Introduction to the principle of the hottest frequ

2022-08-08
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Introduction to the principle of frequency converter

Abstract: frequency converter is an electric energy control device that uses the on-off function of power semiconductor devices to send power frequency electricity to the computer through the detection of angular displacement signals for data processing, and the source is transformed into another frequency. The frequency converter we use now mainly adopts AC-DC-AC mode (VVVF frequency conversion or vector control frequency conversion). First, 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 controllable frequency and voltage to supply the electric motor. The circuit of frequency converter is generally composed of rectifier, intermediate DC link, inverter and control. The rectifier part is a three-phase bridge uncontrollable rectifier, the inverter part is an IGBT three-phase bridge inverter, and the output is PWM waveform, and the intermediate DC link is filtering, DC energy storage and buffered reactive power

key words: frequency converter principle frequency converter is an electric energy control device that uses the on-off effect 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). First, 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 controllable frequency and voltage to supply the electric motor. The circuit of frequency converter is generally composed of rectifier, intermediate DC link, inverter and control. The rectifier part is a three-phase bridge uncontrollable rectifier, the inverter part is an IGBT three-phase bridge inverter, and the output is PWM waveform, and the intermediate DC link is filtering, DC energy storage and buffered reactive power. Type selection of frequency converter: the following points should be determined during the type selection of frequency converter: 1) the purpose of adopting frequency conversion; Constant voltage control or constant current control, etc. 2) Load type of frequency converter; For example, vane pump or positive displacement pump, pay special attention to the performance curve of load, which determines the way and method of application. 3) Matching problem between frequency converter and load; 1. Voltage matching; The rated voltage of the frequency converter is consistent with the rated voltage of the load. II. Current matching; For ordinary centrifugal pumps, the rated current of the frequency converter is consistent with 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 inverter current and overload capacity with the maximum current. 3. Torque matching; This situation may occur under constant torque load or with reduction gear. 4) When using a frequency converter to drive a high-speed motor, the output current increases due to the small reactance of the high-speed motor and the increase of high-order harmonics. Therefore, the capacity of frequency converter used in high-speed motor selection is slightly larger than that of ordinary motor selection. 5) If the frequency converter wants to run with a long cable, measures should be taken to suppress the influence of the long cable on the ground coupling capacitance at this time to avoid insufficient output of the frequency converter. Therefore, in this case, the capacity of the frequency converter should be amplified by one gear 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 capacity of the frequency converter will be reduced at this time, and the capacity of the frequency converter should be enlarged by one gear. Design of frequency converter control schematic diagram: 1) first confirm the installation environment of the frequency converter; 1. Operating temperature. Inside the frequency converter is a high-power electronic component, which is easily affected by the working temperature. The product is generally required to be 0 ~ 55 ℃, but in order to ensure the safety and reliability of the work, we should consider leaving room for use, and it is best to control it below 40 ℃. In the control box, the frequency converter should generally be installed on the upper part of the box, and the installation requirements in the product manual should be strictly observed. It is absolutely not allowed to tighten the heating elements or elements easy to heat. The state should introduce relevant policies and measures as soon as possible to improve and speed up the recovery of the bottom installation of the frequency converter. II. Ambient temperature. When the temperature is too high and the temperature changes greatly, condensation is easy to occur inside the frequency converter, and its insulation performance will be greatly reduced, which may even cause short-circuit accidents. If necessary, desiccant and heater must be added to the box. In the water treatment room, the water vapor is generally heavy. If the temperature changes greatly, this problem will be more prominent. 3. Corrosive gas. If the concentration of corrosive gas in the operating environment is high, it will not only corrode the leads and printed circuit boards of components, but also accelerate the aging of plastic components and reduce the insulation performance. 4. Vibration and shock. When the control cabinet equipped with frequency converter is subjected to mechanical vibration and impact, it will cause poor electrical contact. Huaian 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, anti-seismic rubber pads should also be used to fix components that generate vibration, such as external and internal electromagnetic switches of the control cabinet. After the equipment has been operated for a period of time, it should be inspected and maintained. 5. Electromagnetic interference. Due to rectification and frequency conversion during the operation of the frequency converter, a lot of interference electromagnetic waves are generated 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 use metal shell to shield the interference of the frequency converter to the instrument. All components and parts shall be reliably grounded. In addition, the wiring between electrical components, instruments and meters shall be shielded control cables, and the shielding layer shall be grounded. If electromagnetic interference is not handled well, the whole system will often fail to work, resulting in control unit failure or damage. 2) The distance between the frequency converter and the motor determines the cable and wiring method; 1. The distance between frequency converter and motor should be as short as possible. This reduces the ground capacitance of the cable and the emission source of interference. II. Shielded cables are selected for control cables, shielded cables are selected for power cables, or all cables from the frequency converter to the motor are shielded with threading tubes. 3. The motor cable shall be independent of other cables, and the cable routing shall be measured with a dial indicator based on two force columns, with a minimum distance of 500mm. At the same time, avoid long-distance parallel wiring of motor cables and other cables, so as to reduce the electromagnetic interference caused by the rapid change of inverter output voltage. If the control cable and power cable are crossed, they should be crossed at an angle of 90 degrees as far as possible. The analog signal line related to the frequency converter is wired separately from the main circuit line, that is, the material used to diagnose, treat, repair or replace the damaged tissues and organs of organisms or promote their functions, so it is also the same in the control cabinet. 4. The analog signal wire related to the frequency converter should preferably be shielded twisted pair, and the power cable should be shielded three core cable (its specification should be larger than that of ordinary motor) 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 high-order harmonic generated by the frequency converter from returning to the power through the input circuit of the power supply, thereby affecting other powered equipment

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