☀️The main factors affecting the dissipation factor of film capacitors are frequency, temperature, humidity and so on.
❓What is the relationship between dissipation factor and frequency?
✨If the inductive reactance Ls is not considered, for ƒ <ƒt (where ƒt is the resonant frequency), the equivalent circuit of the capacitor shows that the disipation factor tanδ is the loss factor tanδp of the parallel resistance part, the disipation factor tanδs of the equivalent series resistance part, and the insulation The disipation factor tanδd of the dielectric part is formed. In this way, the overall disipation factor of the film capacitor is
Tanδ = tanδp + tanδs + tanδd
Among them,
And tanδd is the characteristic of the medium itself.
✨Because the diffipation factor of the parallel resistance part is relatively small, it is negligible in the entire frequency range, because due to the existence of insulation resistance (parallel resistance Rp), even at very low frequencies (ƒ<<1kHz), tanδp, In fact, it has no obvious effect on the entire dissipation factor. Therefore, the dissipation factor Tanδ at low frequencies is only determined by the insulation component tanδd. For small-capacity polypropylene film capacitors, before the frequency rises to several MHz, the dissipation factor is almost independent of frequency, about 0.0001.
✨However, as the frequency increases (ƒ>1kHz), the equivalent series resistance (ESR) part of the dissipation factor Tanδs (proportional to the capacitance) increases rapidly, until it becomes the decisive component of the dissipation factor curve, Figure 1.15 is 1nF And the dissipation factor of the polypropylene film capacitor of 100nF, the relationship between the dissipation factor of the equivalent series resistance and the frequency can clearly see this conclusion. The equivalent series resistance part is the sum of the connection resistance (terminal), wire resistance, metal layer and electrode foil resistance.
☀️The relationship between the dissipation factor and frequency of various film dielectric capacitors is shown in Figure 1.26.
✨Among them, due to the insulating properties of polyester film capacitors, a considerable dielectric dissipation tanδd of the insulating component is caused. Especially in the case of low frequency, polyester film capacitors are significantly higher than the total dissipation factor of polypropylene film capacitors and polyethylene film capacitors.
☀️The above is the relationship between the dissipation factor and frequency of the film capacitor. In the next article, we will explain the relationship between the dissipation factor and temperature, humidity and voltage.
