Capacitors are composed of metal parts and non-metallic parts, so the source of heat is also discussed from these two aspects. This article introduces the heat sources of metallized film capacitors in detail.
① Loss of metallized film capacitor
Metallized plates of capacitors, internal metal wires in capacitors, gold-sprayed layers on the ends of capacitor elements, wire connectors or solder joints, metal lead wires and other metal parts all have a certain resistance. These resistors create an additional resistance r in series with the metallized film capacitor, called the series equivalent resistance of the capacitor.
Due to the existence of this series equivalent resistance r, the metallized film capacitor has a loss, which is referred to as the plate resistance loss, metal wire loss, gold spray contact resistance loss, solder joint contact resistance loss, metal lead wire loss, etc. . These losses are collectively referred to as metal losses.
② Dielectric loss
Under the action of voltage, a physical phenomenon occurs in the medium that turns part of the electrical energy into heat, which is called dielectric loss. Dielectric loss is mainly divided into conductivity loss and polarization loss. Conduction loss: Because the medium has a certain conductance, a conductive current must be generated after the voltage is applied. In a good dielectric, the conduction current is small, the insulation resistance is high, and the conductivity loss is small. If the manufacturing process is rough or not strict, the presence of moisture or other impurities in the capacitor will reduce the insulation resistance and increase the conductivity loss. Polarization loss: Under AC voltage, the medium will be subjected to alternating polarization, and electrons, ions, dipoles or other polar groups will be repeatedly displaced. The movement of these particles must overcome the resistance of thermal motion, so it will consume Part of the electrical energy and the conversion of electrical energy into heat. Polarization loss is closely related to polarization mode, temperature, and electric field frequency. The stronger the dielectric polarity, the greater the polarization loss. In summary, the equivalent circuit model of a metallized film capacitor can be expressed as a form of a conductivity capacitance model plus a series resistance.
③ Loss caused by partial discharge
When a metalized film capacitor is applied with a higher voltage, if there is a small air gap remaining between the dielectric layers, the air gap will ionize due to the low electrical strength of the air, causing partial discharge. In addition, due to the concentrated electric field at the edges of the capacitor plates, the field strength is higher than the uniform field strength between the plates, and the capacitor edges are also prone to partial discharge. Because partial discharge consumes active power, it also generates lossy heat. And ozone, nitrogen oxides and other gases accompanying partial discharge cause the medium to be chemically corroded, and the dielectric performance is reduced, so that it gradually deteriorates and breaks down.
The cause of metallized film capacitor heat generation can be caused by the above conditions, everyone should pay attention when using it.
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