Fig. 1,499.—Partially distributed winding. Each coil unit is here divided into two concentric coils of different dimensions and connected in series, as shown in detail in fig. 1,497. This being a "whole coil" winding the several units are so connected that the winding of adjacent units proceeds in opposite directions, that is, one coil is wound clockwise, and the next counter clockwise, etc., so that the induced currents flow in a common direction as indicated by the arrows for the position shown.
Fig. 1,500.—Fully distributed winding. In this type of winding each coil consists of so many sub-coils that the winding occupies the entire surface of the armature core; that is, there are no extensive spaces unoccupied, the spacing being uniform as shown.
Ques. How far is it advisable to spread distributed coils of a single phase alternator?
Ans. There is not much advantage in reducing the interior breadth much below that of the breadth of the pole faces, nor is there much advantage in making the exterior breadth greater than the pole pitch.
Undue spreading of distributed coils lowers the value of the Kapp coefficient (later explained) by reducing the breadth coefficient and makes necessary a larger number of inductors to obtain the same voltage.
The increase in the number of inductors causes more armature self-induction. From this point of view, it would be preferable to concentrate the winding in fewer slots that were closer together. This, however, would accentuate the distorting and demagnetizing reactions of the armature. Accordingly, between these two disadvantages a compromise is made, as to the extent of distributing the coils and spacing of the teeth, the proportions assigned being those which experience shows best suited to the conditions of operation for which the machine is designed.
Fig. 1,501.—Developed diagram of single phase concentrated whole coil winding in two slot stamping for six pole alternator. If the sides of adjacent whole coils be slightly separated by placing the winding in a two slot stamping the electrical result will not differ materially from the monotooth whole coil winding, but if the winding be hemitropic, as in fig. 1,502, and has coils of two sizes as shown, it will be suitable for high voltages.
The Kapp Coefficient.—A volt or unit of electric pressure is defined as the pressure induced by the cutting of 100,000,000 or 108 lines of force per second. In the operation of an alternator the maximum pressure generated may be expressed by the following equation: