This invention relates generally to electrical insulation methods and more particularly to methods of curing resin impregnated insulation structures.
A common object in the insulation of electro-magnetic coils is to obtain a high thermal conductivity of the insulation structure so as to enable an increase in current density in the coils without exposing the organic electrical insulating materials to excessively high temperatures. To this end, the process of vacuum-pressure impregnating (VPI) an insulated coil with a solventless varnish to reduce the number and size of voids in the insulation and thereby enhance its electrical insulation properties is well known in the art. In order to obtain a sufficiently long shelf life time for the resin, subsequent to the coil being impregnated, it is cured by slow curing agents which generally require heat in order to achieve a cure of the varnish within a reasonable time. The heat in turn tends to lower the viscosity of the impregnating varnish before the chemical reaction occurs thereby resulting in the draining out of substantial quantities of varnish due to gravitational forces. In particular, field coils which are cured in a stationary position are more susceptible to this phenomena than are armature coils which are commonly rotated during the curing procedure. As a consequence, many air pockets are formed within the coils and in the interstices between the coil and the pole pieces or laminations, with the air pockets tending to impede heat conductivity from the coil, and thus limiting rated power output.
A known approach to solving the resin drainage problem is that of providing additional resin curing acceleration in the outer portion of the insulation, thereby causing the resin in the outer portion to cure more rapidly and providing an impervious shell which prevents an outward loss of resin and allows a slow controlled cure of the resin in the inner portions of the insulation. This is shown in U.S. Pat. No. 3,778,536 issued to Robert W. Smearing on Dec. 11, 1973, and assigned to the assignee of the present invention. In such a process it is important that the cure acceleration is not only reactive with the resin but that it is insoluble therewith. If it were soluble with the resin it would tend to leach out into the inner portion of the resin impregnant and also into the VPI tank to contaminate the resin therein.
With the use of epoxy resins there are various known accelerators which fulfill the requirements of insolubility. However, there are certain applications where the use of epoxy resins as varnish impregnants of insulation is not desirable for other reasons. For example, when insulating the coils of a motor armature, the entire assembly of coils and rotor is normally placed in the VPI tank and impregnated with the resin. If epoxy resin is used, the coils will tend to be bonded to the armature, thereby precluding any subsequent rewinding of the coils, a process which in some industries is commonly done every few years, as for example, in the rail vehicle traction motor industry.
Another disadvantage of the use of an epoxy resin as the impregnating varnish is that it tends to cure to a hard and brittle state and therefore has a tendency to crack under stress. Further, with many cured epoxy resin compositions, they tend to increase in viscosity with shelf life, a characteristic which is undesirable for an insulation structure.
It is therefore an object of this invention to provide an insulated coil with improved thermal conductivity characteristics utilizing the superior properties of polyester resin varnishes.
Another object of this invention is the provision in an insulation assembly for preventing the drainage of varnish impregnant between the impregnating stage and the curing stage.
Yet another object of this invention is the provision for an insulated coil structure which has a long shelf life.
Still another object of this invention is the provision for accelerating the cure of the impregnant periphery without contamination of the VPI tank.
A further object of this invention is the provision for curing a varnish impregnated coil without bonding it to its support structure.
Another object of this invention is the provision for a solid insulation structure which is soft and pliable and not prone to cracking.
Yet another object of this invention is the provision for a coil insulation structure which is economical to manufacture and extremely effective in use.
These objects and other factors and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.