ARMATURE WINDINGS 



three-phase generator is equivalent, so far as the external circuit is 

 concerned, to interchanging the connections of two of the terminals. 



From a manufacturing point of view, armature coils may be 

 divided into two classes: former-wound and hand-wound. Former- 

 Nvound coils require entirely open slots, whereas hand- wound coils 

 may be placed in closed or semi-closed slots. Former-wound coils 

 are cheaper, and are more easily replaced than hand-wound coils. On 

 the other hand, in fitting a former-wound coil into position there is 

 some risk of damaging the insulation of the coil or slots. The use 

 of open slots results in an irregular wave-shape (unless some such 

 expedient as skewing the pole-pieces is adopted, see Fig. 72), and 

 necessitates, as a rule, the use of laminated pole-shoes in order to 

 prevent excessive eddy-current loss. Owing to the fact that thoroughly 

 reliable insulation is more easily obtained with hand-wound coils 

 (the insulating lining of the slot in this case consisting of a seamless 

 tube), this type would appear to be preferable in the case of generators 

 designed for very high voltages. 



The insulating lining of the slots consists of some form of mica 

 insulation, such as micanite or " reconstructed mica," and in the case 

 of hand-wound coils seam- 

 less tubes of the insulating 

 material are fitted into the 

 slots, as shown in Fig. 77. 

 In order to prevent dis- 

 ruptive discharge between 

 the coils and the frame, 

 these tubes are carried well 

 beyond the core-ends, as 

 shown in the illustration. 

 Since the end coils i.e. 

 those nearest the machine 

 terminals are subject to 

 the greatest dielectric stress, 

 it is advisable to provide 

 these coils with additional 

 insulation. 



The actual thickness of the slot insulation depends, of course, on 

 the voltage of the machine, and, according to Mr. H. M. Hobart,* the 

 following table may be taken as representing modern practice 



500 1000 2000 4000 8000 12,000 16,000 

 30 1-75 2-47 3-35 4'6 5*67 6'67 



MICANITE TUBES 



FIG. 77. Showing Armature Coils in Position. 



rf\K 



alternator ... j 

 Thickness of slot in- 

 sulation, in mm. 



* Electrical Iteview, vol. hi. p. 681 (1905). 



