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Medium and low-power general-purpose variable frequency drives (VFD frequency inverters) are generally voltage-type inverters that use an AC-DC-AC working mode. When the VFD is first powered on, due to the very large capacitance of the filter capacitor on the DC side, it is equivalent to a short circuit for the current at the moment of charging, resulting in a very large inrush current. If no charging resistor is added between the rectifier bridge and the electrolytic capacitor, it is equivalent to a direct short circuit of the 380V power supply to ground, and the instantaneous infinite current through the rectifier bridge will cause the rectifier bridge to explode. After adding a charging resistor for current limiting, if no relay or other components are connected in parallel, the power consumption of the charging resistor will be very large.

For example, for a 22kW VFD, there is at least 45A of current on the PN terminal (DC bus). If there is a problem with the "control circuit connection" part (such as quality issues with relays or thyristors, etc.), the charging resistor will be damaged due to excessive heating after the VFD runs for a while. Therefore, the charging resistor is connected in series in the charging circuit to limit the current during power-on to protect components in the input circuit such as the rectifier. It is also called a buffer resistor or starting resistor in some books. The start-up circuit of the Siemens 6SE701G VFD is shown in the attached diagram.

After charging is completed, the control circuit shorts the resistor through the contacts of a relay or a thyristor to complete the power-up process of the VFD frequency inverter. If the AC input power of the VFD is frequently switched on and off, or if the contacts of the bypass contactor are poorly connected or the conduction resistance of the thyristor increases, repeated charging or excessively long charging time will cause the charging resistor to burn out. Therefore, before replacing the charging resistor, the cause must be identified before the VFD can be put back into use.

However, in some VFD frequency inverters, the CPU performs voltage detection and frequency reduction during startup. If the lead terminals of the contactor coil are loose and cause poor contact, and the contactor fails to engage, the large current during startup will create a significant voltage drop across the charging resistor. The sudden drop in the DC voltage of the main circuit is detected by the voltage detection circuit, and the CPU will issue a frequency reduction command. When the VFD is unloaded or lightly loaded, the detection circuit will "promptly report" an undervoltage fault, and the CPU will immediately stop the machine for protection. In this case, the resistor does not have time to burn out before the VFD stops and protects itself.

01. How to choose the resistance value of the charging resistor?

After the 380V AC power is rectified, it charges the electrolytic capacitor through the charging resistor. When the voltage reaches a certain value (e.g., DC 200V), the auxiliary power supply starts to provide power to the control board, enabling it to operate. Subsequently, the relay or thyristor is activated, bypassing the charging resistor. At the moment of startup, the smaller the resistance value of the charging resistor, the larger the current flowing through the rectifier bridge. Novice VFD frequency inverter repair technicians often call to inquire if replacing the charging resistor with a smaller one could cause the rectifier bridge to blow immediately upon startup. The answer is no.

In fact, during startup, a blown rectifier bridge is typically not caused by a too-small charging resistor R but rather by a too-large R. When the VFD starts, current flows through the charging resistor to charge the capacitor. Once the voltage is sufficient to activate the auxiliary power supply (e.g., 200V), the CPU sends a signal to close the relay or trigger the thyristor. If the voltage at point b of the relay is low (but higher than 200V) while the voltage at point a, directly rectified from 380V AC (approximately DC 540V), a significant voltage difference exists between points a and b. During the instant of triggering and conduction, the current is extremely high—similar to applying several hundred volts across a very small resistor. This surge in current far exceeds the rectifier bridge's rated capacity, causing it to fail.

For higher-power VFD frequency inverters, the charging resistor is smaller. Higher power requires larger electrolytic capacitors, which in turn demand longer charging times. Since the RC time constant determines the charging duration, reducing the resistance R minimizes this time. Generally, the charging resistor should be selected such that its maximum value does not exceed 300Ω and its minimum value is at least 10Ω. Larger resistors are used for low-power VFDs, while smaller resistors are employed for high-power units.

02. Selection of energy storage capacitor capacity

The general rule of thumb for capacitor selection is ≥60μF/A. For example, a 15kW VFD frequency inverter with a rated current of 30A requires a capacitance of ≥60μF/A × 30A, which is at least 1800μF. Therefore, four 2200μF capacitors (two in parallel and two in series) or two 4700μF capacitors (in series) are typically chosen. The brand of the capacitors must also be considered, as quality can vary significantly among different manufacturers.

Some technicians replace only the damaged inverter module when repairing a VFD frequency inverter, only to find the module fails again shortly afterward. They may blame poor module quality or harsh operating environments, but the root cause is often that they fail to identify why the module failed in the first place, leaving underlying issues unresolved.

Internal factors leading to inverter module damage include capacitor degradation, such as reduced capacitance or complete failure, which can be just as critical as external factors like prolonged overload, inadequate cooling, or lightning strikes. The consequences of capacitor issues should not be underestimated. A slight reduction in capacitance may manifest as poor load handling, triggering DC bus undervoltage trips under heavy loads. Severe capacitor failure can fatally damage the inverter module before the voltage detection circuit can react.

When capacitors degrade (e.g., reduced capacitance), the VFD frequency inverter may appear normal under light loads but fail under full load. The DC bus loses its energy storage and filtering capabilities, resulting in a 300Hz pulsating DC. During motor startup, increased current draw exacerbates these pulsations. This explains why selecting an overly small charging resistor harms high-voltage capacitors, while an oversized resistor risks blowing the rectifier bridge. Additionally, if the motor's back electromotive force (EMF) or the VFD's output carrier frequency aligns with the pulsating DC frequency, resonance can occur. Combined with parasitic inductance and capacitance in the circuit, this resonance generates dangerous overvoltages. Although IGBTs and voltage clamping diodes in the inverter module have voltage ratings with safety margins, they cannot withstand these transient spikes, leading to breakdowns. Even sophisticated protection circuits may fail to react quickly enough to such rapid voltage transients.

Capacitor issues are often insidious, presenting as "soft faults" that are easily overlooked. Some capacitors may test within specifications but still pose risks during operation. In high-power VFDs, capacitors subjected to years of harsh conditions may develop corroded terminals due to high-frequency charging and discharging. While capacitance measurements may appear normal, increased internal resistance causes voltage drops during operation, misleading technicians. To reiterate: capacitor degradation can trigger resonant overvoltages, which are a primary cause of module failure.