After charging is completed, the control circuit short-circuits the resistor through the contacts of the relay or the thyristor to complete the power-on process of the inverter. If the AC input power of the frequency converter is frequently switched on and off, or the contacts of the bypass contactor are in poor contact or the conduction resistance of the thyristor becomes large, repeated charging or charging for too long will cause the charging resistor to burn out. Therefore, before replacing the charging resistor, the cause must be found out before the inverter can be put into use.

　　However, during the startup of some inverters, the CPU has a voltage detection and frequency reduction action. If the contactor coil lead terminal is loose and causes poor contact, the contactor fails to close, and the larger current during startup will form a larger current on the charging resistor. A large voltage drop or a sharp drop in the main circuit DC voltage is detected by the voltage detection circuit, and the CPU will issue a frequency reduction command. When there is no load or light load, the detection circuit will "timely report" the undervoltage fault, and the CPU will shut down immediately. Protect. The resistor has no time to burn out, and the inverter has shut down for protection.

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

　　After the 380V AC is rectified, the electrolytic capacitor is charged through the charging resistor. When the charge reaches 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 power-on, the smaller the charging resistance, the greater the current flowing through the rectifier bridge. Beginner inverter repairmen often call for consultation. After replacing the charging resistor, the rectifier bridge is blown up as soon as the inverter is turned on. Is it because the charging resistor is too small? The answer is no.

　　In fact, when the power is turned on, the rectifier bridge is usually blown up when the power is turned on, not because the selected charging resistor R is too small, but because R is too large and the rectifier bridge is blown up. Because after the inverter is turned on, the current is charged through the charging resistor. 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 to turn on. At the moment when the relay is turned on, the voltage at point b of the relay is very low (larger than 200V), while the voltage at point a is AC380V which is directly rectified and 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 there is a small resistor between a and b, and a voltage of several hundred volts is applied instantly. In this way, the current flowing through the rectifier bridge is much greater than the rated current of the rectifier bridge, so the rectifier The bridge blew up.

　　The greater the power of the frequency converter, the smaller the charging resistance. Because the greater the power of the frequency converter, the greater the capacity of the electrolytic capacitor required, and the greater the capacity of the capacitor, the longer it takes to charge. RC determines the charging time. If you want the charging time to be as short as possible, you can only make the charging resistor R smaller. General charging resistor selection: the maximum value should not exceed 300Ω, and the minimum value should be greater than or equal to 10Ω. For high-power inverters, choose a small charging resistance, and for small-power inverters, choose a large charging resistance.

　　2. Energy storage capacitor

　　Selection of energy storage capacitor capacity: Generally, the empirical value is ≥60μF/A. For example, a 15kW inverter with a rated current of 30A requires a capacitance of ≥60μF/A×30A, which is at least 1800μF. Therefore, four 2200μF (two in parallel and two series) or two 4700μF capacitors (two in series) are generally selected. ). Of course, the brand of the selected capacitor must also be considered. Different brands have great differences in quality.

　　Some people repair the inverter and only replace the damaged inverter module. It often doesn't take long for the module to be damaged again. When this happens, there will be complaints about poor module quality, poor user environment, etc. In fact, the most important reason is that they did not find out the cause of the damage to the inverter module and did not completely eliminate the hidden fault.

　　In addition to long-term overloading, poor heat dissipation and lightning strikes, the damage to the inverter module is due to internal causes such as capacitor capacity reduction, capacitance loss and failure, which are the fatal killers of its damage! Its harmfulness cannot be ignored. The capacity decreases, which may manifest as poor load carrying capacity. When the load increases, it often causes a DC link undervoltage gate failure. When the capacitor is further damaged, it will cause a fatal blow to the inverter module. At this time, the voltage detection circuit has no time to make a decision. reaction, a fault is reported, causing damage to the inverter module.

　　After the capacitor is defective or failed (or the capacity becomes smaller), there will be no abnormality on the surface when running with a small power load (large horse-drawn cart), but the situation will be different after a larger power load is connected (full load operation). At this time, the DC circuit of the frequency converter has completely (or partially) lost its energy storage filtering capability. The DC loop is a pulsating DC with a frequency of 300Hz. The current drawn when the motor is started increases the pulsating component of the pulsating current. This is one of the reasons why choosing a small resistor is detrimental to the high-voltage capacitor, and choosing a large resistor is one of the reasons why it is easy to explode. In addition, if the back electromotive force of the motor winding or a certain output carrier of the frequency converter happens to fall within the range of the pulsating DC, the two superimpose each other, and the sharp change of the pulsating current in the entire system happens to fall at 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 cause the dynamic energy in the circuit to rise sharply, and an instant dangerous resonant overvoltage appears at this time! In the inverter module The IGBT tubes and the peak voltage absorbing diodes in the circuit have a certain or even large withstand voltage under normal conditions, but under the high voltage impact that is several times higher than the withstand voltage value, there is no The ability to parry also appears to be very fragile. It is not surprising that overvoltage explosions and breakdown short circuits occur. Although the frequency converter has a complete voltage or current protection detection circuit, if it often faces such instantaneous voltage distortion, it will appear incompetent, or sometimes it will not be able to respond in a timely manner.

　　However, faults in energy storage capacitors are often hidden and can be said to be soft faults that are easily overlooked. Some capacitors seem to have no problem when measuring their capacity and can operate, but they are a major hidden danger during operation. Especially for the capacitors in high-power frequency converters, if the environment is harsh and the capacitors are operated for a long time, the lead electrodes will be subjected to large current charging and discharging impacts of hundreds of hertz over the years, resulting in varying degrees of corrosion and oxidation. When measured with a capacitance meter, the capacity will be found to be zero. If it is abnormal, but it is connected in the circuit, the internal resistance of charging and discharging will increase, causing the DC circuit voltage to drop, and the inverter will not work normally, causing the maintenance personnel to make misjudgments and take detours. Let me emphasize again: after the energy storage capacitor loses its capacity, it is very easy for the module to burst due to resonant overvoltage.

　　The advantages of PTC thermistor charging resistor over traditional charging resistor are:

　　Built-in self-protection function

　　Under normal operating conditions, the PTC ICL acts as an ordinary resistor. When the power is turned on and the component temperature is the same as the ambient temperature, the resistance of the PTC ICL varies between 20 ohms and 500 ohms depending on the model. This is enough to limit the peak inrush current. Once the DC link capacitors are fully charged, the PTC ICL is bypassed,

　　If the charging circuit fails, the special function of the PTC thermistor comes into play to protect the circuit. When current passes through this component, the temperature of the PTC thermistor will increase and the resistance will also increase significantly. Therefore, thanks to its self-protection function, PTC thermistor has inherent advantages in the following failure modes:

　　– Capacitor short circuit

　　– The current limiting element is not bypassed (switching element fails) when the DC link capacitors are charged.

　　All these failure modes have one thing in common: thermal stress on the current limiting element. There are two ways to ensure that the ICL component will not be damaged in similar situations: use a power resistor with a sufficient power rating or use a PTC thermistor. The design of SINOCHIP PTC ICL allows it to work when directly connected to the supply voltage of the maximum rated voltage without the need for additional current limiting measures because the PTC ICL has a self-protection function. In the event of excessive current flow, such as a short circuit, the PTC temperature will increase, causing its resistance to rise significantly, allowing the PTC thermistor itself to limit the current to non-critical levels (Figure 4).

　　Figure 4: Current curve when capacitor is short-circuited

　　If a capacitor short circuit occurs, the current through the PTC ceramic resistor will quickly drop to a non-critical value (blue). However, if a normal resistor is used, the current will remain at a higher constant value (red).

　　SINOCHIP PTC thermistors have some key advantages as active surge current limiting ICL components in some applications:

　　– Its ICL functionality is not affected by extreme operating temperatures.

　　– Effective inrush current limitation is achieved as soon as the load is switched off and cooling is already taking place during normal operation.

　　– Has self-protection function against current overload caused by circuit fault.

　　Charging control circuit for energy storage capacitor

　　Self-protecting charging resistors based on PTC (positive temperature coefficient) ceramics are used to smooth capacitors in power supplies. When a short circuit occurs, they limit the current flow to a safe level.

　　Ordinary resistors are often used to limit current when charging capacitors. However, this often involves technical risks. For example, when a capacitor is shorted, the resistor will be continuously exposed to high power levels if the capacitor shorts or if the relay fails. This can lead to damage to the resistor or the entire system. Huaju Electronics adopts the new BPFLY series charging resistors based on PTC ceramics and has now developed a professional solution:

　　1. Main technical parameters