The circuit of frequency converter is generally composed of rectifier, intermediate DC link, inverter and control. The rectifier is a three-phase bridge uncontrollable rectifier, the inverter is an IGBT three-phase bridge inverter, and the output is PWM waveform. The middle DC link is filtering, DC energy storage and buffering reactive power. Frequency converter is a kind of power control device which uses the on-off function of power semiconductor devices to convert the power frequency power supply to another frequency. The frequency converter we use now mainly adopts AC-DC-AC mode (VVVF frequency conversion or vector control frequency conversion). First, the power frequency AC power supply is converted into DC power supply through rectifier, and then the DC power supply is converted into AC power supply which can be controlled by frequency and voltage to supply the motor. The circuit of frequency converter is generally composed of rectifier, intermediate DC link, inverter and control. The rectifier is a three-phase bridge uncontrollable rectifier, the inverter is an IGBT three-phase bridge inverter, and the output is PWM waveform. The middle DC link is filtering, DC energy storage and buffering reactive power.
Design and analysis of inverter control schematic diagram:
1) Firstly, confirm the installation environment of the inverter;
1. Operating temperature. The inverter is a high-power electronic component, which is easily affected by the working temperature. The general requirement of the product is 0 ~ 55 ℃. However, in order to ensure the safety and reliability of the work, we should consider leaving room when using, and it is better to control below 40 ℃. In the control box, the frequency converter should be installed on 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 the heating element or easy heating element close to the bottom of the frequency converter. 2. Ambient temperature. When the temperature is too high and the temperature changes greatly, the internal frequency converter is prone to condensation, and its insulation performance will be greatly reduced, and may even lead to short circuit accidents. If necessary, desiccant and heater must be added to the tank. In the water treatment room, the water vapor is generally heavy. If the temperature changes greatly, this problem will be more prominent. 3. Corrosive gas. If the concentration of corrosive gas is high, 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. 4. Vibration and shock. When the control cabinet with frequency converter is subject to mechanical vibration and impact, it will cause poor electrical contact. Huaian thermal power has such a problem. At this time, in addition to improving the mechanical strength of the control cabinet and keeping away from the vibration source and shock source, the anti-seismic rubber pad should also be used to fix the vibration components such as the electromagnetic switch outside and inside the control cabinet. After a period of operation, the equipment shall be inspected and maintained. 5. Electromagnetic interference. Due to rectification and frequency conversion in the work of frequency converter, a lot of interference electromagnetic waves are produced around, which have certain interference to nearby instruments. Therefore, the instrument and electronic system in the cabinet should use metal shell to shield the interference of frequency converter to the instrument. All components shall be reliably grounded. In addition, shielded control cables shall be selected for the wiring among electrical components, instruments and meters, and the shielding layer shall be grounded. If the electromagnetic interference is not handled properly, the whole system will be unable to work, resulting in the control unit failure or damage.
2) The distance between inverter and motor determines the cable and wiring method;
1. The distance between inverter and motor should be as short as possible. In this way, the ground capacitance of the cable is reduced, and the emission source of interference is reduced. 2. The control cable shall be shielded cable, the power cable shall be shielded cable, or the frequency converter and motor shall be shielded with conduit. 3. The motor cable shall be independent of other cables, with a minimum distance of 500mm. At the same time, the motor cable and other cables should be avoided to run in parallel for a long distance, so as to reduce the electromagnetic interference caused by the rapid change of inverter output voltage. 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 is separated from the main circuit line, even in the control cabinet. 4. It is better to select shielded twisted pair for analog signal line related to frequency converter, and select shielded three core cable for power cable (whose specification is larger than that of ordinary motor cable) or follow the user manual of frequency converter.
3) Inverter control schematic diagram;
I. Main circuit: the role of reactor is to prevent the high-order harmonic generated by frequency converter from returning to the power grid through the input circuit of power supply, thus affecting other power receiving equipment. It needs to decide whether to add reactor according to the capacity of frequency converter. The filter is installed at the output end of frequency converter to reduce the high-order harmonic output of frequency converter. When the distance between frequency converter and motor is long, it should be adjusted The filter should be installed. Although the frequency converter itself has various protection functions, the lack of phase protection is not perfect. The circuit breaker plays an overload role in the main circuit and lacks equal protection. When selecting the type, it can be selected according to the capacity of the frequency converter. The thermal relay can be replaced by the overload protection of the inverter itself. II. Control circuit: it has manual switching of power frequency frequency conversion, so that it can manually switch power frequency operation in case of frequency conversion failure. Because the output terminal can not add voltage, fixed power frequency and frequency conversion should be interlocked.
4) Grounding of frequency converter;
The correct grounding of frequency converter is an important means to improve system stability and restrain noise. The smaller the grounding resistance of the grounding terminal of the inverter, the better. The cross section of the grounding wire shall not be less than 4mm and the length shall not be more than 5m. The grounding of frequency converter shall be separated from the grounding point of power equipment, and shall not be grounded in common. 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 floating. The inverter is electrically connected with the control cabinet.
Common fault analysis:
1) Over current fault: over current fault can be divided into acceleration, deceleration and constant speed over current. It may be due to the acceleration and deceleration time of the inverter is too short, load mutation, uneven load distribution, output short circuit and other reasons. At this time, we can extend the acceleration and deceleration time, reduce the sudden change of load, add energy consumption braking components, carry out load distribution design and check the line. If the load inverter is disconnected or there is an over-current fault, the inverter circuit of the inverter is closed 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, the acceleration time, braking time and grid voltage can be extended. If the load is too heavy, the selected motor and frequency converter can not drive the load, or it may be caused by poor mechanical lubrication. If the former, the high-power motor and frequency converter must be replaced; if the latter, the production machinery must be overhauled. 3) Under voltage: it indicates that there is something wrong with the power input part of the frequency converter, which can be operated only after checking.