When choosing the type of converter, the following points should be determined:
1. The purpose of frequency conversion; constant voltage control or constant current control.
2. The load type of frequency converter, such as vane pump or volume pump, pays special attention to the load performance curve, which determines the way of application.
3. The matching problem between frequency converter and load;
1) Voltage matching; the rated voltage of the converter is in accordance with the rated voltage of the load.
2) Current matching; Normal centrifugal pump, the rated current of frequency converter is consistent with the rated current of motor. For special loads such as deep-water pumps, motor performance parameters are needed to determine the current and overload capacity of the converter with the maximum current.
3) Torque matching; this may occur under constant torque loads or deceleration devices.
4. When using frequency converter to drive high-speed motor, the output current value increases due to the small reactance of high-speed motor and the increase of high-order harmonics. Therefore, the capacity of the frequency converter used in high-speed motors is slightly larger than that of ordinary motors.
5. If the frequency converter wants to run with long cable, measures should be taken to restrain the influence of long cable on ground coupling capacitance at this time, so as to avoid the insufficient output of the frequency converter. Therefore, in this case, the capacity of the frequency converter should be enlarged by one gear or 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 converter will be reduced at this time, and the capacity of the converter should be enlarged in one gear.
Inverter control principle:
1. First confirm the installation environment of the converter.
1) Working temperature. Inverter is a high-power electronic component, which is vulnerable to the influence of working temperature. Generally, the product is required to be 0-55 C. However, in order to ensure the safety and reliability of the work, the use of the converter should be considered to leave room, preferably controlled below 40 C. In the control box, the converter should generally be installed on the upper part of the box, and strictly abide by the installation requirements in the product instructions. It is absolutely not allowed to install heating elements or easily heating elements close to the bottom of the converter.
2) Corrosive gases. If the concentration of corrosive gas is high in the environment, it will not only corrode the lead and printed circuit board of components, but also accelerate the aging of plastic devices and reduce the insulation performance.
3) Environmental temperature. When the temperature is too high and the temperature varies greatly, the dew phenomenon is easy to occur inside the converter, and its insulation performance will be greatly reduced, and may even lead to short-circuit accidents. If necessary, desiccants and heaters must be added to the box. In the water treatment room, water vapor is generally heavy, if the temperature changes greatly, this problem will be more prominent.
4) Electromagnetic interference. Because of rectification and frequency conversion, the frequency converter produces a lot of interference electromagnetic waves around it. These high-frequency electromagnetic waves have some interference to nearby instruments and instruments. Therefore, the instrument and electronic system in cabinet should choose metal shell to shield the interference of frequency converter to the instrument. All components should be grounded reliably. In addition, shielding control cables should be selected for the wires between electrical components, instruments and instruments, and the shielding layer should be grounded. If the electromagnetic interference is not handled properly, the whole system will often be unable to work, resulting in the failure or damage of the control unit.
5) Vibration and shock. When the control cabinet equipped with frequency converter is subjected to mechanical vibration and shock, it will cause bad electrical contact.  Huai'an Thermal Power Plant has such a problem. At this time, besides improving the mechanical strength of the control cabinet and keeping away from the source of vibration and shock, the anti-seismic rubber pad should also be used to fix the components that generate vibration, such as the external and internal electromagnetic switches. After the equipment has been running for a period of time, it should be checked and maintained.
2. The Distance between Inverter and Motor to Determine Cable and Wiring Method
1) Shielded cables are used for control cables and shielded cables for power cables or threading pipes are used for shielding from frequency converters to motors.
2) The distance between the converter and the motor should be as short as possible. This reduces the ground capacitance of the cable and the source of interference.
3) Shielded twisted pair cable is the best choice for analog signal line related to frequency converter, shielded three-core cable for power cable (its specification is larger than that of ordinary motor cable) or user manual for frequency converter.
4) Motor cables should be independent of other cables, with a minimum distance of 500 mm. At the same time, it is necessary to avoid the long-distance parallel traveling between the motor cable and other cables, so as to reduce the electromagnetic interference caused by the fast change of the output voltage of the frequency converter. If the control cables and power cables 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 separated from the main circuit line, even in the control cabinet.
3. Control Principle of Inverter
1) Control loop: Manual switching with power frequency conversion, in order to cut power frequency manually when the frequency conversion fails, because the output voltage can not be added, fixed power frequency and frequency conversion must be interlocked.
2) Main circuit: The function of the reactor is to prevent the high-order harmonic generated by the converter from returning to the power grid through the input circuit of the power supply, thus affecting other electrical equipment. It is necessary to decide whether to add the reactor according to the capacity of the converter. The filter is installed at the output end of the converter to reduce the high-order harmonic produced by the converter. When the converter is far away from the motor, Filters should be installed. Although the converter itself has a variety of protection functions, but the lack of phase protection is not perfect, circuit breaker in the main circuit overload, lack of equal protection, type selection can be made according to the capacity of the converter. The thermal relay can be replaced by the overload protection of the converter itself.
4. Grounding of frequency converter;
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