Selection
1) the purpose of using frequency; constant pressure control or constant current control.
2) The load type of the inverter; such as vane pump or positive displacement pump, pay special attention to the performance curve of the load, the performance curve determines the way of application method.
3) Matching of frequency converter with load;
I. Voltage matching; The rated voltage of the inverter is in accordance with the rated voltage of the load.
II. Current Matching; Ordinary centrifugal pump, the rated current of the inverter is in accordance with the rated current of the motor. For special loads such as deep water pumps, etc., refer to the motor performance parameters to determine the inverter current and overload capacity with the maximum current.
III. Torque Matches; This may occur when a constant torque load or a deceleration device is present.
4) When using the inverter to drive the high-speed motor, the output current value increases due to the small reactance of the high-speed motor and the increase of the harmonic. So for high-speed motor inverter selection, its capacity to slightly larger than the selection of ordinary motor.
5) If the inverter to run long cable, this time to take measures to suppress the long cable to the ground coupling capacitor, to avoid the inverter output is insufficient, so in this case, the inverter capacity to enlarge a file or in the inverter Outputs the output reactor.
6) for some special applications, such as high temperature, high altitude, this time will cause the inverter derating, the inverter capacity to enlarge a block.
Control schematic design
1) first confirm the inverter installation environment;
I. Operating temperature. Inverter is a high-power electronic components inside, very vulnerable to the impact of the working temperature, the product generally requires 0 ~ 55 ℃, but in order to ensure safe and reliable work, use should be considered leave room, the best control below 40 ℃ The In the control box, the inverter should be installed in the upper part of the box, and strictly abide by the installation instructions in the product manual, absolutely not allowed to heat the element or easy to heat the components close to the bottom of the inverter installation.
II. Ambient temperature. When the temperature is too high and the temperature changes greatly, the condensation inside of the inverter is prone to occur, and its insulation performance will be greatly reduced, and may even lead to short circuit accidents. If necessary, the desiccant and the heater must be added to the tank. In the water treatment, the general water vapor are relatively heavy, if the temperature changes, then the problem will be more prominent.
III. Corrosive gases. Use of the environment If the corrosive gas concentration, not only will corrode the components of the lead, printed circuit boards, but also to accelerate the aging of plastic devices, reducing insulation performance.
IV. Vibration and Shock. The control cabinet with the inverter is subject to mechanical vibration and shock, which can cause poor electrical contact. Huai'an thermal power on the emergence of such a problem. At this time in addition to improve the mechanical strength of the control cabinet, away from the vibration source and the impact of the source, but also the use of anti-seismic rubber pad fixed control cabinet and internal electromagnetic switch and other components of vibration. After the equipment has been in operation for a period of time, it should be inspected and maintained.
V. Electromagnetic interference. Inverter in the work due to rectification and frequency conversion, the surrounding produce a lot of interference with electromagnetic waves, these high-frequency electromagnetic waves on the nearby instruments, instruments have some interference. Therefore, the cabinet instrumentation and electronic systems, should use the metal shell, shielding the inverter interference on the instrument. All the components should be reliable grounding, in addition, the electrical components, equipment and instrumentation between the connection should be shielded control cable, and the shield should be grounded. If the processing is not good electromagnetic interference, often make 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 methods;
I. The distance between the inverter and the motor should be as short as possible. This reduces the ground capacitance of the cable and reduces the source of the interference.
II. Control cable Use shielded cable, power cable use shielded cable or from the inverter to the motor all with a threading machine shield.
III. The motor cable shall be independent of the other cable traces with a minimum distance of 500 mm. At the same time should avoid the motor cable and other cables long distance parallel alignment, so as to reduce the inverter output voltage caused by rapid changes in electromagnetic interference. If the control cable and the power cable cross, as far as possible so that they cross the 90 degree angle. The analog signal lines associated with the frequency converter are routed separately from the main circuit, even in the control cabinet.
IV. The analog signal cable related to the inverter is preferably shielded twisted pair cable, the power cable selection shielded three-core cable (its specifications than ordinary motor cable large file) or follow the inverter user manual. 3) Inverter control schematic;
I. The main circuit: the role of the reactor is to prevent the inverter generated by the high-order harmonic through the power supply input circuit to return to the power grid and thus affect the other power equipment, the size of the inverter according to the size of the need to decide whether the need for reactor; The filter is installed at the output of the inverter to reduce the higher harmonics of the inverter output. When the distance from the inverter to the motor is far away, the filter should be installed. Although the inverter itself has a variety of protection functions, but the lack of protection is not perfect, the circuit breaker in the main circuit to play overload, lack of protection, selection can be selected in accordance with the capacity of the inverter. You can replace the thermal relay with the overload protection of the inverter itself.
II. Control loop: with the frequency of frequency manual switching, in order to change the frequency of failure can manually cut frequency operation, because the output can not add voltage, solid frequency and frequency to have interlock.
4) grounding of the inverter;
Proper grounding of the inverter is an important means to improve the system stability and suppress the noise capacity. The grounding resistance of the grounding terminal of the inverter is as small as possible. The cross section of the grounding conductor is not less than 4mm and the length is not more than 5m. The grounding of the inverter should be separated from the grounding point of the power equipment. One end of the shield of the signal line is connected to the ground terminal of the inverter and the other end is floating. The inverter is electrically connected to the control cabinet.
Control cabinet design
The inverter should be installed inside the control cabinet, the control cabinet in the design should pay attention to the following questions:
1) Heat dissipation problem:
The heat of the frequency converter is generated by the internal loss. In the inverter part of the main loss in the main circuit, accounting for about 98%, control circuit accounted for 2%. In order to ensure the normal and reliable operation of the inverter, we must drive the inverter. We usually use fan cooling. The built-in fan of the inverter can take away the inside of the box of the inverter. If the fan can not work normally, High-power inverter also need to add fan in the control cabinet, control cabinet air duct to design a reasonable, all the air inlet to set the dust network, exhaust ventilation, to avoid the formation of vortex in the cabinet, in a fixed position to form dust accumulation; According to the inverter manual ventilation to select the matching fan, fan installation should pay attention to shock problems.
2) Electromagnetic interference problem:
I. Inverter due to rectification and frequency conversion, the surrounding produce a lot of interference with electromagnetic waves, these high-frequency electromagnetic waves on the nearby instruments, instruments have some interference, and will produce high harmonics, this high-order harmonic Through the power supply circuit into the entire power supply network, thus affecting other instruments. If the power of the inverter accounts for more than 25% of the total system, it is necessary to consider the anti-jamming measures of the control power supply.
II. When the system has high frequency impact load such as welding machine, electroplating power, the inverter itself will be protected because of interference, then consider the entire system power quality problems.
3) protection issues need to pay attention to the following:
I. Waterproof anti-condensation: If the inverter on the scene, need to pay attention to the inverter cabinet above the pipe flange or other leaks, in the vicinity of the inverter can not have splashing water flow, in short, the cabinet protection level IP43 the above.
II. Dust: all the air inlet to set the dust barrier barrier floc debris into the dust network should be designed to be removable to facilitate the cleaning and maintenance. The grid of the air filter is determined according to the specific situation of the site. The combination of the dustproof network and the control cabinet should be handled closely.
III. Corrosive gases: In the chemical industry, this situation is more common, then you can put the inverter cabinet in the control room.
Wiring specification
Signal line and power line must be separated from the line: the use of analog signals for remote control of the inverter, in order to reduce the analog by the inverter and other equipment from the interference, please control the inverter signal line and strong circuit (main circuit And the sequential loop). Distance should be above 30cm. Even in the control cabinet, the same to maintain such wiring specifications. The maximum length of the control loop between the signal and the frequency converter should not exceed 50 m. Signal lines and power lines must be placed in different metal pipes or metal hose inside: the PLC and the inverter connected to the signal line if not placed in the metal pipe, easily affected by the inverter and external equipment interference; No built-in reactor, so the inverter input and output level power line on the outside will have a strong interference, so put the signal line of metal pipe or metal hose has been extended to the control terminal of the inverter to ensure that the signal line Separate from the power line.
1) The analog control signal cable should use double stranded shielded wire, wire size is 0.75mm2. In the wiring must be noted that the cable stripping to be as short as possible (5-7mm or so), while the shield after the use of insulating tape wrapped with insulating tape to prevent the introduction of shielded cable and other equipment to introduce interference.
2) In order to improve the simplicity and reliability of the wiring, it is recommended to use the wire rod terminal on the signal line.
parameter settings
The parameters of the inverter settings, each parameter has a certain range of choice, often encountered in the use of individual parameters set improperly, resulting in the inverter can not work properly.
Control mode: speed control, torque control, PID control or other means. To take control, generally according to the control accuracy, the need for static or dynamic identification.
Minimum operating frequency: the minimum speed of the motor running, the motor running at low speed, the poor heat dissipation, the motor running for a long time at low speed, will cause the motor burned. And low speed, the current in the cable will increase, will lead to cable heating.
Maximum operating frequency: the general frequency converter to the maximum frequency of 60Hz, and some even to 400 Hz, high frequency will make the motor high-speed operation, which for ordinary motor, the bearing can not be a long time ultra-rated speed operation, the motor rotor Can withstand such centrifugal force.
Carrier frequency: the higher the carrier frequency setting, the higher the harmonic component, which is closely related to the length of the cable, the motor heating, the heating of the cable heaters.
Motor parameters: The frequency converter sets the power, current, voltage, speed and maximum frequency of the motor in the parameters. These parameters can be obtained directly from the motor nameplate.
Frequency hopping: at a certain frequency, there may be resonance phenomenon, especially in the whole device is relatively high; in the control of the compressor, to avoid the compressor surge point.
Common Fault Analysis
1) over-current fault: over-current fault can be divided into acceleration, deceleration, constant speed over-current. It may be due to the acceleration and deceleration time of the inverter is too short, the load mutation, uneven load distribution, output short circuit and other reasons. At this time generally by extending the acceleration and deceleration time, reduce the load mutation, plus energy consumption brake components, load distribution design, the line to check. If you turn off the load inverter or overcurrent fault, indicating that the inverter inverter circuit has been ring, you need to replace the inverter.
2) Overload fault: Overload faults include variable frequency overload and motor overload. Which may be caused by the acceleration time is too short, the grid voltage is too low, overload and other reasons. Generally by extending the acceleration time, extend the braking time, check the grid voltage and so on. The load is too heavy, the selected motor and inverter can not drag the load, it may be due to poor mechanical lubrication caused. Such as the former must replace the high-power motor and inverter; the latter will have to repair the production machinery.
3) Undervoltage: Note The inverter power input part of the problem, need to check before running.