Capacitors of the type utilized with ballasts in AC applications are commonly made from a metallized film, such as polypropylene. To form the capacitor, the film is typically wound into one or more roll-shaped capacitor sections which are housed within a sealed metal container. Such containers often include a mechanical interrupter which disconnects the capacitor from external terminals, in the event that there is excessive pressure or temperature within the container. In addition, the capacitor is impregnated with oil and the entire container is filled with oil, in order to seal out air and moisture. This avoids the possibility that air or moisture inside the capacitor might ionize and produce electrical arcing.
The problem with oil-filled capacitors is that the oils used are considered to present an environmental and health hazard. Accordingly, great care must be taken to avoid leakage of the oil from the capacitor casing while the capacitor is installed. In addition, disposal of capacitors that are no longer usable requires that they be subjected to special toxic waste treatment. In some instances, metallized capacitor sections for fluorescent lighting applications have been dipped in wax. However, wax exhibits a relatively high degree of shrinkage and has a tendency to develop cracks. In high voltage applications, air and/or moisture is likely to enter these cracks and results in early failure of the capacitor, owing to electrical arcing. Also, wax tends to be a relatively expensive material for use in capacitors.
Broadly, it is an object of the present invention to avoid the shortcomings of existing capacitors used for a AC applications. It is specifically contemplated that the invention provide a capacitor used for relatively high voltage AC applications which does not require a liquid impregnant in order to seal the capacitor against the entry of air and moisture.
It is another object of the present invention to provide a metal encased capacitor which does not require a liquid fill material, yet is resistive to failures due to the leakage of air and/or moisture thereinto, and which will continue to function reliably in relatively high voltage applications for extended periods of time.
It is also an object of the present invention to provide a capacitor for AC applications which is reliable, safe and convenient in use, yet relatively inexpensive in construction.
It is also an object of the present invention to provide a method for manufacturing a capacitor useful in AC applications which meets one or more of the foregoing objects.
In accordance with a preferred embodiment of the present invention, a capacitor for an AC application is manufactured from zinc alloy metallized polypropylene sheet material. The sheet material is wound into one or more sections having a roll-shape, which sections are placed within a metal container and provided with electrical terminals for connecting to the exterior of the container. The metal container is then filled with a potting compound comprising 80%-90% blown asphalt and 10%-20% wax additive. When the potting compound sets, the capacitor sections are sealed therein so as to be protected from air, moisture and thermal and electrical shock. The container is then covered with a top which includes a pressure interrupter, and it is sealed. This capacitor exhibits improved resistance to deterioration due to environmental factors over long periods of time, and it permits operation with higher voltages than was previously possible.