Frequency converter selection:
When selecting a frequency converter, the following points should be determined:
1) adopts frequency conversion; Constant voltage control or constant current control, etc.
2) Load type of frequency converter; Such as vane pump or displacement pump, we should pay special attention to the performance curve of load, which determines the application mode and method.
3) Matching between inverter and load;
First, voltage matching; The rated voltage of inverter is consistent with the rated voltage of load.
Two. Current matching; For ordinary centrifugal pumps, the rated current of the inverter is the same as that of the motor. For special loads such as deep water well pumps, it is necessary to refer to the motor performance parameters to determine the inverter current and overload capacity with the maximum current.
Three. Torque matching; This may happen when there is a constant torque load or deceleration device.
4) When the inverter is used to drive the high-speed motor, the high-order harmonics increase and the output current value increases due to the small reactance of the high-speed motor. Therefore, the capacity of high-speed motor is slightly larger than that of ordinary motor.
5) If the frequency converter runs with a long cable, measures should be taken to suppress the influence of the long cable on the coupling capacitance, so as to avoid insufficient output of the frequency converter. Therefore, in this case, the capacity of the frequency converter should be expanded by a step 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 inverter will be reduced, and the capacity of the inverter will be enlarged by one step.
Design of frequency converter control schematic diagram;
1) First confirm the installation environment of the inverter;
First, the working temperature. Inverter is a high-power electronic component, which is easily affected by working temperature. Generally, the product is required to be 0 ~ 55℃, but in order to ensure safe and reliable operation, it should be considered to leave room when using, and it is best to control it below 40℃. In the control box, the frequency converter should generally be installed at the upper part of the box, and the installation requirements in the product manual should be strictly observed. It is absolutely not allowed to install heating elements or easily heating elements near the bottom of the frequency converter.
Two. Ambient temperature. When the temperature is too high and the temperature changes greatly, condensation is easy to occur inside the inverter, which greatly reduces its insulation performance and may even lead to short circuit accidents. When necessary, desiccant and heater must be added in the box. In the water treatment room, water vapor is generally heavy, and this problem will be more prominent if the temperature changes greatly.
Three. Corrosive gas. If the concentration of corrosive gas is high, it will not only corrode component leads, printed circuit boards, etc. It will also accelerate the aging of plastic devices and reduce the insulation performance.
Four. Vibration and shock. When the control cabinet equipped with frequency converter is subjected to mechanical vibration and impact, it will cause poor electrical contact. Huai 'an Thermal Power Company 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 the electromagnetic switches and other components that generate vibration outside and inside the control cabinet. After the equipment has been running for a period of time, it should be inspected and maintained.
Electromagnetic interference. Due to the rectification and frequency conversion during the operation of the inverter, a large number of interference electromagnetic waves are generated around it, and these high-frequency electromagnetic waves have certain interference to nearby instruments and meters. Therefore, the instrument and electronic system in the cabinet should adopt metal shell to shield the interference of frequency converter to the instrument. All components shall be reliably grounded. In addition, shielded control cables should be used for wiring between electrical components and instruments, and the shielding layer should be grounded. If the electromagnetic interference is not handled well, it will often make the whole system unable to work, resulting in the failure or damage of the control unit.
2) The distance between the inverter and the motor determines the cable and connection mode;
I the distance between inverter and motor should be as short as possible. This reduces the capacitance of the cable to the ground and reduces the interference source.
Two. Shielded cables shall be used for control cables, and shielded cables for power cables or all cables from frequency converters to motors shall be shielded through threading pipes.
Three. The motor cable shall be routed independently of other cables with a minimum distance of 500mm. At the same time, long-distance parallel wiring of motor cables and other cables should be avoided to reduce the electromagnetic interference caused by the rapid change of inverter output voltage. If the control cable and power cable cross, try to cross at 90 degrees. Analog signal lines related to the frequency converter are wired separately from the main circuit, even in the control cabinet.
Four. It is best to use shielded twisted pair for analog signal lines related to the frequency converter, and shielded three-core cable (whose specification is larger than that of ordinary motor cable) for power cables 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 harmonics generated by the frequency converter from returning to the power grid through the input circuit of the power supply, thus affecting other power receiving equipment. It is necessary to decide whether to add reactors according to the capacity of the frequency converter; The filter is installed at the output end of the frequency converter to reduce the higher harmonics output by the frequency converter. When the frequency converter is far away from the motor, a filter should be installed. Although the inverter itself has various protection functions, the lack of phase protection is not perfect. Circuit breaker plays the role of overload and lack of equality protection in the main circuit, and can be selected according to the capacity of inverter. Thermal relay can be replaced by overload protection of frequency converter itself.
Two. Control loop: it has manual switching power frequency conversion, so as to manually switch power frequency when frequency conversion fails. Because no voltage can be applied to the output terminal, constant power frequency and frequency conversion should be interlocked.
4) grounding of frequency converter;
Correct grounding of frequency converter is an important means to improve system stability and suppress noise. The smaller the grounding resistance of the inverter grounding terminal, the better. The section of grounding conductor is not less than 4 mm and the length is not more than 5 m. The grounding of frequency converter should be separated from the grounding point of power supply equipment, not * * *. One end of the shielding layer of the signal line is connected to the grounding end of the frequency converter, and the other end is suspended. The frequency converter is electrically connected with the control cabinet.
Design of inverter control cabinet;
The frequency converter should be installed in the control cabinet, and the following problems should be paid attention to when designing the control cabinet.
1) heat dissipation: the heat of the inverter is caused by internal loss. The main circuit accounts for about 98% and the control circuit accounts for about 2% of the loss of each part of the frequency converter. In order to ensure the normal and reliable operation of the inverter, the inverter must be cooled. We usually use a fan to cool down. The built-in fan of the inverter can take away the heat dissipation inside the inverter box. If the fan can't work normally, stop the inverter immediately. The high-power inverter also needs to add a fan to the control cabinet, and the air duct of the control cabinet should be designed reasonably. All air inlets shall be provided with dust screens, and the air exhaust shall be smooth, so as to avoid vortex formation in the cabinet and dust accumulation in a fixed position. Select the matching fan according to the ventilation volume in the frequency converter manual, and pay attention to the shockproof problem when installing the fan.
2) Electromagnetic interference:
1. During the operation of the frequency converter, due to rectification and frequency conversion, a large number of interfering electromagnetic waves are generated around it. These high-frequency electromagnetic waves have certain interference to nearby instruments and meters, and also produce higher harmonics, which enter the whole power supply network through the power supply loop, thus affecting other instruments and meters. If the power of the inverter accounts for more than 25% of the whole system, it is necessary to consider the anti-interference measures of the control power supply.
Two. When there are high-frequency impact loads such as welding machine and electroplating power supply in the system, the inverter itself will be protected by interference, so the power quality of the whole system should be considered.
3) Protection issues need to pay attention to the following points:
1. Waterproof and dewing prevention: When the inverter is placed on the site, it should be noted that there should be no pipeline flange or other leakage points above the inverter cabinet, and there should be no splashing water near the inverter. In short, the protection level of the cabinet on site should be above IP43.
Two. Dust-proof: All air inlets shall be equipped with dust-proof nets to prevent flocculent impurities from entering, and the dust-proof nets shall be designed to be detachable for easy cleaning and maintenance. The grid of the dust screen shall be determined according to the specific site conditions, and the joint between the dust screen and the control cabinet shall be strictly handled.
Three. Anticorrosive gas: This situation is common in the chemical industry. At this time, the frequency conversion cabinet can be placed in the control room.
Wiring specification of frequency converter:
The signal wire and power wire must be routed separately: when using analog signals to remotely control the frequency converter, in order to reduce the interference of analog signals of frequency converter and other equipment, please route the signal wire for controlling the frequency converter separately from the strong current loop (main loop and sequence control loop). The distance should be more than 30cm. Even in the control cabinet, this wiring specification should be maintained. The longest control loop between the signal and the frequency converter shall not exceed 50m.
The signal wire and power wire must be placed in different metal tubes or metal hoses: if the signal wire connecting PLC and frequency converter is not placed in metal tubes, it will be easily interfered by frequency converter and external equipment; At the same time, because the inverter has no built-in reactor, the power lines of the input stage and output stage of the inverter will have strong interference to the outside world. Therefore, the metal pipe or hose for placing the signal wire should extend all the way to the control end of the frequency converter to ensure that the signal wire is completely separated from the power wire.
1) The analog control signal line should adopt double-stranded shielded wire with the wire specification of 0.75mm2 When wiring, it is necessary to pay attention to the cable stripping as short as possible (about 5-7mm), and at the same time, the stripped shielding layer should be wrapped with insulating tape to prevent the shielded wire from contacting other equipment and introducing interference.
2) In order to improve the simplicity and reliability of wiring, it is suggested to use the wire pressing rod terminal on the signal line.
Operation of frequency converter and setting of relevant parameters;
There are many setting parameters of frequency converter, 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 mode: speed control, torque control, PID control or other modes. After the control mode is adopted, static or dynamic identification is generally needed according to the control accuracy.
Minimum operating frequency: the lowest rotating speed of the motor. When the motor is running at low speed, the heat dissipation performance is very poor. If the motor runs at low speed for a long time, it will burn out. Moreover, at low speed, the current in the cable will also increase, which will also lead to the cable heating.
Maximum working frequency: the maximum frequency of general frequency converters reaches 60Hz, and some even reach 400 Hz. High frequency will make the motor run at high speed. For ordinary motors, their bearings cannot run at a fixed speed for a long time. Can the rotor of the motor withstand such centrifugal force?
Carrier frequency: The higher the carrier frequency is set, the greater the harmonic component, which is closely related to cable length, motor heating, cable heating and inverter heating.
Motor parameters: The inverter sets the power, current, voltage, speed and maximum frequency of the motor in the parameters, which can be obtained directly from the motor nameplate.
Frequency hopping: at a certain frequency point, vibration may occur, especially when the whole equipment is relatively high; When controlling the compressor, the surge point of the compressor should be avoided.
Common fault analysis:
1) overcurrent fault: overcurrent fault can be divided into acceleration, deceleration and constant-speed overcurrent. It may be caused by too short acceleration and deceleration time of frequency converter, sudden load change, uneven load distribution, output short circuit and other reasons. At this time, it is generally possible to extend the acceleration and deceleration time, reduce the sudden change of load, increase energy-consuming braking parts, carry out load distribution design and check the line. If the load inverter is disconnected or overcurrent, it means that the inverter circuit of the inverter has been looped and the inverter needs to be replaced.
2) Overload fault: Overload fault includes frequency conversion overload and motor overload. It may be caused by too short acceleration time, too low grid voltage and too heavy load. Generally, you can extend the acceleration time, extend the braking time and check the grid voltage. The load is too heavy, so the selected motor and frequency converter can't drag the load, or it may be caused by poor mechanical lubrication. If it is the former, the high-power motor and frequency converter must be replaced; In the latter case, the production machine should be overhauled.
3) Undervoltage: It means that there is something wrong with the power input part of the inverter, and it can only be operated after inspection.