The life of a winding depends upon keeping it in its original condition as long as possible. In a new machine, the winding is snug in the slots, and the insulation is fresh and flexible and has been treated to be resistant to the deteriorating effects of moisture and other foreign matter.
Moisture is one of the most subtle enemies of the machine insulation. Insulation should be kept clean and dry. Certain modern types of the insulation are inherently moisture proof and require infrequent varnish treatment, but the great majority, if exposed to a damp atmospheric place, should be given special moisture-resisting treatment.
One condition that frequently hastens winding failure is movement of the coils caused by vibration during operation. After insulation dries out, it loses its flexibility. Mechanical stresses caused by starting and plugging, as well as natural stresses in operation under load, sometimes precipitate short circuits in the coils and possibly failures from coil to ground, usually at the point where the coil leaves the slot.
Periodic varnish treatment and curing, correctly done so as to fill all spaces caused by drying and shrinkage of the insulation, will provide an effective seal against moisture and should be a matter of routine electrical maintenance. Varnish treatment and curing of rotating electrical equipment follow a logical procedure.
The wound apparatus should be dried in an oven held at a temperature of 115°C–125°C (239°F–257°F) for 6–12 h or until the insulation resistance becomes practically constant. If a vacuum is used, the drying time may be reduced.
The apparatus should be brought up to temperature slowly because excessive moisture may be present in the windings. If heated rapidly, this moisture may vaporize quickly enough to rupture the insulation.
Before treatment, the apparatus should be cooled to within 10°C (50°F) above room temperature, but never to a temperature lower than 25°C (77°F). If the apparatus is cooled to room temperature and allowed to stand, it will take up moisture quickly. If placed in the varnish at a temperature higher than that specified, the varnish will tend to harden.
The selection of varnish is dependent upon the operating conditions to which the motor is subjected; also, the type of environmental conditions (i.e., moisture, corrosion, chemical, abrasion) should be taken into consideration.
Varnish must be compatible with the insulation system with which it is to be used. If it is incompatible, it may not adhere and may not give the desired protection. For most applications, the selection of a general-purpose high bonding, yet resilient, synthetic resin varnish is recommended. The varnish can be either class A, B, or F, depending upon the insulation system rating. On large AC stators using class A insulation, the use of a flexible asphalt or oleoresinous varnish is suggested; then, if it becomes necessary to lift a coil, the coil will not be destroyed.
Many types of varnishes are available, and when applying the insulating varnish, the recommendation of the manufacturer should be followed with respect to specific gravity, viscosity, and curing cycle for the particular varnish in question. After the varnish has been adjusted to give the desired film build and drainage characteristics, the specific gravity and viscosity readings should be recorded; then at periodic intervals the varnish should be examined for either specific gravity or viscosity, or both, and adjustments should be made to bring it within the original limits.