The frequency converter is a power control device that utilizes the on-off action of power semiconductor devices to convert the industrial frequency power supply to another frequency. We are now using the frequency converter is mainly used in AC-DC-AC mode (VVVF frequency conversion or vector control frequency conversion), the first frequency AC power through the rectifier into DC power, and then DC power into frequency, voltage can be controlled AC power to supply the motor. The circuit of inverter is generally composed of four parts: rectifier, intermediate DC link, inverter and control. Rectifier part of the three-phase bridge uncontrollable rectifier, inverter part of the IGBT three-phase bridge inverter, and the output for the PWM waveform, the intermediate DC link for filtering, DC energy storage and buffer reactive power.
Frequency converter control schematic design:
1) Firstly, confirm the installation environment of the frequency converter;
I. Operating temperature. Frequency converter is a high-power electronic components inside, very susceptible to the influence of the operating temperature, the product is generally required to 0 ~ 55 ℃, but in order to ensure safe and reliable work, the use of should be considered to leave a margin of error, it is best to control the temperature below 40 ℃. In the control box, the frequency converter should generally be installed in the upper part of the box, and strictly comply with the installation requirements of the product specification, absolutely do not allow the heat-generating components or easy to heat the components installed close to the bottom of the frequency converter.
II. Ambient temperature. When the temperature is too high and the temperature change is large, the inverter is prone to internal condensation phenomenon, its insulation performance will be greatly reduced, and may even cause a short-circuit accident. If necessary, desiccants and heaters must be added to the box. In the water treatment room, the general water vapor is heavier, if the temperature changes, this problem will be more prominent.
III. Corrosive gases. Use of the environment if the concentration of corrosive gases is large, not only will corrode the component leads, printed circuit boards, etc., but also accelerate the aging of the plastic device, reducing insulation properties.
IV. Vibration and shock. The control cabinet equipped with frequency converter is subjected to mechanical vibration and impact, which will cause poor electrical contact. Huai'an thermoelectric on such a problem. In addition to improve the mechanical strength of the control cabinet, away from the source of vibration and shock sources, should also use anti-vibration rubber pads to fix the control cabinet outside and inside the electromagnetic switches and other components that generate vibration. After a period of operation, the equipment should be inspected and maintained.
V. Electromagnetic interference. Frequency converter in the work due to rectification and frequency conversion, around the generation of a lot of interference electromagnetic waves, these high-frequency electromagnetic waves on the nearby instruments, instruments have a certain degree of interference. Therefore, the cabinet instrumentation and electronic systems, should be selected metal shell, shielding the frequency converter on the interference of the instrument. All components should be reliably grounded, in addition to the electrical components, instruments and meters between the connection should be selected shielded control cable, and the shield should be grounded. If the electromagnetic interference is not handled well, often the whole system can not work, resulting in control unit failure or damage.
2) The distance between the inverter and the motor to determine the cable and wiring method;
I. The distance between the inverter and the motor should be as short as possible. This reduces the capacitance of the cable to ground and reduces the source of interference emission.
II. Control cables use shielded cables, power cables use shielded cables or from the inverter to the motor all shielded with a penetrating pipe.
III. motor cable should be independent of other cable routing, the minimum distance of 500mm. at the same time, should avoid the motor cable and other cables long distance parallel routing, so as to reduce the frequency converter output voltage rapid changes in electromagnetic interference. If the control cable and power cable cross, they should be crossed at an angle of 90 degrees as far as possible. The analog signal line related to the frequency converter and the main circuit line are routed separately, even in the control cabinet.
IV. It is better to use shielded twisted-pair cable for the analog signal line related to the frequency converter, and use shielded three-core cable for the power cable (whose specification should be larger than that of the ordinary motor cable) or follow the user's manual of the frequency converter.
3) Inverter control schematic;
I. Main circuit: the role of the reactor is to prevent the frequency converter generated by the high harmonics through the power supply input circuit to return to the power grid, thereby affecting other powered equipment, need to be based on the size of the capacity of the inverter to determine the need to add the reactor; filter is installed in the output of the frequency converter, to reduce the output of the frequency converter of the high harmonics when the distance from the frequency converter to the motor is relatively large. The filter should be installed when the distance from the inverter to the motor is far. Although the inverter itself has a variety of protection functions, but the phase loss protection is not perfect, the circuit breaker in the main circuit plays a role in overload, phase loss and other protection, selection can be made in accordance with the capacity of the inverter. You can use the overload protection of the inverter itself instead of the thermal relay.
II. Control circuit: with frequency inverter manual switching, in order to frequency inverter failure can be manually cut frequency operation, because the output can not add voltage, solid frequency and frequency to have interlock.
4) Grounding of the inverter;
The correct grounding of the inverter is an important means of improving system stability and suppressing noise. The smaller the grounding resistance of the grounding terminal of the frequency converter, the better, the cross-section of the grounding wire is not less than 4mm, and the length is not more than 5m. The grounding of the frequency converter should be separated from the grounding point of the power equipment, and can not be **** ground. One end of the shield of the signal line is connected to the grounding end of the inverter, and the other end is floating. The inverter and the control cabinet are electrically connected to each other.
Frequency converter control cabinet design:
Frequency converter should be installed inside the control cabinet, the control cabinet should pay attention to the following issues in the design
1) Heat dissipation: the heat of the frequency converter is generated by the internal loss. In the frequency converter in each part of the loss is mainly in the main circuit, accounting for about 98%, the control circuit accounts for 2%. In order to ensure the normal and reliable operation of the frequency converter, the frequency converter must be dissipated we usually use fan cooling; frequency converter internal fan can be the frequency converter box internal heat dissipation away, if the fan can not work properly, you should immediately stop the frequency converter operation; high-power frequency converter also need to add a fan on the control cabinet, the control cabinet air ducts should be reasonably designed, all the inlet should be set up to prevent dust nets, exhaust smooth, avoid the formation of eddy currents in the cabinet, and avoid the formation of eddy currents in the fixed cabinet. cabinet to form eddy currents, the formation of dust accumulation in a fixed location; according to the inverter manual ventilation to select the matching fan, fan installation should pay attention to the problem of anti-vibration.
2) Electromagnetic interference problems:
I. Inverter in the work due to rectification and frequency conversion, around the generation of a lot of interference electromagnetic waves, these high-frequency electromagnetic waves on the nearby meters, instruments have a certain degree of interference, and will produce high harmonics, this high harmonic will be through the power supply circuit into the entire power supply network, thus affecting other instruments. If the power of the frequency converter is very large accounting for more than 25% of the whole system, it is necessary to consider the anti-interference measures of the control power supply.
II. When the system has a high-frequency shock load such as welding machine, plating power supply, the frequency converter itself will be protected because of the interference, then consider the power quality of the whole system.
3) Protection issues need to pay attention to the following:
I. Waterproof condensation: If the inverter is placed on the site, you need to pay attention to the inverter cabinet above the pipeline flange or other leakage, in the vicinity of the frequency converter can not be a splash of water, in short, the site cabinet protection level should be in IP43 or more.
II. Dustproof: all air inlets should be set up to block the dust net flocculent debris into the dust net should be designed as a removable type, in order to facilitate cleaning, maintenance. Dust net grid according to the site-specific conditions to determine the dust net around the combination with the control cabinet.
The place should be handled tightly.
III. Anti-corrosive gases: this is more common in the chemical industry, this time you can put the frequency converter cabinet in the control room.
Frequency inverter wiring specifications:
Signal line and power line must be separated from the line: the use of analog signals for remote control of the frequency converter, in order to reduce the analog by the interference from the frequency converter and other equipment, please control the frequency converter signal line and the strong power circuit (the main circuit and the downstream control circuit) separate line. The distance should be more than 30cm. The same wiring specification should be maintained even inside the control cabinet. The maximum length of the control loop line between this signal and the frequency converter shall not exceed 50m.
Signal lines and power lines must be placed in different metal pipes or inside metal hoses: the signal lines connecting PLC and frequency converter are extremely susceptible to interference by the frequency converter and external equipment if they are not placed in the metal pipes; at the same time, since the frequency converter does not have any built-in reactors, the input and output stages of the frequency converter Power line to the outside will produce extremely strong interference, so place the signal line of the metal pipe or metal hose has to be extended to the control terminals of the frequency converter, in order to ensure that the signal line and the power line of the complete separation.
1) analog control signal line should use double stranded shielded wire, wire specifications for the 0.75mm2. In the wiring must pay attention to, cable stripping should be as short as possible (5-7mm or so), and at the same time after the stripping of shielding layer should be wrapped with insulating tape to prevent shielding line and other equipment contact to introduce interference.
2) In order to improve the simplicity and reliability of the wiring, it is recommended to use the crimp bar terminal on the signal line.
Operation of the frequency converter and the setting of the relevant parameters:
The frequency converter has many setting parameters, each parameter has a certain selection range, often encountered in the use of individual parameters are not set properly, resulting in the frequency converter can not work properly.
Control mode: that is, speed control, pitch control, PID control or other ways. After taking the control mode, generally according to the control accuracy, static or dynamic identification is required.
Minimum operating frequency: i.e., the minimum speed at which the motor operates, when the motor operates at low speed, its heat dissipation performance is very poor, and the motor running at low speed for a long time will lead to motor burnout. And at low speed, the current in its cable will also increase, which will also cause the cable to heat up.
Maximum operating frequency: the general frequency converter maximum frequency to 60 Hz, some even to 400 Hz, high frequency will make the motor high-speed operation, which is for ordinary motors, its bearings can not be a long time to run over the rated speed, the motor's rotor can withstand such a centrifugal force.
Carrier frequency: the higher the carrier frequency is set, the higher the harmonic component, which is closely related to the length of the cable, the motor heat, the cable heat inverter heat and other factors.
Motor parameters: inverter in the parameters set motor power, current, voltage, speed, maximum frequency, these parameters can be obtained directly from the motor nameplate.
Frequency hopping: at a certain frequency point, there is a possibility of *** vibration phenomenon, especially when the whole device is relatively high; in the control of the compressor, to avoid the compressor's wheezing point.
Common failure analysis:
1) overcurrent fault: overcurrent fault can be divided into acceleration, deceleration, constant speed overcurrent. It may be due to the frequency converter acceleration and deceleration time is too short, sudden change of load, uneven load distribution, output short circuit and other reasons. This can generally be done by extending the acceleration and deceleration time, reduce the sudden change of load, plus energy braking elements, load distribution design, check the line. If disconnect the load inverter or overcurrent fault, indicating that the inverter inverter circuit has been ring, need to replace the inverter.
2) overload fault: overload faults include frequency conversion overload and motor overload. It may be acceleration time is too short, the grid voltage is too low, the load is too heavy and other reasons. Generally by extending the acceleration time, extend the braking time, check the grid voltage. The load is too heavy, the selected motor and inverter can not drag the load, may also be caused by poor mechanical lubrication. If the former must be replaced with high-power motors and inverters; if the latter to overhaul the production machinery.
3) undervoltage: the inverter power input part of the problem, need to check before you can run.