1. Field of the Invention
The present invention relates to metallized film capacitors which are improved by comprising a number of separate electrode segments made of evaporated metal film provided with fusing proof function.
2. Prior Arts
Conventional AC capacitors grouped into metallized film capacitors have structures that metallized roll films are rolled to a drum or a flat shape. FIG. 1 is a perspective view of a conventional winding type capacitor. FIG. 2 is an enlarged elevation view of one constitutional segment used for the capacitor of FIG. 1. A roll foil 1 coated with evaporated metal films 6 and 6' on both surfaces thereof is placed adjacent another roll foil 2 serving as an insulation film. Both foils 1 and 2 are than rolled into a drum shape or a non-circular shape as shown in FIG. 1.
Metal spray is applied to the side faces of the rolled electrode layers thus forming electrode lead-out portions 3. External lead wires are soldered to the metal-sprayed side faces 3.
When such rolled capacitors are used for long hours, or they are subject under a high temperature condition, thermal characteristics of the insulative foils 1 and 2 are gradually degraded. Thus, self-healings defined as below occasionally occur. The self-healing is a phenomenon that when dielectric breakdown of the foils occurs locally, the breakdown places are healed, i.e. insulation of the capacitors is soon restored by flowing off the metal film portions around the short-circuited foil films. The increase of self-healing times resultantly leads to an increase of a loss angle tan .delta., thereby increasing consuming electric energy. This energy increase triggers detachment of the sprayed metal contact layers from the evaporated metal electrode layers. And in worst cases, the sprayed metal contact layers can not maintain sufficient electric contact with the evaporated metal electrode layers, and dielectric breakdown exceeding the limit of the self-healing is triggered ultimately. At the last stage, the metallized film capacitors may emit smoke, or sometimes they are burned.
In order to avert such fuming or burning of the capacitors, the conventional capacitors are provided with some kinds of proof means, for example temperature fuse, current fuse, etc. The provision of such proof means in the casing of the capacitors increases their production costs.
The reason of the production of fire can be explained as the following way. Since the electrode layers are formed long on the long rolled single foil, and besides since the sprayed metal contact layers are in contact with the electrode layers in length, excessive current can flow through well-preserved current paths until the fire production, when self-healing is not sufficient, thereby leading to generation of badly insulated places.