An engine running 240 revolutions per minute will turn
240 ÷ 60 = 4 revolutions per second.
A frequency of 60 requires
60 ÷ 4 = 15 cycles per phase per revolution, or
15 × 2 = 30 poles per phase. Hence for a two phase alternator the total number of poles required is
30 × 2 = 60.
It is thus seen that a considerable length of spider rim is required to attach the numerous poles, the exact size depending upon their dimensions and clearance.
Fig. 1,436.—Three Crocker-Wheeler 75 kva., slow speed alternators direct connected to high speed engines. The alternator is styled slow speed although connected to a high speed engine, because what is considered high engine speed is slow speed for alternator operation. The alternators have direct connected exciters which are plainly seen in the illustration placed on an extension of bearing pedestal. Direct connected exciters on units of this kind do not, as a rule, assume too bulky proportions, because of the high engine speed.
Fly Wheel Alternators.—The diameter of the revolving fields on direct connected alternators of very large sizes becomes so great that considerable fly wheel effect is obtained, although the revolutions be low. By giving liberal thickness to the rim of the spider, the rotor then answers the purpose of a fly wheel, hence no separate fly wheel is required. In fact, the revolving element resembles very closely an ordinary fly wheel with magnets mounted on its rim, as illustrated in fig, 1,437.