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Solution to the interference problem of frequency converter
In the field, the disturbance of the frequency converter is more and more serious, and even the control system can not be put into use. The working principle of the frequency converter is doomed to produce strong electromagnetic interference.
The converter includes rectifier circuit and inverter circuit. The input AC is converted to DC voltage through rectifier circuit and harmonic circuit, and then the DC voltage is converted to different width pulse voltage by inverter, which is called pulse width modulation voltage (PWM). Using this PWM voltage to drive the motor, the motor torque and speed can be adjusted. This principle of work leads to the following three kinds of electromagnetic interference:
1, harmonic interference: rectifier circuit will produce harmonic current, the harmonic current generated voltage drop in the power supply system impedance, leading to voltage distortion, the voltage distortion for many electronic devices to form interference (because most of the electronic equipment can only work under sinusoidal voltage conditions), voltage distortion is common sine wave flat top. When the harmonic current is constant, the voltage distortion is more serious under the condition of a weak power supply. The characteristic of this interference is to interfere with the equipment that uses the same power grid, but it has nothing to do with the distance between the device and the inverter.
2, radio-frequency conduction and emission interference: as the load voltage pulse shape, so the lessons from the power grid inverter current is the pulse shape, pulse current which contains a large number of high frequency components, the formation of radio frequency interference, the interference characteristic is the formation of interference to use the same power equipment, but not between equipment and inverter distance;
3, radiofrequency radiation interference: radio frequency radiation interference from the input cable and output cable from the frequency converter. In the case of radio frequency conduction and emission interference, if the radio frequency interference current exists on the input and output cable of the inverter, because the cable is equivalent to the antenna, it will generate electromagnetic radiation and produce radiation interference. The PWM voltage transmitted on the output cable of the frequency converter also contains rich components of high frequency, which will produce electromagnetic wave radiation and form radiation interference. The characteristic of radiation interference is that when other electronic devices are close to the frequency converter, the interference phenomenon becomes serious.
According to the basic principles of electromagnetics, the formation of electromagnetic interference must have three elements: electromagnetic interference source, electromagnetic interference, and electromagnetic interference sensitive system. In order to prevent interference, hardware anti-interference and software anti-interference can be used. Among them, the hardware anti-interference is the most basic and the most important anti-jamming measure, generally from the two aspects of resistance and release to suppress the interference, the general principle is to suppress and eliminate the interference source, cut off the interference to the system coupling channel, and reduce the sensitivity of the system interference signal. The concrete measures can be used in engineering, such as isolation, filtering, shielding, grounding and other methods. The following are the main steps to solve field interference:
1, the use of software anti-interference measures: specifically speaking, the frequency of the inverter is lowered by the man-machine interface of the inverter, and the value is reduced to a suitable range. If this method does not work, we can only take the following hardware anti-interference measures.
2, the correct grounding: through the specific field investigation we can see that the ground grounding is not ideal. The correct grounding can not only effectively suppress external interference, but also reduce the interference of the device itself to the outside world, which is the most effective measure to solve the problem of inverter interference. Specifically, it is to do the following points:
(1) the main circuit terminal PE (E and G) of the inverter must be grounded. The grounding can be shared with the motor carried by the inverter, but it cannot be shared with other devices. The grounding pile must be soled separately, and the location should be kept away from the location of the weak electrical equipment as far as possible. At the same time, the intercepting area of the grounding wire of the frequency converter should not be less than 4mm2, and the length should be controlled within 20m.
(2) in the ground lines of other mechanical and electrical equipment, the protective grounding and working grounding should be separately connected to the ground pole, and finally to the electrical connection place of the distribution cabinet. The shields of the control signals and the shielding of the main circuit wires should also be separately connected to the ground poles and finally remitted to the electrical connection locations of the distribution cabinet.
3, shielding the interference source: the shielding interference source is a very effective method to suppress interference. Usually the inverter itself with iron shielding, can not let the leaked electromagnetic interference, but the output line frequency converter is best to use steel shielding, in particular to an external signal (4~20mA signal output from the controller) control of inverter, the control signal line is as short as possible (usually less than 20m), and must use the shielded twisted pair and the main power line and the control line (AC380) and (AC220V) completely separated. In addition, the circuit of the electronic sensitive equipment in the system also requires the use of shielded twisted pair lines, especially the pressure signals. And all the signal lines in the system can never be placed in the same pipe or line slot with the main power line and control line. In order to make the shielding effective, the shielding layer must be reliably grounded.
4, reasonable wiring: the specific methods are:
(1) the power line and signal line of the equipment should be as far away from the input and output line of the inverter as possible.
(2) the power lines and signal lines of other equipment should be avoided in parallel with the input and output lines of the frequency converter.
If the above method is still not able to work, then follow the following measures:
5, interference isolation: the so-called interference isolation, refers to the circuit from the interference source and the vulnerable parts of the isolation, so that they do not occur electrical connections. The isolation transformer is usually used in the power line between the power supply and the controller, the transmitter and other amplifier circuits to avoid interference, and the power supply is separated.