38 ALTERNATING GENERATORS .AND SYNGHRONOUS. MOTORS. 
mover, when the load is varied from full load to zero, 
and consequently when designing the excitation, one has 
not to make a considerable provision to cover this drop 
in speed. | Br ws 
69. Frequency.—The frequency of the alternations of the 
current is the principal characteristic of the alternating 
current system. For instance, in connection with a certain 
prime mover of, say, 500 revolutions per minute, a 25 
eyele generator will be of much smaller diameter, but 
of considerably larger width than a 50 eycle. machine. 
The reason for this is: the 50 cycle alternator has double 
the number of poles of that of the 25 cycle machine, 
consequently the first machine must have a larger bore. 
In the design of alternating current machines it is necessary 
to consider, step by step, how the different items are in- 
fluenced by the frequency. For instance, the permissible 
iron densities will be higher for a low frequency gener- 
ator, than for one of higher frequency, when considering 
a certain value of’iron loss. 
70. ‘Law of Speed and Frequency.—For a fixed frequency 
the numberof applicable speeds is limited, because the 
number of cycles is equal to the number of pole pairs p, 
multiplied by the number of revolutions per second N; 
that is: 
71. Example 4.—A generator running at the speed of 600 
revolutions per minute N, having a field system of 10 
poles, that is, 5 pole pairs p, will generate a current at 
50 cycles. See Formula (7). 
Example 5.—Take the case of a 12 pole machine in 
stock, which has to generate a current at 75 cycles. Under 
_. these circumstances, at what speed must the generator be 
driven? 
= a pa 
Se ee ee 
