88 



ALTERNATING CURRENTS 



43. Armature Windings of Alternators 



As regards the armature winding, by far the greatest number of 

 alternators are provided with six slots per pole (though sometimes as 

 many as twelve slots per pole are used). One advantage of choosing 

 this number lies in the possibility of standardizing the core-plates, 

 so that the same core may be used for single-, two-, or three-phase 

 machines. If the machine is to be a single-phaser, only three or four 

 slots per pole would be utilized for the armature winding, the remain- 

 ing slots being left empty. These vacant slots tend to improve the 

 ventilation of the machine, and also offer the further advantage that 

 a supplementary winding may, if necessary, be placed in them, thus 

 enabling the machine to supply a small two-phase load in addition 

 to the main single-phase load.* 



FIG. 74. Illustrating Two Types of Armature Winding. 



If we agree to use four out of the six slots per pole for a single- 

 phase winding, then there are two ways of arranging the armature 

 coils. These two methods are shown in Fig. 74. The first method, 

 which we may describe as the ordinary method, consists in using one 

 coil per pole, each coil being distributed over four slots. This method 

 is illustrated by the left-hand side of Fig. 74. The second method, 

 for which Prof. S. P. Thompson has suggested the term hemi-tropic 

 winding, consists in using one coil per pair of poles. Each coil is 

 now embedded in eight slots, as shown by the right-hand side of 



* If a single-phase generating station is required to supply current to a short 

 tramway line, the transformation from single-phase to continuous current may be 

 effected by means of a synchronous motor-generator set. A single-phase rotary con- 

 verter, on account of its unsatisfactory performance, would not be used. But two-phase 

 converters have been used for this purpose, the alternating currents corresponding to 

 the second phase being obtained from the supplementary winding in one or more of the 

 main generators, the winding being placed in some of the unused slots. 



