ALTERNATING-CURRENT WINDING 173 



to the title of a winding only signifies that it has four slots per pole 

 per phase. Such a winding is not necessarily " quadruple-coil," as 

 we shall see. The broad division between windings, as regards 

 their distribution, is " single-coil " and " multi-coil." The latter 

 describes all windings where the coil is distributed over a number 

 of slots greater than unity. 



The " single-coil " winding is often used for single-phase alter- 

 nators, as this permits of the most effective disposition of the 

 armature conductors as regards generation of electromotive force. 

 If more slots or coils are used, or, in the case of face windings, if 

 the conductors are more evenly distributed over the face of the 

 armature, the electromotive forces generated in the various con- 

 ductors are in different phases, and the total electromotive force 

 is less than the algebraic sum of the effective electromotive forces 

 induced in each conductor. But, on the other hand, the sub- 

 division of the conductor in several slots or angular positions per 

 pole, or, in the case of face windings, their more uniform distribu- 

 tion over the peripheral surface, decreases the self-induction of 

 the windings with its attendant disadvantages. It also utilises 

 more completely the available space, and tends to bring about a 

 better distribution of the necessary heating of core and conductors. 



The advantage of the " single-coil " winding is that it gives the 

 maximum obtainable total electromotive force for a given number 

 of conductors, whereas the electromotive force obtained from the 

 same number of conductors but distributed or arranged as a 

 " multi-coil " winding is considerably less. The armature reactions 

 are, however, for a given number of conductors and a given 

 current greatest when these conductors are arranged as a single- 

 coil grouping, which is an undesirable feature. 



While the single-coil type is used considerably for single-phase 

 armatures, the windings for polyphase armatures are almost 

 always multi-coil, having a number of slots per pole per phase 

 varying from two up to as high as ten in turbo-alternators, where 

 the high speed renders only few poles necessary. 



In graphically representing single-coil windings it is not con- 

 venient to draw out a number of conductors concentrated together, 

 and it is customary to represent a single concentrated coil of 

 many turns by a thick line with a thin line at its beginning and 

 end, representing the terminals. 



Thus Fig. 211 shows graphically a coil of a single-coil winding, 

 which may represent a coil of any number of turns. This will 

 be employed to represent the coils in any subsequent single-coil 



