Electricity: electrical systems and devices – Electrostatic capacitors – With protection or compensating means
Patent
1995-09-18
1997-06-10
Ledynh, Bot L.
Electricity: electrical systems and devices
Electrostatic capacitors
With protection or compensating means
3612741, 361534, H01G 216
Patent
active
056382502
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a capacitor provided with internal protection and made of a metallized plastic film, said capacitor comprising at least two metallized plastic films wound around a core tube to form a capacitor cylinder, a first and a second metallized contact surface formed at the ends of the capacitor cylinder, a first lead connected to said first contact surface, and a second lead which is conducted through the core tube of the capacitor cylinder and is in connection with the second contact surface of the capacitor cylinder, whereby the second lead is connected to the second contact surface by a separate conductor, and a thermal fuse is provided between the lead and the conductor.
Capacitors are manufactured by winding metallized plastic films into cylindrical coils, and spraying contact and fixing surfaces of zinc or some other suitable material to the ends of the cylinders so as to form two or more overlapping spiral capacitor surfaces, some of them being secured to the contact surface of one end of the cylinder and others being secured to the contact surface of the opposite end.In the event that a fault occurs in a capacitor, it is sometimes necessary to isolate it from the network or remove it from operation. Damage done to a capacitor is usually repaired by means of self-repair.
A self-repairing capacitor is based on the idea that a current peak penetrating the plastic film serving as an insulator evaporates the metallic film around the breakdown point so that the insulating strength becomes sufficiently high. If, however, the breakdown continues or the number of breakdowns increases, the plastic begins to warm up gradually, and the electric breakdown starts to heat the capacitor, whereby the temperature begins to rise, which exerts pressure on the capacitor. Consequently the electrical strength of the insulating plastic reduces, and the destruction accelerates as it is in a way self-progressive. This means that the higher the temperature gets, the lower is the voltage required to effect breakdown. Finally, when the temperature has become high enough, the insulating plastic begins to melt. If the destruction continues, the capacitor may catch fire, which may result in the entire capacitor being destroyed unless it is isolated.
The object of the present invention is to provide a capacitor with protection both against heating up and, if necessary, against other damages so that the capacitor can be isolated from a power source before it catches fire.
The capacitor of the invention is characterized in that said thermal fuse consists of a joint in which an electrical connection is formed between the lead and the conductor by means of a connecting material the melting point of which is lower than that of the plastic film of the capacitor cylinder.
An essential feature of the invention is that the second lead, extending to the middle of the capacitor, is connected to the conductor extending to one end of the capacitor by means of a connecting material, such as a solder or a soldering paste, having a melting point lower than that of the plastic used as insulating material in the capacitor. Another feature of the invention is that the melting point of the core tube in the capacitor is lower than that of the insulating plastic of the capacitor but higher than that of the soldering paste or solder. Still another feature of the invention is that the core tube is preferably air- or gas-filled so that the molten plastic mass may penetrate it and come in direct contact with the joint which is designed to be melted. The solder or paste is allowed to melt and escape from the joint between the lead and the conductor, whereby the lead and the conductor are disconnected from each other. The preferred embodiment of the invention is characterized in that the conductor extending from one end of the capacitor into the core tube is a rigid uninsulated copper or aluminium conductor with good conductivity of heat and electricity, and that a separate fuse wire is connected between the rigid copper conductor and the se
REFERENCES:
patent: 3573565 (1971-04-01), Grunert
patent: 3579061 (1971-05-01), Waldon
patent: 3619742 (1971-11-01), Rud, Jr.
patent: 4635163 (1987-01-01), Voglaire
patent: 4748536 (1988-05-01), Robinson
patent: 4791529 (1988-12-01), Duncan et al.
Ledynh Bot L.
Nokia Kondensaattorit Oy
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