CHAPTER X 



THE INSULATING OF ARMATURE CONDUCTORS 1 



IN order to obtain a high " space factor " in armature slots, it is 

 essential not only to select the very best of materials for the 

 insulation of the groups of conductors, but careful attention must 

 also be given to the order of their arrangement, so that those 

 materials most suitable for withstanding mechanical strains shall 

 be so disposed as to shield the mechanically more delicate, but 

 dielectrically stronger materials. We must consider the reasons 

 leading to the use of particular materials in various cases. 



It lias for many grades of work been found preferable in 

 practice that a tough fibrous material, suitably treated to render it 

 moisture-proof, should come next to the iron walls of the slot, in 

 order to protect from mechanical injuries the conductors and their 

 high-grade insulating wrappings. Nevertheless, where motors are 

 manufactured in quantities, the slot lining is often dispensed with, 

 and the form-wound coil, after being hydraulically compressed, is 

 forced into the unlined slot. 



The following materials have been employed for slot linings. 

 They are arranged in the order of their toughness: 



Horn Fibre. 



Leatheroid. 



Presspahri. 



Red Rope Paper, and 



Manilla Paper. 



These and similar materials may be obtained of almost any 

 thickness. It is generally preferable to use several layers of thin 

 material in making up a slot lining of a required thickness. Thus, 

 for example, sheets of presspahn, each 0'25 mm. thick, used in two 

 or three layers, are far more flexible and give a somewhat higher 

 total disruptive strength than single sheets of equivalent thickness. 



1 Part of the material contained in this chapter is drawn from Chap. XIV. of 

 The Insulation of Electric Machines (Turner & Hobart), to which the reader is 

 referred for a full treatment of the subject. 



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