CHAPTER XIX
THEORY OF THE ARMATURE
Current Distribution in Ring and Drum Armatures.—In studying the actions and reactions which take place in the armature, the student should be able to determine the directions of the induced currents. The basic principles of electromagnetic induction were given in chapter X, from which, for instance, the distribution of current in the gramme ring armature, shown in [fig. 279], is easily determined by the application of Fleming's rule.
Tracing the current from the negative to the positive brush, it will be seen that it divides, half going through coils 1, 2, 3, and half through coils I, II, III, these two currents ascend to the top of the ring, uniting at the positive brush.
Ques. In the Gramme ring armature ([fig. 279]) what is the distribution of armature currents?
Ans. There are two paths in parallel as indicated in [fig. 279].
Ques. How does the voltage vary in the coils?
Ans. It varies according to the position of the coils, being least when vertical and greatest when horizontal in a two pole machine arranged as in [fig. 279].
The upper and lower coils in the right hand half of the ring armature, [fig. 279], will have about the same electromotive force induced in them, say 2 volts each, while the two coils between them will have a higher electromotive force, at the same instant, say 4 volts each, since they occupy nearly the positions of the maximum rate of change of the magnetic lines threading through them. These eight coils may be represented by two batteries connected in parallel, each battery consisting of two 2 volt cells and two 4 volt cells as shown in [fig. 280]. The voltage of each battery then will be
2 + 4 + 4 + 2 = 12 volts
| 2 | + | 4 | + | 4 | + | 2 | = 12 volts |