ARMATURE WINDINGS AND SLOT INSULATION 95 



tween the conductors in one coil cannot be very high. The 

 copper conductors are usually insulated with cotton spun upon 

 the wire in two layers. Cotton braiding is sometimes used on 

 large conductors of rectangular section; and a silk covering is 

 used on very small wires where a saving of space may be effected 

 and an economical design obtained notwithstanding the high 

 price of the silk covering. A triple cotton covering is occasionally 

 used when the potential difference between turns exceeds 20 

 volts. Conductors of large cross-section may be insulated by a 

 covering of cotton tape put on when the coil is being wound. 



In additipn to the comparatively small amount of insulation 

 on the wires, a substantial thickness of insulation must be pro- 

 vided between the armature core and the winding as a whole. 

 The materials used for slot lining are: 



1. Vulcanized Fiber; Leatheroid or Fish Paper; Manilla Paper; 

 Pressboard; Presspahn; Horn Fiber; etc.; all of which, being 

 tough and strong, are used mainly as a mechanical protection 

 because they are more or less hygroscopic and cannot be relied 

 upon as high-pressure insulators, especially when moisture is 

 present. 



2. Mica; Micanite; Mica Paper or Cloth. These materials 

 are good insulators, and the pure mica or the micanite sheet will 

 withstand high temperatures. Sheet micanite is built up of 

 small pieces of mica split thin and cemented together by varnish. 

 The finished sheet is subjected to great pressure at high tem- 

 peratures in order to expel the superfluous varnish. Mica is 

 hard and affords good protection against mechanical injury; 

 but it is not suitable for insulating corners or surfaces of irregular 

 shape. 



3. Treated Fabrics, such as Varnished Cambric; Empire 

 Cloth, etc. These provide a means of applying a good insulat- 

 ing protection to coils of irregular shape. Linseed oil is very 

 commonly used in the preparation of these insulating cloths 

 and tapes because it has good insulating properties and remains 

 flexible for a very long time. 



It is common practice to impregnate the finished armature 

 coil with an insulating compound, and press it into shape at a 

 fairly high temperature. When this is done, ordinary untreated 

 cotton tape is used in place of the varnished insulation. 



Fig. 32 shows a typical slot lining for a 500-volt winding. 

 This can, however, be modified in many respects, and the reader 



