Views:0 Author:Site Editor Publish Time: 2018-04-17 Origin:Site
Charge resistance low power general frequency converter is generally voltage type inverter, using AC-DC-AC mode of operation. When the converter is just powered on, because the capacity of the DC side filter capacitance is very large, the current is equivalent to short circuit at the moment of the new charging, and the current will be very large. If there is no charge resistance between the rectifier bridge and the electrolytic capacitor, it is equivalent to the 380V power supply direct short circuit to the ground, the instantaneous rectifier bridge passing through the infinite current causes the rectifier bridge to blow up. After adding charge resistance limit current, if do not combine relay or other component, charge resistance also consumes power very much. For example, for a 22kW converter, there is at least 45A current on the PN (DC bus). If the "connection control circuit" part of the problem (such as relay or thyristor and other quality problems), running in the converter for a while charge resistance will be too hot and broken. So the charging resistor is connected in the charging circuit to charge the current in a moment to protect the rectifier and other input circuit devices. Some books are also called buffer resistance or start resistance. Siemens 6SE701G frequency conversion startup circuit as shown in the attached figure.
After charging, the control circuit shortens the resistance through the contact of the relay or thyristor to complete the power up process of the inverter. If the AC input power supply of the converter is frequently switched off, or the contact contact of the bypass contactor is poor or the conduction resistance of the thyristor becomes larger, repeated charging or charging time too long will cause the charging resistance to burn out. Therefore, before replacing the charging resistor, it is necessary to find out the reason before the inverter can be put into use.
However, some frequency converters have a voltage detection and frequency reduction action during startup. If the contact of the contactor coil lead terminal is loosened, the contactor fails to be sucked, and the larger current during startup forms a larger voltage drop on the charge resistance. The sharp drop of DC voltage in the main circuit is detected by the voltage detection circuit and the CPU will make the frequency drop instruction. When there is no load or light load, the detection circuit "reports the under-voltage fault in time" to the CPU immediately to stop protection. Resistance too late to burn, the inverter has been shut down to protect.
So, how to choose the resistance of the charging resistor?
. The smaller the charge resistance, the greater the current flowing through the bridge. . the answer is negative.
In fact, at the moment of booting, under normal circumstances, the next step is to blow up the rectifier bridge because the selected charging resistor R is small, but R is too large to cause the rectifier bridge to blow up. . . .
The greater the inverter power, the smaller the charging resistance. Because the inverter power is greater, the capacity of the electrolytic capacitor is required to be larger, and the larger the capacity of the capacitor is, the longer the charging time is required. The RC determines the charging time. If the charging time is as short as possible, only the charging resistor R should be small.