258 PRINCIPLES OF ELECTRICAL DESIGN 



made clear in the vector diagram, Fig. 98. The winding is sup- 

 posed to be distributed in a very large number of slots, and the 

 diameter of the semicircle represents the resultant generated 

 e.m.f. if all the slots are filled with conductors (connected in 

 series). If, as is usual in practice, only 75 per cent, of the slots 

 are utilized, the spread of the single-phase winding will be about 

 135 electrical degrees; 1 the resultant e.m.f. will be AB, which is 

 not much shorter than AC] but the length and weight of copper 



in the two cases are in the proportion A pr , The fact that, 



arc A.LJ\J 



in polyphase machines, the whole of the armature surface is 

 available for the windings, while only a portion of this surface 

 is utilized in the single-phase alternator, accounts for the fact 

 that the output of the latter is less than that of the polyphase 

 machine for the same size of frame. Given a three-phase machine, 

 it is merely necessary to omit one of the phase windings entirely 



Fia. 98. Vector diagram illustrating "spread" of armature winding in 

 single-phase alternator. 



and connect the two remaining phases in series, to obtain a 

 single-phase generator. The modified machine will be capable 

 of giving something more than two-thirds of the output of the 

 polyphase generator, the limit being reached when the copper 

 losses become excessive. 



80. Insulation of Armature Windings. With very high 

 voltages, such as are used on many power transmission schemes, 

 a special study has to be made of the problems of insulation. 

 These problems then become of extreme importance, and manv 

 difficulties have to be overcome that do not trouble the designer 

 who is dealing with pressures of the order of 5.000 to 10,000 

 volts. 



The reader is referred to Art. 28 of Chap. V, where slot insula- 

 tion was discussed, and since the same insulating materials 

 are used in alternators as in dynamos, there is little to be added 

 here. It is the practice of some manufacturers to have the 



1 A spread of 120 degrees is very common because this is obtained by con- 

 necting in series two of the phase windings of a three-phase generator. 



