Proper preparation of the stator after burnout and stripping is very important. Cleaning the stator and removing burrs and sharp edges helps protect winding insulation. Varnishing the stator before rewinding will seal the laminations and prevent oxidation.

Inspection and repair of the stator

After coil removal has been completed, inspect the stator for the following conditions.

• Charred but adhering varnish
• Sprung laminations
• Sharp metal edges on laminations
• Debris in lamination slots

Old varnish can irritate new winding insulation if it is not removed. Remove the old varnish by any of the following methods.

• Chemical cleaning with approved solvents if facilities are available
• Wire brushing
• Sandblasting (Sandblasting must be done carefully to prevent loosening stator laminations.)

Tighten any loose or sprung laminations by wrapping the lamination group with heavy copper wire. Use a ball peen hammer to peen a single lamination at an angle. Remove any sharp edges or burrs on laminations by filing by hand; this will prevent cutting of the slot insulation material and nicking wire varnish.

Do NOT clean stator slots with compressed air since this could result in personnel injury or contamination of the stator from moisture in the air system.

Remove any debris from stator slots with a round stiff brush, similar to a rifle bore brush, after all cleaning and repair are completed.

Conduct a core loss test.

Reinspect the stator to ensure that it has been properly cleaned and is ready for varnishing.

Varnishing preparation

Insert the bolts in stator bolt holes as applicable. Attach a dipping harness or wire rope slings to the stator so the stator bore will be in a vertical position during dipping.

Place the stator in the bake oven which has been preheated to 302°F (150°C) ± 10 percent. Bake the stator for 2 to 4 hours, depending on its size.

Always wear thermo-insulated gloves when handling heated components.

Wired Laminations, Views of Both Sides

Do not dip the stator if it is too hot. Remove the stator from the oven at the end of allotted baking time and allow it to cool to approximately 104°F  (40°C).

The following applies only to solvent type varnish.

Click on image to enlarge.

Stators Cooling after First Bake

Use a Zahn Varnish Cup No. 2 or Demmler Varnish Cup No. 1 to measure varnish viscosity in accordance with NAVSEA 0900-LP-060-2010 Series, Volume 1, Chapter 11, paragraphs 11-11 through 11-15.

Thin the varnish in accordance with NAVSEA 0900-LP-060-2010 Series, Volume 1, Chapter 11, paragraph 11-16 through 11-17.

Varnish viscosity should be between 150 and 250 centipoise. Varnish over 250 centipoise will be thinned with xylene.

When solventless varnish is used follow varnish manufacturers' instructions.

Lift the stator with the varnish tank chainfall and position it over the center of the varnish tank.

Varnishing and baking

Lower the stator and immerse it in the varnish until the bubbling stops. The stator should be just below the surface, fully submerged, but still visible.

Raise the stator clear of the varnish and allow it to drain over the tank for about 5 minutes.

Close the cover on the varnish tank as soon as the stator is moved.

Position the stator over a shallow pan lined with paper and allow it to drain and air dry for 1 hour. This is very important for solventless varnish.

Wipe the varnish from the end bell mating surfaces, the tops of the laminations inside the stator bore, and the bottom of the mounting feet, if applicable, with a lint-free cloth dipped in xylene for solvent varnish. A clean lint-free rag is best for solventless varnish. The best way is to coat the surfaces with a release agent available from the manufacturer.

Ensure that the stator is always positioned so that the bore is vertical.

Rotate the stator periodically during the air drying process to keep the varnish from pocketing.

Place the stator in a bake oven that has been preheated to 302°F (150°C) ± 10 percent. Bake for 6 to 8 hours, depending on stator size.

Always wear thermo-insulated gloves when handling heated components.

Remove the stator from the bake oven and cool it to room temperature.

Inspect the stator for any varnish lumps; remove them by carefully scraping.

Varnish Viscosity Chart, No. 2 Zahn Cup and No. 1 Demmler Cup