TRANSFORMERS 441 



placed be ween the core and the low-voltage coil, and between the 

 high- and low-voltage coils the same as used for the other types 

 This construction is necessary for transformers of the higher 

 voltages, requiring a greater number of turns and an increased 

 amount of turn insulation. Therefore, it can be seen that, at a 

 given capacity a point will be reached where it will be impossible 

 to use cylindrical edge-wound coils because the conductor will 

 become too thin to wind on edge, while, on the other hand, the 

 losses may limit the use of such a construction before the mechanical 

 considerations. As the edge-wound conductor becomes thin on 

 account of increased turns and insulation, the width, which is the 

 thickness of the cylinder, must be increased sufficiently to give 

 the proper current carrying capacity. This width is perpen- 

 dicular to the leakage flux and eddy current losses are accordingly 

 set up in the conductors. It is quite possible to reach a point 

 where an increase in the width of the conductor will give an 

 increased total loss in the copper on account of the eddy current 

 losses increasing faster than the PR loss decreases, due to a layer 

 conductor. The disc coils effectively overcome these difficulties, 

 first, because the width of the coils is sufficient to accommodate 

 the required turns, and second, because the width of the rect- 

 angular conductor is parallel to the leakage flux and, therefore, 

 does not increase the eddy current loss. 



After the coils are wound they are clamped to dimensions and 

 thoroughly baked and vacuum treated to insure the complete 

 elimination of moisture. Numerous treatments in insulating 

 compounds are then applied, sealing up all interstices and cement- 

 ing each coil into a solid structure. The coils are then subjected 

 to further baking, after which the clamps are removed and the 

 proper number of tapings applied, followed by the final series 

 of treatments and bakings, after which the coils are ready for 

 assembly. 



The taps are always placed in the coils located in the central 

 portions of the winding where the potential strains are at a mini- 

 mum. To facilitate the bringing up of several leads from the taps, 

 a new arrangement is being used in modern transformers. It 

 consists of multi-conductor leads, two or more insulated cables 

 being bound together and heavily wrapped with varnished cambric, 

 forming a stiff solid structure that is easily supported and well 

 insulated from ground (Fig. 276). Each element of the group 



