(1) Field of the Invention
This invention relates to a capacitor. More particularly, the invention relates to a new capacitor which is characterized in that at least a part of the dielectric material thereof is made of plastics and is impregnated with 1-phenyl-1-(benzylphenyl)ethane.
(2) Description of the Prior Art
In recent years, oil-filled capacitors are made small in size and durable to high voltages. With this tendency, various kinds of plastic materials have been used singly or together with the conventional insulating paper.
The plastic materials will be still more used as the component materials of oil-filled capacitors. In some instances, all of the conventional insulating paper was replaced by a plastic material. As the electrical insulating oil used for the electrical appliances in which a plastic material is used, the insulating oil must have compatibility with the plastic material. In other words, when an electrical insulating oil comes into contact with a plastic material, if the oil dissolves or swells the plastic material, the dielectric strength of electrical appliances will be impaired. In the case that the viscosity of electrical insulating oil is too high, the dielectric strength is also lowered owing to insufficient impregnation. Accordingly, it is necessary for the electrical insulating oil that it is well compatible with plastic materials and the viscosity of the insulating oil is low.
Meanwhile, metallized film capacitors (hereinafter referred to as "MF capacitors") are known in which a metal-deposited film that is made by vacuum-depositing a metal such as aluminum or zinc, is wound as an electrode. MF capacitors of this kind are used widely because they have self healing action and can be made small in sizes with high dielectric strength even when they are not provided with any insulating films interposed among layers of electrodes. Furthermore, biaxially stretched polypropylene film is widely used for these MF capacitors because it is not expensive as compared with other films such as polyester film, in addition, the temperature characteristic in dielectric loss can be made good.
The MF capacitors presently used are mainly the so-called dry-type ones in which any electrically insulative impregnating agent such as insulating oil is not used.
Generally not only in electric capacitors but also in other various electrical appliances, the potential gradient can be made high when electrodes or electric conductors are wholly surrounded by an electrically insulative impregnating agent. That is, it is advantageous because the dielectric strength of electrical appliances can be improved. Furthermore, if the impregnating agent is properly selected, the high-voltage withstanding property can be much improved.
Accordingly, the so-called oil-impregnated or oil-filled MF capacitors are more preferable than dry-type ones. However, when a metal-deposited film with a polypropylene base film is impregnated with an electrically insulative impregnating agent, the sizes of the film are changed or the impregnating agent permeates through the boundary between the base film and the vacuum-deposited metallic layer. Accordingly, the metallic layer is cracked, and what is worse, the metallic layer peels off resulting into dielectric breakdown. In addition, as the capacitor is impregnated with an impregnating agent after the film is wound, if the selection of impregnating agent is not adequately done, the impregnating agent cannot penetrate sufficiently into the clearances among film layers, from which aimed effect of impregnation cannot be expected.
In the dry-type capacitors, dielectric breakdown is often liable to occur in the marginal edges or peripheries of wound metallic layers. So that, the improvement in the dry-type MF capacitors is intended to some extent as semi-dry MF capacitors, by impregnating the edge and peripheral portions of wound film with an impregnating agent leaving internal portions unimpregnated.
There is, however, a limit in the improvement effect in the semi-dry MF capacitors of the above type because it is impregnated only partially and most portions of electrode layers are left unimpregnated, and it may not be denied that the semi-dry MF capacitors are not satisfactory as compared with fully impregnated capacitors. Accordingly, various MF capacitors of impregnated type have been proposed.
For example, proposed in Japanese Laid-Open Patent Publication No. 55-36972 is an impregnated MF capacitor in which the expansion rate of metallized film that was swollen by an impregnating agent is made 0.5% or less. In British Patent No. 1,451,499, an oil-filled capacitor is disclosed in which the percentage of change in length of polypropylene film owing to an insulating oil is not more than 0.5% at 80.degree. C. and the diffusion quantity of the insulating oil into the polypropylene film is not more than 10% at 100.degree. C., and this capacitor can be an MF capacitor.
However, the above-described MF capacitors do not always fit for practical uses.
In European Patent No. A1-104786 is disclosed a metal-deposited polypropylene film capacitor that is impregnated with 1,2-diarylethane.
As described above, several attempts to improve capacitors have been made in connection with their base films and impregnating oils, however, there has never been proposed any satisfactory capacitor.