ARMATURE WINDINGS 259 



question of insulation studied by experts who decide upon the 

 most suitable materials to withstand the particular conditions 

 under which the machine will have to operate, and then advise 

 the designer of the machine regarding the space to be allowed 

 to accommodate this insulation. If high-class insulating materials 

 are used, the slot lining, i.e., the total thickness of insulation 

 between the conductors and the side or bottom of the slot, 

 should have the following values: 



Terminal voltage Thickness of .insulation 



(one side) 



500 0.045 in. 



1,000 0.060 in. 



2,000 0.080 in. 



4,000 0.12 in. 



8,000 0.19 in. 



12,000 0.27 in. 



81. Current Density in Armature Conductors. Although the 

 armature may be stationary, the permissible current density 

 in the conductors will depend to some extent upon the peripheral 

 speed of the rotating field magnets, because the fanning effect 

 will be greater at the higher velocities. The cooling effect of 

 the air thrown against the conductors by the rotation of the 

 field magnets is not so great as when the armature rotates, and 

 moreover, the air is warmed to some extent in passing over the 

 heated surface of the field coils. The current density in alter- 

 nator armatures usually lies between 1,500 and 3,000 amp. per 

 square inch of cross-section. The formula previously used in 

 the design of dynamo armatures requires some modification, 

 and the writer proposes the following empirical formula for 

 current density in the armature windings of alternators with 

 rotating field system, up to a peripheral speed of 8,000 ft. per 

 minute : 



A = f + I (96) 



The symbols have the same meaning as in formula (51) on page 

 97; the peripheral velocity, v, being calculated by assuming 

 that the armature is rotating instead of the field. 



82. Tooth and Slot Proportions. In deciding upon the 

 number of teeth on the armature, a compromise must be 

 made between a very small number of teeth which involves 

 the bunching of conductors, with consequent high internal 



