor should be installed at the output end of the inverter.
6) For some special applications, such as high temperature and high altitude, the inverter capacity will be reduced at this time, and the inverter capacity should be increased at this time.
Design of frequency converter control schematic diagram;
1) First confirm the installation environment of the inverter;
First, the working temperature. The high-power electronic components in frequency converters are easily affected by the working temperature, and the product generally requires 0 ~ 55℃, but in order to ensure the safety and reliability of the work, the allowance should be considered when using, and it is best to control it below 40℃. In the control box, the inverter should be installed at the upper part of the box, and the installation requirements in the product manual should be strictly observed. It is absolutely allowed to install heating elements or easily heated elements near the bottom of the frequency converter.
Two. Ambient temperature. When the temperature is too high and the temperature changes greatly, condensation is easy to occur inside the inverter, which greatly reduces its insulation performance and even leads to short circuit accidents. When necessary, desiccant and heater must be added in the box. In the water treatment room, water vapor is usually heavy, and this problem will be more prominent if the temperature changes greatly.
Three. Corrosive gas. If the corrosive gas concentration in the environment is high, it will only corrode component leads, printed circuit boards, etc. And it will also accelerate the aging of plastic devices and reduce the insulation performance.
Four. Vibration and shock. When the control cabinet equipped with frequency converter is subjected to mechanical vibration and impact, it will cause good 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 keeping away from vibration sources and impact sources, the vibration components such as electromagnetic switches outside and inside the control cabinet should also be fixed with anti-seismic rubber pads. After the equipment has been running for a period of time, it should be inspected and maintained.
Electromagnetic interference. Because of the rectification and frequency conversion in the operation of the frequency converter, there are many interference electromagnetic waves around, which will definitely interfere with nearby instruments and meters. Therefore, the instrument and electronic system in the cabinet should adopt metal shell to shield the interference of frequency converter to the instrument. All components should be connected. In addition, shielded control cables should be used for wiring between electrical components and instruments, and the shielding layer should be connected. If the electromagnetic interference is not handled properly, it will often lead to the failure of the whole system and the failure or damage of the control unit.
2) Cable and wiring method for determining the distance between the frequency converter and the motor;
I The distance between the inverter and the motor should be as short as possible. This reduces the capacitance of the cable pair and reduces the interference to the emission source.
Two. Shielded cables shall be used for control cables, and shielded cables for power cables or all cables from frequency converters to motors shall be shielded through threading pipes.
Three. The motor cable shall be routed independently of other cables with a minimum distance of 500mm. At the same time, long-distance parallel wiring of motor cables and other cables should be avoided to reduce the electromagnetic interference caused by the rapid change of inverter output voltage. If the control cable and power cable cross, try to cross at 90 degrees. The analog signal line connected with the frequency converter is wired separately from the main circuit, even in the control cabinet.
Four. It is best to use shielded twisted pair for analog signal lines connected with the frequency converter, and shielded three-core cable (whose specifications are larger than ordinary motor cables) for power cables or follow the user manual of the frequency converter.
3) Control schematic diagram of frequency converter;
1. Main circuit: The function of the reactor is to prevent the inverter from generating higher harmonics, which will return to the power grid through the power input circuit and affect other electrical equipment. It is necessary to decide whether to add reactors according to the capacity of the inverter; The filter is installed at the output end of the frequency converter to reduce the higher harmonics output by the frequency converter. When the frequency converter is far away from the motor, a filter should be installed. Although the inverter itself has various protection functions, the lack of phase protection is not perfect. The main circuit of the circuit breaker plays the role of overload and lack of equal protection, and is selected according to the capacity of the inverter. So as to replace that thermal relay with the overload protection of the frequency convert itself.
Two. Control loop: it has the function of manually switching power frequency conversion. When the frequency conversion fails, it can be operated by manually switching power frequency. Because voltage can be applied to the output terminal, constant power frequency and frequency conversion should be interlocked.
4) frequency converter connection;
The correct connection of frequency converter is an important means to improve system stability and suppress noise. The smaller the resistance of the inverter terminal, the better. The cross section of the connecting wire is less than 4 mm, and the length is more than 5 m. The frequency converter is separated from the power equipment contact, which can be * * *. The shielding layer of the signal line is connected to the inverter terminal, and the other end is suspended. The frequency converter is electrically connected with the control cabinet.
Design of inverter control cabinet;
The frequency converter should be installed in the control cabinet, and the following problems should be paid attention to when designing the control cabinet.
1) heat dissipation problem: the heat generated by the inverter is caused by internal loss. The main circuit accounts for 98% and the control circuit accounts for 2%. In order to ensure the normal operation of the inverter, the inverter must be cooled. We usually use a fan to cool down. The built-in fan of the inverter takes away the heat dissipation in the inverter box. If the fan can work normally, please stop the operation of the inverter immediately. High-power inverter also needs to add a fan to the control cabinet, the air duct of the control cabinet should be designed reasonably, and the air inlet should be provided with a dust-proof net, so that the air can be discharged smoothly, so as to avoid vortex formation in the cabinet and dust accumulation in a fixed position; Select the matching fan according to the ventilation volume in the frequency converter manual, and pay attention to the shockproof problem when installing the fan.
2) Electromagnetic interference:
First, during the operation of the frequency converter, many interfering electromagnetic waves are generated around it. These high-frequency electromagnetic waves will certainly interfere with nearby instruments and meters, and will produce higher harmonics, which will enter the whole power supply network through the power supply loop, thus affecting other instruments and meters. If the power of inverter accounts for more than 25% of the whole system, anti-interference measures of control power supply should be considered.
Two. When high-frequency impact loads such as welding machine and electroplating power supply appear in the system, the inverter itself will be protected by interference, so the power quality of the whole system should be considered.
3) Protection issues need to pay attention to the following points:
1. Waterproof and dewing prevention: When the inverter is placed on the site, attention should be paid to the pipeline flange or other leakage points above the inverter cabinet, and water can be sprayed near the inverter. In short, the protection level of the cabinet on site should be above IP43.
Two. Dust-proof: set a dust-proof net at the air inlet to prevent flocculent impurities from entering. The dust-proof net should be detachable to facilitate cleaning and maintenance. The grid of dust screen should be determined according to the specific site conditions, and the joint between dust screen and control cabinet should be strictly handled.
Three. Anti-corrosive gas: This situation is common in the chemical industry, so the frequency conversion cabinet should be placed in the control room at this time.
Wiring specification of frequency converter:
The signal line and power line must be wired separately: when using analog signals to remotely control the frequency converter, in order to reduce the interference of analog signals of the frequency converter and other equipment, please wire the signal line for controlling the frequency converter separately from the strong current loop (main loop and sequence control loop). The distance should be more than 30cm. Even in the control cabinet, this wiring specification should be maintained. The longest control loop between the signal and the frequency converter exceeds 50 meters.
The signal wire and power wire must be placed in the same metal tube or metal hose respectively: if the signal wire connecting PLC and frequency converter is placed in the metal tube, it will be easily interfered by frequency converter and external equipment; At the same time, because the inverter has no built-in reactor, the power lines of the input stage and the output stage of the inverter will cause strong interference to the outside world, so the metal pipe or metal hose where the signal line is placed should extend to the control end of the inverter to ensure that the signal line is completely isolated from the power line.
1) The analog control signal line shall adopt double-stranded shielded wire with the wire specification of 0.75mm2 When wiring, attention must be paid to the cable stripping as short as possible (about 5-7mm), and at the same time, the stripped shielding layer shall be wrapped with insulating tape to prevent the shielded wire from interfering with other equipment.
2) In order to improve the simplicity and reliability of wiring, it is suggested to use the wire pressing rod terminal on the signal line.
Inverter operation and related parameter settings:
Many parameters are set in the frequency converter, and each parameter has a selection range. In use, it is often encountered that the inverter cannot work normally due to improper setting of different parameters.
Control mode: speed control, torque control, PID control or other modes. After the control mode is adopted, static or dynamic identification is generally needed according to the control accuracy.
Minimum operating frequency: the lowest rotating speed of the motor. When the motor runs at low speed, the heat dissipation performance is very poor, and running at low speed for a long time will burn the motor. Moreover, at low speed, the current in the cable will also increase, which will also lead to the cable heating.
Maximum working frequency: Generally, the maximum frequency of frequency converter reaches 60Hz or even 400 Hz. High frequency will make the motor run at high speed. For an ordinary motor, its bearings can run over speed for a long time, so can the motor rotor withstand such centrifugal force?
Carrier frequency: The higher the carrier frequency is set, the greater the harmonic component, which is closely related to the cable length, motor heating, cable heating, inverter heating and other factors.
Motor parameters: motor power, current, voltage, speed and maximum frequency are set in the inverter parameters, which can be obtained directly from the motor nameplate.
Frequency hopping: at a certain frequency point, vibration may occur, especially when the whole equipment is relatively high; When controlling the compressor, the compressor surge point should be avoided.
Common fault analysis:
1) overcurrent fault: overcurrent fault includes acceleration, deceleration and constant-speed overcurrent. It can be caused by too short acceleration and deceleration time of frequency converter, sudden load change, uneven load distribution and output short circuit. At this time, it is common to extend the acceleration and deceleration time, reduce the sudden change of load, increase energy-consuming braking components, design load distribution and check the line. If the load inverter is still overcurrent when disconnected, it means that the inverter circuit of the inverter has been looped and the inverter needs to be replaced.
2) Overload fault: Overload fault includes frequency conversion overload and motor overload. Its acceleration time is too short, the grid voltage is too low and the load is too heavy. Generally, by extending the acceleration time, extending the braking time and checking the grid voltage, etc. If the load is too heavy, the selected motor and frequency converter will drag the load, or it may be caused by good mechanical lubrication. If it is the former, the high-power motor and frequency converter must be replaced; In the latter case, the production machine should be overhauled.
3) Undervoltage: It means that the power input part of the inverter needs to be checked before operation.
I won't send you an email. See for yourself. This article is very suitable for answering your question.