Ques. In what respect do commercial machines differ mostly from the elementary alternator shown in fig. 1,370, and why?
Ans. They have a large number of poles and inductors in order to obtain the desired frequency, without excessive speed, and electromagnets instead of permanent magnets.
Fig. 1,371.—Developed view of elementary single phase four pole alternator and sine curve showing the alternating current or pressure generated during one revolution. The armature is here shown as a flat surface upon which a complete view of the winding is seen. If M be any position of an inductor, by projecting up to the curve gives N, the corresponding value of the current or pressure. Magnetic lines are shown at the poles representing a field decreasing in intensity from a maximum at the center to zero at points half way between the poles, this being the field condition corresponding to the sine form of wave. In actual machines the variation from the sine curve is considerable in some alternators. See figs. 1,247 and 1,248.
Ques. In actual machines, why must the magnet cores be spaced out around the armature with considerable distance between them?
Ans. In order to get the necessary field winding on the cores, and also to prevent undue magnetic leakage taking place, laterally from one limb to the next of opposite sign.
Ques. Is there any gain in making the width of the armature coils any greater than the pole pitch, and why?
Ans. No, because any additional width will not produce more voltage, but on the contrary will increase the resistance and inductance of the armature.
