Fault analysis of frequency converter caused by th

2022-08-21
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Fault analysis of frequency converter caused by charging resistance

small and medium power general frequency converter is generally voltage type frequency converter, which adopts AC-DC-AC working mode. When the frequency converter is just powered on, because the filter capacitor capacity on the DC side is very large, the current at the moment of just charging is equivalent to a short circuit, and the current will be very large. If the charging resistance is not added between the rectifier bridge and the electrolytic capacitor, it is equivalent to the 380V power supply being directly short circuited to the ground, and the rectifier bridge will blow up instantly through the infinite current

after adding charging resistance current limiting, if there is no relay or other components, the charging resistance will consume a lot of power. For example, for a 22KW frequency converter, there is at least 45A current on the PN end (DC bus). If there is a problem with the part of "connecting to the control circuit" (such as the quality of the relay or thyristor, etc.), the charging resistance will be broken due to too much heating after the inverter runs for a while. Therefore, the charging resistance is connected in series in the charging circuit to limit the current at the moment of energization. The experimental method is to cut the width of 15mm or other dimensions at the heat sealing part of the plastic packaging bag to charge, so as to protect the role of some input circuit devices such as rectifier. Some books also call it buffer 4. Calculation formula resistance or starting resistance of scanning time and scanning rate

after charging, the control circuit shortens the resistance through the contact of the relay or the thyristor to complete the power on process of the frequency converter. If the AC input power of the frequency converter is frequently turned on and off, or the contact of the bypass contactor is poor, or the conduction resistance of the thyristor becomes larger, repeated charging or complex data analysis can be carried out, and the charging resistance will be burned out if the charging time is too long. Therefore, before replacing the charging resistance, the reason must be found out before putting the frequency converter into use

however, some frequency converters have a voltage detection and frequency reduction action during startup. If the lead terminal of the contactor coil is loose, resulting in poor contact, the contactor fails to pull in, the large current during startup forms a large voltage drop on the charging resistance, and the sharp drop of the DC voltage of the main circuit is detected by the voltage detection circuit, the CPU will make a frequency reduction command. When no-load or light load, The detection circuit will "report the undervoltage fault in time", and the CPU will shut down immediately for protection. The resistance is too late to burn out, and the experimental software of frequency converter 16 and digital electronic tensile testing machine under the full Chinese Windows platform has been shut down for protection

then, how to choose the resistance value of the charging resistance

380v AC rectifies and charges the electrolytic capacitor through the charging resistance. When it is charged to a certain value (such as dc200v), the auxiliary power supply starts to supply power to the control board, so that the control board works, so that the relay or thyristor is connected, and the charging resistance will no longer work. At the moment of startup, the smaller the charging resistance, the greater the current flowing through the rectifier bridge. There are often novice frequency converter maintainers who call for advice and replace the charging resistance. As soon as the frequency converter is turned on, the rectifier bridge will be blown up. Is the charging resistance too small? The answer is No

in fact, at the moment of power on, generally speaking, the reason why the rectifier bridge is blown up is not that the selected charging resistance R is small, but that R is too large. After the frequency converter is started, the current is charged through the charging resistance. When the charged power is enough to start the auxiliary power supply (such as 200V), the CPU works and sends a signal to the relay or thyristor thyristor to make it conductive. At the moment when the relay is turned on, if the voltage at point B of the relay is very low (greater than 200V), and the voltage at point a is AC380V, the direct rectification is about DC540V, so the voltage difference between terminals a and B is very large. At the moment of triggering and conduction, the current is very large, just like a small resistance between a and B. the voltage of hundreds of volts is added instantly, so the current flowing through the rectifier bridge is far greater than the rated current of the rectifier bridge, so the rectifier bridge is blown up

the greater the power of the frequency converter, the smaller the charging resistance. Because the larger the power of the frequency converter, the larger the capacity of the electrolytic capacitor is required, and the larger the capacity of the capacitor is, the longer the charging time is required. RC determines the charging time. If you want to keep the charging time as short as possible, you can only reduce the charging resistance R. General charging resistance selection: the maximum value should not exceed 300 Ω, and the minimum value should be greater than or equal to 10 Ω. The charging resistance of high-power inverter is small, and the charging resistance of low-power inverter is large

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