Cleaning and Varnishing of Machine Windings
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.
Some machines are exposed to accumulations of materials, such as talc, lint, or cement dust, which although harmless by themselves may obstruct the ventilation. The machine will then operate at higher temperatures than normal, and the life of the insulation will be decreased. Such materials can sometimes be blown out with clean dry compressed air.
The most harmful types of foreign materials include carbon black, metallic dust and chips, and similar substances that not only impair the ventilation but also form a conductive film over the insulation and increase the possibility of insulation failure. Metallic chips may also work themselves into the insulation because of the ventilation and magnetic fields. When windings are cleaned, inspection should be made for any signs of deterioration.
Epoxy-encapsulated windings, a construction finding increasing favor, are sealed against contaminants. They need little attention other than removing dirt accumulations. The common practice when such windings are damaged is replacement with a new winding.
It is extremely important that all wound stators and rotors be perfectly clean before varnish treatment and curing. Unless all conducting dirt and grease are removed, the varnish treatment will not be fully effective. Also, after varnish treatment, the leakage path caused by conducting materials will be difficult to uncover and remove. Correct cleaning involves the following steps:
• Dirt should be removed from all coil surfaces and mechanical parts.
Air vent ducts should be clear. As an alternative, clean, dry air at a pressure of not more than 50 psi may be used. Higher air pressure may damage windings. Do not use air if dust from the machine can damage critical equipment nearby.
• As much oil, grease, and dirt as possible should be removed by wiping the windings with clean, dry cloths and then with clean cloths that have been moistened with a solvent recommended by the coil manufacturer. If the original varnish on the windings is cracked, a brush should be dipped in solvent and used to clean all conducting particles from the cracks.
• For cleaning, armatures or wound rotors should be placed in a vertical position with the commutator or collector ring end up, and a pressure spray gun with solvent should be used to clean under the collecting device and through vent holes. The same procedure should be repeated with the opposite end up, and then repeated again with the commuta- tor or collector ring end up. Most large DC armatures are ventilated through open commutator risers at the front end. The solvent spray should be directed through these risers to reach the inner surface of the armature coils and inner commutator vee-ring extensions.
• Silicone-insulated equipment can be cleaned by the same methods used with other insulation systems. If a liquid cleaner is found to be necessary, the recommendations of the coil manufacturer should be followed.
• For windings other than silicone, there are a number of good commercial cleaners on the market. The manufacturer can recommend the one most suitable for the conditions. Plant safety rules concerning the use of flammable and toxic solvent should be observed and followed.
• Caution should be exercised to remove all liquid cleaners.
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.
The units should be cured in a correctly ventilated forced-air circulating oven to remove the solvent vapors. The oven can be either gas fired or electrically heated. Infrared heat can be used if desired.
For the most part, the time and temperature of the cure should follow the varnish manufacturer’s recommendations. The time of cure will vary from short bakes of several hours up through 16–24 h, based on the physical dimensions and makeup of the units, and taking into consideration the particular characteristics of the type of varnish that has been applied to the equipment.
Curing temperatures will vary from 75°C to 125°C (167°F to 257°F) for oleoresinous-type varnishes to 135°C to 155°C (275°F to 311°F) for classes B and F varnishes. Silicone varnishes usually require a cure temperature range of 185°C–200°C (365°F–392°F) or higher.
Complete rewinding jobs should receive at least two coats of varnish. Baking time can usually be reduced on the first or impregnated coat, with an extended period of time used on the final coat. The use of additional
coats is based on what is expected of the unit after it is in operation. If severe conditions are to be encountered, multiple-coat systems are recommended. Also, apparatus such as high-speed armatures should receive multiple coats for the maximum bonding of the conductors. One coat is all that is necessary on older units that have been cleaned up on which no rewind work has been done.
In the case of large stators or rotors where the size is such that dipping is not possible, the varnish must be sprayed on the windings. Old winding surfaces must be completely coated.
For most applications, conventional dip methods are recommended. Other accepted methods are brushing and flooding. However, if the length or depth of the slots is great and the windings tightly packed, it may be necessary to use a vacuum impregnation system.
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