ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 39 
Solution: Applying Formula (7) we have p= ie = 6; 
n= 75. Therefore the speed is 750 r.p.m. Ans. 
Example 6.—Supposing it is required to design an alter- 
nator to give a specified output at a frequency of 40, for 
direct coupling to a steam engine and the engine maker 
limits the applicable speeds to between 270 and 310 revolu- 
tions, what would be the most favorable speed ? 
Solution: For the generator in question, there are two 
suitable numbers of revolutions, namely 267 and 300. 
The first speed requires an 18 pole generator, the second 
speed corresponds to a machine of 16 poles. Under these 
conditions one would decide in favor of the higher speed, 
that is 300 r. p. m. 3 
72. Standard Frequencies.—Practice has adopted the 40 
, to 60 eyele generators for lighting as well as for power 
work, but 25 cycle alternators are used with advantage 
where the load consists only of motor work. 
Low Frequency: In the design of low frequency fly- 
wheel generators, one should bear in mind the difficulties 
which are presented, as with a comparatively small number 
of poles the diameter of the machine is of a very moder- 
ate size, and it becomes almost impossible to make an 
economical design under such circumstances, when the 
flywheel effect has to be placed in the magnet wheel. 
High Frequency: On the other hand, a low speed 
high frequeney alternator leads to an abnormally narrow 
and big machine, and in such eases the question of 
rigidity must be earefully considered. High frequencies 
of about 100 eyeles are to be found in single phase systems, 
but there, too, it has become a rule to employ 40 to 60 
eyele machines, and for such work, nearly all the makers 
have built their standard line of alternators. 
Frequency and Speed: With the present. large field 
of frequencies and with the great range of available 
