Application of AC filter capacitors for PCB in inverters

Advantages of metallized polypropylene film dielectrics

High dielectric constant, powerful energy storage

Metallized polypropylene film has a high dielectric constant, which allows capacitors to store more charge in a limited space. In the inverter filter circuit, sufficient charge storage helps to more efficiently cope with current fluctuations, quickly absorb and release charges, stabilize voltage output, and ensure smooth operation of the inverter. For example, in the application scenario where the motor is frequently started and stopped, the high dielectric constant metallized polypropylene film AC filter capacitor can quickly respond to current changes, avoid sudden rise and fall of voltage, and protect the motor and other circuit components.

Low dielectric loss, energy saving and high efficiency

Its dielectric loss is extremely low, which means less energy loss during charging and discharging. When the inverter is running continuously for a long time, the low loss characteristic can reduce the heat generation of the capacitor itself, improve the efficiency of electric energy utilization, and reduce unnecessary energy waste. Taking the case of many inverters running simultaneously in a large industrial production line as an example, the large-scale use of low dielectric loss AC filter capacitors can significantly reduce energy consumption in the long run, saving enterprises considerable electricity expenses.​
High insulation resistance, stable and reliable​
The high insulation resistance characteristics of metallized polypropylene film effectively prevent leakage current from being generated and ensure stable performance of capacitors. In the complex electromagnetic environment of the inverter, high insulation resistance can prevent external interference from affecting the operation of the capacitor, ensure consistent filtering effect, and enhance the reliability of the inverter system. For example, in an industrial environment with strong electromagnetic interference, AC filter capacitors based on metallized polypropylene film can operate stably and provide reliable filtering protection for the inverter. ​
Good self-healing characteristics, extended life​
The medium has a unique self-healing ability. When a local breakdown occurs inside the capacitor, the metallized film around the breakdown point will evaporate instantly, isolating the fault point and restoring the capacitor to normal operation. This feature greatly extends the service life of the capacitor in the inverter and reduces the equipment downtime and maintenance time and cost caused by capacitor failure. For example, in chemical production equipment that operates uninterruptedly, the self-healing characteristics of the AC filter capacitor can ensure the long-term stable operation of the inverter and ensure production continuity.​
Working mechanism in the inverter​
Build LC filter circuit to accurately filter out harmonics​
In the inverter circuit, the LC filter circuit composed of AC filter capacitors and reactors is the core structure for filtering out harmonics. In principle, capacitors and reactors present different impedance characteristics to currents of different frequencies. The two are like tacit partners, using these characteristics to build a precise "harmonic barrier". ​
Capacitors have the characteristics of "passing high frequencies and blocking low frequencies". When a current containing a large number of harmonics passes through, high-frequency harmonic currents can pass through the capacitor relatively easily. The reactor is just the opposite. It "passes low frequencies and blocks high frequencies". It presents high impedance to high-frequency harmonic currents and hinders their passage. When the two are connected in series or in parallel to form an LC filter circuit, harmonics in a specific frequency range can be accurately filtered. ​
In actual operation, the harmonic components generated by the inverter are complex, covering high-order harmonics of multiple frequencies. The LC filter circuit can guide the harmonic current of a specific frequency to the filter branch according to the design requirements. For high-order harmonics of a certain frequency, the LC filter circuit adjusts the parameters of the capacitor and the reactor to make it present extremely low impedance to the harmonics of this frequency, so that the harmonic current will be attracted to the filter branch instead of entering the power grid or interfering with the motor, thereby effectively improving the output waveform of the inverter and reducing the impact of harmonics on the system. ​
In addition, the design of the LC filter circuit also needs to consider the operating frequency range, load characteristics and harmonic distribution of the inverter. In different application scenarios, the frequency and amplitude of harmonics are different, so the parameters of the LC filter circuit need to be carefully calculated and debugged to ensure that it can play the best filtering effect in the entire operating frequency range. ​
Smooth DC bus voltage and stabilize power supply​
AC Filter Capacitor for PCB is connected to the DC bus side of the inverter and plays an indispensable role in stabilizing the DC voltage. The working process of the inverter is an AC-DC-AC conversion process. In the rectification link of converting AC to DC, and the subsequent inverter link of converting DC to AC, the DC bus voltage will be affected by many factors and fluctuate.​
On the one hand, the AC input voltage itself may be unstable, such as voltage amplitude fluctuations, frequency offsets, etc. These changes will be directly reflected in the DC bus voltage. On the other hand, the switching action of the power devices inside the inverter will also cause ripples in the DC bus voltage. For example, when the power devices are quickly turned on and off, instantaneous current changes will occur on the DC bus, which will cause voltage fluctuations. ​
At this time, the AC filter capacitor is like a "stabilizer", using its own charging and discharging characteristics to smooth these voltage fluctuations. When the DC bus voltage reaches a peak, the capacitor quickly absorbs excess charge and stores the electrical energy in the form of electric field energy; when the voltage reaches a valley, the capacitor will release the stored charge in time to supplement the voltage, so that the DC bus voltage remains at a relatively stable level. ​
Stable DC bus voltage is crucial for the normal operation of the inverter. It provides a stable power supply foundation for the subsequent inverter link, ensuring that the inverter circuit can output a stable AC voltage, thereby driving the motor to run smoothly. If the DC bus voltage is unstable, the AC voltage output by the inverter will also fluctuate, resulting in unstable motor torque, vibration, heating and other problems, and may even damage the motor and other circuit components. Therefore, the role of AC filter capacitors in smoothing the DC bus voltage is a key link in ensuring the reliable operation of the inverter. ​
Specific application scenarios​
Industrial motor drive system​
In various motor drive equipment in the factory, the inverter is used to adjust the motor speed to meet different production needs. AC filter capacitors can effectively filter out harmonics, reduce motor heating and vibration, improve motor operation efficiency and stability, and extend the service life of the motor. For example, in the looms of textile factories and machine tools in machinery factories, AC filter capacitors ensure the smooth operation of the motor and improve product quality and production efficiency. ​
Fan and water pump control system​
Fans and water pumps are common energy-consuming equipment in the industrial field. Energy saving can be achieved by adjusting their speed through inverters. AC filter capacitors can suppress harmonics and prevent harmonics from damaging fan and water pump motors, while improving power factor and reducing grid losses. In the ventilation systems of large shopping malls and office buildings, as well as in industrial cooling water circulation systems, AC filter capacitors ensure stable and efficient operation of fans and water pumps, thus achieving energy conservation and consumption reduction. ​
Elevator drive system​
The smooth operation of elevators is crucial to passenger safety and comfort. The inverter controls the acceleration, deceleration and smooth operation of the elevator motor. The AC filter capacitor filters out harmonics, reduces the motor torque pulsation, makes the elevator run more smoothly, starts and stops more smoothly, and improves the passenger riding experience. In the elevator system of high-rise buildings, AC filter capacitors are key components to ensure the stable operation of elevators. ​
Selection points​
Capacitance value matching​
Determine the capacitance value based on the inverter power, operating frequency and load characteristics. Generally speaking, the greater the power and the more frequent the load changes, the greater the required capacitance value. If the capacitance value is too small, the filtering effect is poor; if the capacitance value is too large, it may cause excessive starting current, increasing cost and volume. For example, there is a significant difference in the required capacitance value for light-load equipment driven by a small-power inverter and heavy-load equipment driven by a large-power inverter.​
Rated voltage adaptation​
The rated voltage must be higher than the maximum AC voltage peak that may occur when the inverter is working, and a certain safety margin must be reserved. If the actual working voltage exceeds the rated voltage, the capacitor may be broken down and damaged. For common three-phase inverters, an AC filter capacitor with a suitable rated voltage should be selected. ​
Frequency characteristics match​
Different metallized polypropylene film AC filter capacitors have different performances in different frequency ranges. It is necessary to select products with frequency characteristics adaptation according to the main frequency range of harmonics generated by the inverter to ensure effective filtering throughout the entire operating frequency range. For example, some capacitors have good filtering effects in the low frequency band, while others perform well in the high frequency band. The selection should be based on the actual harmonic frequency distribution. ​
Temperature characteristics consideration​
When the inverter is working, it will generate heat, which will increase the ambient temperature. Therefore, it is necessary to select AC filter capacitors with good temperature characteristics and stable operation in high temperature environments to ensure that the capacitor performance is stable under different temperature conditions and maintain the normal operation of the inverter.