THE INSULATING OF ARMATURE CONDUCTORS 251 



The following is a concise outline of a good treatment for 

 insulating form-wound coils. It presupposes that one of the 

 permanently plastic varieties of varnish is employed. Such 

 varnishes have now been placed on the market by several firms. 



1. Wind the coils on a former to the required shape (see 

 Chapter XL). 



2. Place the form-wound coils (either double cotton covered or 

 braided) in the baking oven, which should preferably be of the 

 vacuum type, for one hour at 90 C. 



3. Immediately on removing from the baking oven, while the 

 coils are still hot, they should be immersed in insulating varnish 

 and kept so immersed until air bubbles cease to rise. 



4. Return the coils to a baking oven, now filled with air instead 

 of being exhausted, and bake for ten to twelve hours at 90 C. 



5. Tape the coils with impregnated tape, applying it with one- 

 third overlap. 



The tape may be prepared and impregnated in the following 

 manner : 



(a) Dish the rolls of tape by pushing their centres outward, 

 rendering them concave in order to expose a maximum of surface. 



(b) String these rolls on a small rod passing through the 

 centre of the rolls. 



(c) Dry the rolls of tape in a baking oven (preferably a vacuum 

 oven) at a temperature of 90 C. for two or three hours. 



(d) Remove the rolls of tape from the oven and immerse them 

 immediately while hot in a suitable insulating varnish. Allow 

 them to remain about twelve hours in the varnish. 



The specific gravity of the varnish should be maintained at 

 about 0'85. Benzine is a suitable thinning material for some 

 varnishes. 



(e) Remove the rolls of tape from the varnish and suspend 

 them in the air to dry. The tape should now be of a permanently 

 plastic nature, and have no tendency to harden. 



The coils so taped up are now ready for assembling on the 

 armature of which the slots and end shields are insulated. This 

 process is referred to in Chapter XL 



In Fig. 287 is given a 500-volt winding for the stator of an 

 induction motor with a wide open slot. 



For equivalent windings for lower voltages, the insulation can, 

 from mechanical considerations, only be .decreased very slightly, 

 since, although the insulation thickness is unnecessarily great for 

 the required disruptive strength, there is great risk of mechanical 



