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Introduction of frequency converter application, frequency converter principle and energy saving principle of frequency converter

Aug 21, 2020

Introduction of frequency converter application, frequency converter principle and energy saving principle of frequency converter

Abstract: Inverter is an electric energy control device that uses the on-off action of power semiconductor devices to transform industrial frequency power into another frequency. The inverters we use now mainly adopt the AC-DC-AC method (VVVF frequency conversion or vector control frequency conversion). First, the industrial frequency AC power supply is converted into a DC power supply through a rectifier, and then the DC power supply is converted into a controllable frequency and voltage. AC power to supply the motor. The circuit of the frequency converter is generally composed of four parts: rectification, 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 a PWM waveform, and the intermediate DC link is filtering, DC energy storage and buffering reactive power.


  Inverter is an electric energy control device that uses the on-off action of power semiconductor devices to transform industrial frequency power into another frequency. The frequency converters we use now mainly adopt the AC-DC-AC method (VVVF frequency conversion or vector control frequency conversion). First, the industrial frequency AC power supply is converted into a DC power supply through a rectifier, and then the DC power supply is converted into a frequency and voltage controllable AC power to supply the motor. The circuit of the frequency converter is generally composed of four parts: rectification, 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 a PWM waveform, and the intermediate DC link is filtering, DC energy storage and buffering reactive power.

   Inverter selection:

   The following points should be determined when selecting the inverter:

  1) The purpose of using frequency conversion; 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. The performance curve determines the method of application.

  3) The matching problem between the inverter and the load;

  I. Voltage matching; the rated voltage of the inverter 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 inverter current and overload capacity with the maximum current.

  III. Torque matching; this situation may occur under constant torque load or with a deceleration device.

  4) When the inverter is used to drive a high-speed motor, the high-order harmonics increase due to the low reactance of the high-speed motor, resulting in an increase in the output current value. Therefore, the capacity of the inverter used in high-speed motors is slightly larger than that of ordinary motors.


5) If the inverter needs to run with a long cable, measures should be taken at this time to suppress the impact of the long cable on the ground coupling capacitance to avoid insufficient output of the inverter. Therefore, in this case, the inverter capacity should be enlarged by one gear or Install output reactor at the output end.


  6) For some special applications, such as high temperature and high altitude, the inverter will be derated at this time, and the inverter capacity should be enlarged by one gear.


  Inverter control principle diagram design:


  1) First confirm the installation environment of the inverter;


  I. Working temperature. There are high-power electronic components inside the inverter, which are easily affected by the operating temperature. The product generally requires 0~55℃, but in order to ensure safe and reliable work, you should consider leaving room when using it, and it is best to control it below 40℃ . In the control box, the inverter should generally be installed in the upper part of the box, and strictly abide by the installation requirements in the product manual. It is absolutely not allowed to install heating elements or components that are prone to heat close to the bottom of the inverter.


  II. Ambient temperature. When the temperature is too high and the temperature changes greatly, condensation is likely to occur inside the inverter, and its insulation performance will be greatly reduced, and it may even cause a short circuit accident. When necessary, desiccant and heater must be added to the box. In the water treatment room, the water vapor is generally heavier. If the temperature changes greatly, this problem will be more prominent.


  III. Corrosive gas. If the concentration of corrosive gas is high in the use environment, it will not only corrode the leads of components and printed circuit boards, but also accelerate the aging of plastic devices and reduce the insulation performance.


  IV. Vibration and shock. When the control cabinet equipped with the inverter is subjected to mechanical vibration and impact, it will cause poor electrical contact. Such a problem occurred in Huaian Thermal Power. At this time, in addition to improving the mechanical strength of the control cabinet and staying away from vibration sources and shock sources, vibration-proof rubber pads should also be used to fix the vibration-producing components such as electromagnetic switches outside and inside the control cabinet. After the equipment has been running for a period of time, it should be inspected and maintained.


  V. Electromagnetic wave interference. Due to the rectification and frequency conversion of the frequency converter during operation, a lot of interference electromagnetic waves are generated around, and these high frequency electromagnetic waves have certain interference to nearby instruments and instruments. Therefore, the instrument and electronic system in the cabinet should use metal casing to shield the interference of the inverter to the instrument. All components should be grounded reliably. In addition, shielded control cables should be used for wiring between electrical components, instruments and meters, and the shielding layer should be grounded. If electromagnetic interference is not handled properly, the entire system will often fail to work, leading to failure or damage to the control unit.


   2) The distance between the inverter and the motor determines the cable and wiring method;


  I. The distance between the inverter and the motor should be as short as possible. This reduces the cable's capacitance to ground and reduces the source of interference.


  II. Use shielded cables for control cables, shielded cables for power cables or use conduits for shielding from the inverter to the motor.


  III. The motor cable should be routed independently of other cables, and the minimum distance is 500mm. At the same time, avoid the long-distance parallel routing of the motor cable and other cables, so as to reduce the electromagnetic interference caused by the rapid change of the inverter output voltage. If the control cables and power cables cross, they should be crossed at an angle of 90 degrees as much as possible. The analog signal wires related to the inverter are routed separately from the main circuit wires, even in the control cabinet.


  IV. It is better to use shielded twisted-pair cables for analog signal wires related to the inverter, and use shielded three-core cables for power cables (its specifications are larger than those of ordinary motor cables) or follow the inverter user manual.

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  3) Inverter control principle diagram;


I. Main circuit: The function of the reactor is to prevent the high-order harmonics generated by the inverter from returning to the grid through the input circuit of the power supply, thereby affecting other power receiving equipment. It is necessary to decide whether to add a reactor according to the capacity of the inverter; The filter is installed at the output end of the inverter to reduce the higher harmonics output by the inverter. When the distance between the inverter and the motor is far, the filter should be installed. Although the inverter has various protection functions, the lack of phase protection is not perfect. The circuit breaker acts as an overload and lack of equal protection in the main circuit. The selection can be made according to the capacity of the inverter. The thermal relay can be replaced by the overload protection of the inverter itself.


  II. Control circuit: with manual switching of power frequency frequency conversion, so that the power frequency can be manually cut when the frequency conversion fails. Because the output terminal cannot be voltage, the fixed power frequency and the frequency conversion must be interlocked.


  4) Grounding of the frequency converter;


  The correct grounding of the inverter is an important means to improve the stability of the system and suppress the noise. The grounding resistance of the grounding terminal of the inverter is as small as possible. The cross-section of the grounding wire shall not be less than 4mm and the length shall not exceed 5m. The grounding point of the frequency converter should be separated from the grounding point of the power equipment, not the same ground. One end of the shielding layer of the signal wire is connected to the ground terminal of the inverter, and the other end is floating. There is electrical communication between the inverter and the control cabinet.


   Inverter control cabinet design:


  The inverter should be installed inside the control cabinet. The following issues should be paid attention to when designing the control cabinet


  1) Heat dissipation problem: The heat generated by the inverter is caused by internal losses. The main circuit mainly accounts for 98% of the losses in each part of the frequency converter, and the control circuit accounts for 2%. In order to ensure the normal and reliable operation of the inverter, the inverter must be dissipated by a fan. The built-in fan of the inverter can take away the heat from the cabinet of the inverter. If the fan cannot work normally, the inverter should be stopped immediately; High-power inverters also need to add a fan to the control cabinet, the air duct of the control cabinet should be designed reasonably, and all air inlets should be equipped with dust-proof nets, and the exhaust air should be smooth to avoid the formation of eddy currents in the cabinet and the accumulation of dust in a fixed position; Choose the matching fan according to the ventilation volume of the inverter manual. The fan installation should pay attention to the problem of vibration.


  2) Electromagnetic interference problem:


I. Due to the rectification and frequency conversion of the inverter during operation, a lot of interference electromagnetic waves are generated around. These high-frequency electromagnetic waves have a certain interference to nearby instruments and instruments, and will produce high-order harmonics. This high-order harmonics will Enter the entire power supply network through the power supply loop, thereby affecting other meters. If the power of the inverter accounts for more than 25% of the entire system, you need to consider anti-interference measures for the control power supply.


  II. When there are high-frequency shock loads in the system, such as electric welders and electroplating power supplies, the inverter itself will be protected due to interference, so consider the power quality of the entire system.


  3) The protection issues need to pay attention to the following points:


I. Waterproof and anti-condensation: If the inverter is placed on site, it should be noted that there are no pipe flanges or other leaks above the inverter cabinet, and there should be no splashing water near the inverter. In short, the protection level of the on-site cabinet should be IP43 the above.


  II. Dust-proof: All air inlets should be equipped with dust-proof nets to prevent flocculent debris from entering. The dust-proof nets should be designed to be detachable to facilitate cleaning and maintenance. The grid of the dust-proof net is determined according to the specific conditions of the site, and the joints around the dust-proof net and the control cabinet should be handled strictly.


  III. Anti-corrosive gas: This situation is more common in the chemical industry. At this time, the frequency conversion cabinet can be placed in the control room.


  Inverter wiring specification:


The signal line and the power line must be routed separately: When using analog signals to remotely control the inverter, in order to reduce the analog signal interference from the inverter and other equipment, please connect the signal line of the control inverter to the strong current circuit (main circuit). And sequence control loop) separate wiring. The distance should be above 30cm. Even in the control cabinet, the same wiring specifications must be maintained. The longest control circuit line between this signal and the inverter shall not exceed 50m.


The signal line and power line must be placed in different metal pipes or metal hoses: if the signal line connecting the PLC and the inverter is not placed in the metal pipe, it is very susceptible to interference from the inverter and external equipment; at the same time, due to the inverter There is no built-in reactor, so the input and output power lines of the inverter will cause strong interference to the outside. Therefore, the metal pipe or metal hose for the signal line must be extended to the control terminal of the inverter to ensure the signal line Complete separation from the power line.


  1) The analog control signal wire should use twisted-pair shielded wire, the wire size is 0.75mm2. When wiring, pay attention to 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 the shielding wire from contacting other equipment and causing interference.


  2) In order to improve the simplicity and reliability of wiring, it is recommended to use crimping bar terminals on signal wires.


   Inverter operation and related parameter settings:


   There are many setting parameters of the inverter, and each parameter has a certain selection range. In use, it is often encountered that the inverter cannot work normally due to improper setting of individual parameters.


  Control method: namely speed control, torque control, PID control or other methods. After adopting the control method, it is generally necessary to perform static or dynamic identification according to the control accuracy. 


  Minimum operating frequency: the minimum speed at which the motor runs. When the motor runs at a low speed, its heat dissipation performance is very poor. If the motor runs at a low speed for a long time, it will cause the motor to burn. And at low speed, the current in the cable will increase, which will also cause the cable to heat up.


Maximum operating frequency: The maximum frequency of general inverters is up to 60Hz, and some even up to 400 Hz. High frequency will make the motor run at high speed. For ordinary motors, the bearings cannot run at super-rated speed for a long time. Whether the rotor of the motor Can withstand such centrifugal force.


  Carrier frequency: the higher the carrier frequency setting, the higher the harmonics<