I. Advantages and Applications of Film Capacitors
Film capacitors are capacitors made by overlapping metal foil as electrodes with plastic films such as polyethylene terephthalate (PET), polypropylene (PP), or polystyrene (PST) at both ends and then winding them together, with or without inductance. Because they use PET, PPT, or PST as the dielectric, they are also called plastic film capacitors.
Film capacitors are widely used in various industries, including electronics, home appliances, communications, and power, due to their advantages such as high insulation resistance, good frequency characteristics, non-polarity, and low dielectric loss. They are also used in high-frequency filtering, high-frequency bypassing, and analog circuits. So, what roles do film capacitors play in different circuits? Let's take a look.
Traditional film capacitors are made by overlapping and winding aluminum or other metal foils, which serve as electrodes, with a plastic film. However, another method for manufacturing film capacitors is called metallized film. This method involves vacuum-depositing a very thin layer of metal onto a plastic film to serve as electrodes. This eliminates the need for thick electrode foils, reducing the volume per unit capacitance of the capacitor. Therefore, film capacitors are easier to manufacture into small, high-capacity capacitors. For example, the common MKP capacitor is a synonym for metallized polypropylene film capacitor, while MKT is a synonym for metallized polyethylene capacitor.
(1) Parallel film capacitors, also known as mobile film capacitors, are mainly used to compensate for the reactive power of inductive loads in power systems, improve the power factor, improve voltage quality, and reduce line losses in circuits.
(2) Series film capacitors are connected in series in power frequency high voltage transmission and distribution lines to compensate for the distributed inductive reactance of the line, improve the static and dynamic stability of the system, improve the voltage quality of the line, and greatly increase the transmission distance and improve the transmission capacity.
(3) Electrothermal film capacitors are used in electrothermal equipment systems with frequencies of 40 to 24000 Hz to improve the power factor and improve the voltage or frequency characteristics of the circuit.
(4) Coupled film capacitors are mainly used for high-frequency communication, measurement, control and protection of high-voltage power lines.
(5) Circuit breaker film capacitor, formerly known as voltage equalization capacitor, is connected in parallel to the high voltage circuit breaker break to play the role of voltage equalization, so that the voltage between each break of the high voltage circuit breaker is uniform during the segmentation process and when it is disconnected, which can improve the arc extinguishing characteristics of the circuit breaker and improve the breaking capacity.
II. What factors easily cause film capacitor failure?
Film capacitors are capacitors made by winding metal foil as electrodes and plastic films such as polyethylene, polypropylene or polystyrene, as well as other materials. They are externally encapsulated with epoxy resin and have good flame retardant properties.
Film capacitors are widely used in various electronic fields and are frequently used in analog circuits due to their excellent characteristics such as non-polarity, high insulation resistance, good frequency characteristics, and low dielectric loss. However, film capacitors can also malfunction and become unusable for various reasons.
1. High working environment temperature
Film capacitors can work normally in a temperature range of -40℃ to +105℃, but once the operating temperature range is exceeded, the thermal aging of film capacitors will be accelerated, which will not only reduce the working time, but in severe cases, the film capacitors may even explode.
2. Improper selection of operating current
The current value in the circuit must be less than the current value that the film capacitor can pass through; otherwise, the film capacitor will heat up. Over time, the service life of the film capacitor will not only decrease, but in severe cases, it may even explode and ignite.
3. Exceeding the specified operating voltage
The voltage applied to the circuit is too high, far exceeding the rated operating voltage of the film capacitor. Under the influence of high voltage, partial discharge will occur inside the film capacitor, causing it to break down and become unusable.
4. Harmonic currents exist.
The superposition of higher harmonic currents with the fundamental current leads to an increase in the total current flowing into the film capacitor; higher harmonics cause parallel resonance between the system inductive reactance and the capacitive reactance of the film capacitor, increasing the current flowing into the film capacitor; the film capacitor experiences local series resonance with a certain higher harmonic, causing the film capacitor to be overloaded, expand internally, and thus trigger an explosion leading to failure.
These are the reasons why film capacitors malfunction. Before using film capacitors, these factors should be eliminated. When purchasing film capacitors, be sure to choose reputable, high-quality ones.
