ALTERNATING GENERATORS AND SYNCHRONOUS mMoTORS. 119 
193. Example 26. 190, the 300 KVA., 100 r.p.m. 
. alternator, has a total armature ampere conductor value 
of 163,600. Assuming the peripheral density correspond- 
ing to a generator of 300 KVA. capacity, which is 450, 
Equation (37) gives an armature bore of 
163,600 
D= “ 314 144 inches. 
194. Peripheral Speed.—lIt is always advisable to check the 
diameter by calculating the peripheral speed of the magnet 
wheel, which is equal to 
| nF, RL eae Se ee ee (38) 
where D, = the external diameter of the wheel in inches; 
N =the number of revolutions per minute; 
and V = the peripheral speed in feet per minute. 
The value of the peripheral speed indicates the importance 
of safe mechanical dimensions. This data is used only in 
special cases for guidance in the choice of the diameter, 
because nowadays it is not found difficult to cope with 
peripheral speeds as high as 10,000 feet per minute, and 
even more. The peripheral speeds of standard alternators 
vary only within small limits, and on the average it is 
usually 4,500 to 6,000 feet per minute. 
195. Effective Iron Length. 
effective iron length of the armature, it is necessary to be 
guided by the data of average tooth densities for different 
periodicities, which represent a compromise regarding 
the items 2 and 4 of Par. 181. 
The lower the specified efficiency, the higher may the mag- 
netic density be driven, resulting in the length of the 
machine becoming smaller. 
+196. Proportions of Alternators.—In the design of elec- 
