1. Charging resistance

The medium and small power general-purpose frequency converter is generally a voltage type frequency converter, which adopts the AC-DC-AC working mode. When the inverter is just powered on, because the filter capacitor capacity on the DC side is very large, the current is equivalent to a short circuit at the moment of charging, and the current will be very large. If there is no charging resistor between the rectifier bridge and the electrolytic capacitor, it is equivalent to a short circuit of 380V power supply directly to the ground, and the instantaneous rectifier bridge will blow up through infinite current. With the current limiting of the charging resistor, if there is no relay or other components, the charging resistor consumes a lot of power. For example, for a 22kW frequency converter, there is at least 45A of current on the PN terminal (DC bus). If there is a problem with the "control circuit" part (such as the quality of relays or thyristors, etc.), the charging resistor will be broken due to too much heat after the inverter runs for a while. Therefore, the charging resistor is connected in series in the charging circuit to limit the current at the moment of energization to protect some input circuit devices such as rectifiers. Some books are also called buffer resistors or starting resistors. SIEMENS 6SE701G variable frequency starting circuit as shown in the figure.

After the charging is completed, the control circuit short-circuits the resistor through the contact of the relay or the thyristor to complete the power-on process of the inverter. If the AC input power of the inverter is frequently turned on and off, or the contact of the bypass contactor is not in good contact or the conduction resistance of the thyristor becomes larger, repeated charging or charging time is too long will cause the charging resistor to burn out. Therefore, before replacing the charging resistor, the reason must be found out before the inverter can be put into use.

However, some inverters have a voltage detection and frequency reduction action during startup. If the contactor coil lead terminal is loose and causes poor contact, the contactor fails to close, and the large current during startup forms a large voltage drop on the charging resistor. The sharp drop of the DC voltage of the main circuit is detected by the voltage detection circuit, and the CPU will make a frequency reduction instruction under no load or light load, the detection circuit will "report" the undervoltage fault in time, and the CPU will stop immediately for protection. The resistance is too late to burn, and the frequency converter has been shut down for protection.

So, how to choose the resistance of the charging resistor?

After 380V AC is rectified, the electrolytic capacitor is charged through the charging resistor. When the auxiliary power supply is charged to a certain value (such as DC200V), the auxiliary power supply starts to supply power to the control board, allowing the control board to work so that the relay or thyristor is turned on, and the charging resistor no longer works. At the moment of startup, the smaller the charging resistance, the greater the current flowing through the rectifier bridge. There are often beginners inverter repairers call to consult, replace the charging resistor, the inverter is turned on, the rectifier bridge will be blown up immediately, is the charging resistor selection too small.

In fact, at the moment of starting up, it is not because the selected charging resistor R is small, but because R is too large that the rectifier bridge is blown up. Because after the inverter is turned on, the current is charged through the charging resistor. When the charged electricity is enough for the auxiliary power supply to start (for example, 200V), the CPU works and sends a signal to the relay or thyristor to turn on. 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 directly rectified, it is about DC540V, so the voltage difference between the two ends of 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, and a voltage of several hundred volts is added in an instant, 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 inverter power, the smaller the charging resistance. Because the greater the power of the inverter, the greater the capacity of the electrolytic capacitor, and the greater the capacity of the capacitor, the longer the charging time. RC determines the charging time. If you want the charging time to be as short as possible, you can only take the charging resistor R as small as possible. General charging resistance selection: the maximum value is best not to exceed 300 Ω, the minimum value is best greater than or equal to 10 Ω, high-power inverter selection charging resistance is small, small power inverter charging resistance is large.

2. Energy storage capacitor

Selection of storage capacitor capacity: the general choice of empirical value is ≥ 60 μF/A. For example, a 15kW inverter has a rated current of 30A, and the required capacitor capacity is ≥ 60 μF/A× 30A, I .e. at least 1800 μF, so four 2200 μF (two parallel two strings) or two 4700μF capacitors (two series) are generally selected. Of course, the brand of the selected capacitor must also be considered. The quality of different brands will vary greatly.

Some people only replace the damaged inverter module when repairing the inverter, and it often doesn't take long for the module to be damaged again. This situation will complain about the poor quality of the module, the poor user environment, etc. In fact, the most important reason is that they did not find out the reason for the damage of the inverter module and did not completely eliminate the hidden trouble.

The damage of the inverter module, in addition to the load for a long time overload, poor heat dissipation and lightning impulse, the internal reasons, the capacity reduction, loss of capacity and failure of the capacitor, is the fatal killer of its damage! Its harmfulness cannot be ignored. When the capacity decreases, the lighter one shows poor load capacity. When the load increases, it often causes the DC circuit to fail to overvoltage. When the capacitor is further damaged, it will form a fatal blow to the inverter module. At this time, the voltage detection circuit has no time to respond and report the failure, resulting in damage to the inverter module.

After the capacitor is poor or fails (or the capacity becomes smaller), there is no abnormality on the surface when running with a small power load (large horse-drawn trolley), but the situation is different when a large power load is connected (full load operation). At this time, the DC circuit of the frequency converter has completely (or partially) lost the energy storage filtering capacity. The DC circuit is pulsating DC with a frequency of 300Hz, and the current suction when the motor starts increases the pulsating component of the pulsating current. This is one of the reasons why a small resistance is unfavorable to a high-voltage capacitor and a large resistance is easy to explode. In addition, if the back EMF of the motor winding or a certain output carrier of the frequency converter just falls within the variation range of the pulsating DC, the two are superimposed on each other, and the rapid change of the pulsating current in the whole system just falls on a certain frequency point, the distributed inductance and distributed capacitance in the circuit are added from time to time, and the addition and interaction of various unfavorable factors make the dynamic energy in the circuit rise sharply, instantaneous dangerous resonance overvoltage appears at this time! The IGBT tube in the inverter module and the spike voltage absorption diode in the circuit have a certain or even large amount of voltage under normal conditions, but under the high voltage impact several times higher than the voltage resistance at this time, they have no fighting power and are very fragile. It is not surprising that overvoltage burst and breakdown short circuit are not surprising. Although the inverter has a perfect voltage or current protection detection circuit, but if you often have to face such instantaneous voltage distortion, it is powerless, or sometimes unable to make a timely response.

However, the bad faults of energy storage capacitors are often hidden, which can be said to be soft faults and easy to be ignored. Some capacitors seem to have no problem measuring their capacity, and they can also run, but they are a big hidden danger in operation. In particular, if the capacitor in the high-power frequency converter is operated in a harsh environment for a long time, its lead-out electrode will be subjected to large current charging and discharging impact of hundreds of Hz for many years, resulting in corrosion and oxidation phenomena of different degrees. When measured by a capacitance meter, the capacity is not abnormal. However, when connected to the circuit, the DC circuit voltage drops due to the increase of internal resistance of charging and discharging, and the frequency converter cannot work normally, thus causing the maintenance personnel to make misjudgment and detours. Again, it is emphasized that resonant overvoltage is prone to burst after the loss of energy storage capacitor.

Main technical parameters