The present invention relates to electrical insulation, and more specifically to an improved means and method for splicing together the insulation of a pair of cables whose conductors have been joined.
Whenever insulated electrical conductors are joined it is usually necessary to replace the insulation over the exposed, joined sections of conductor. In some cases both the provision of insulation and the mechanical joining of the conductors can be accomplished in one step, as by means of an insulated wire connector or "wire nut." In other cases, particularly with small conductors and in low-voltage applications, preformed insulation devices such as shrinkable polyester sleeves can be applied to the joint, providing a quick and easy means of insulating the exposed conductors.
In other applications, however, the problem is more difficult. When high voltages are present or large conductors used, frequently insulation splices must be hand made by taping the joints and/or applying insulation materials in liquid or paste form. Further, many applications require that a splice be water-tight, and that it be able to withstand many years' hard usage in hot, abrasive, or other hostile environments.
One example of such an application is the attachment of leads to the windings of large electric motors. Owing to the nature of the conductors used for electromagnetic windings they are frequently unsuitable for use as leads, and are terminated within the motor housing and connected to other, flexible cables which are then brought out of the motor to serve as connection leads. Such splices must be physically strong and resistant to heat, water, abrasion and motion, since a failure of the splice would require disassembly of the motor. In the past such splices have conventionally been made by brazing the conductors together, then overtaping the joint with a variety of tapes of types which are commonly known within the electrical insulation field such as Dacron polyester, woven glass, mica tapes, and combinations of the above impregnated with various resins and adhesives. The tape splices are in some cases separately cured by the application of heat, or by the use of an air-dry resin. In other cases, the splices were made before immersing the motor windings in a varnish bath or a vacuum-pressure impregnation (VPI) vessel, so that the splice became thoroughly impregnated and cured.
With the advent of newer, improved insulation materials, particularly synthetic materials, generally improved insulation characteristics have resulted. However, new insulation materials have given rise to additional problems in the manufacture of motors. For instance, cables of the sort which are well adapted to form motor leads are insulated with materials of a type which do not form cohesive bonds with the impregnants, adhesives and resins which are used to form insulation splices. For instance, materials utilizing silicones, fluoroethylenes and polyethylenes frequently do not adhere to the tapes and resins conventionally used to form insulation splices. This results in a joint which is not water-tight, and which does not have the physical integrity of prior art joints. Yet another problem arises when such leads are attached to motor windings before VPI treatment. While the lead insulation material is not porous, it is slightly resilient and the hydraulic forces involved in the VPI process will drive liquid resin into the leads, beneath the insulation and along the conductors throughout the length of the lead. As the resin cures the lead becomes hard and rigid, defeating the flexibility which is ordinarily necessary.
One way of overcoming this problem has been to splice extra-long lead cables to motor windings, and extend the free ends of the cable above the level of the resin during the VPI process so that the resins would impregnate the motor windings and the insulation joint between the windings and leads, but would not enter the leads themselves. This approach, however, is time consuming and wasteful of lead material. Further, it precludes the VPI treatment of large numbers of motors, unless the lower tiers of motors which are to be deeply immersed in the VPI resin are provided with extra-long leads, which must subsequently be cut short, wasting still more of the expensive lead material.
It will therefore be appreciated that it would be highly desirable to provide a method of splicing lead cables to already-impregnated conductors which will provide a water-tight joint, and which does not require subsequent impregnation.
It is therefore an object of the present invention to provide an improved insulation splice for electrical conductors.
Another object is to provide a method of splicing conductor insulations which does not rely upon adhesion to the conductor insulation.
Yet another object is to provide an improved, flexible insulation joint which is water-tight.
Another object of the invention is to provide a method of sealing later-applied insulation material about a cable insulated with a nonadhesive synthetic material.