DIRECT CURRENT DYNAMOS AND MOTORS. 65 
gives 40 cycles. But this is rather too high a frequency, 
and should not be adoptad except for special reasons. 
If the values n, = 1, 2, 3, 4, etc., and f = 10, 15, 25, and 
35 be inserted in the above formula for 7,, the following 
table of dynamo speeds for various numbers of pules is 
obtained. From this table the proper number of poles 
for low, medium, and high speed machines of given 
speeds can be readily taken. 
TABLE 23. 
NUMBER OF MAGNET POLES FOR VARIOUS SPEEDS. 
Wusiies Limits of Speeds. 
< Low Speed Medium-Speed High Speed 
Poles Machines. Machines. Machines 
. f = 10 to 15. f = LAG. f = ~i to 35. 
2 600 to 900 900 to 1500 1500 to 2100 
4 300 ‘* 450 450 **. 750 750 ** 1050 
6 200 ** 300 300 ** 500 500 ** 700 
8 150 ** 225 225 ** BTS B76 “* 625 
10 120 ** 180 180 ** 300 300 *£ 420 
12 100 ** 15! 150. ** "260 Rot Ps BAO 
14 85 **.130 130 ** °215 Shp "= BOO 
16 ro * 115 tae “F90 THe f4.. 260 
18 ees S'C) 100.170 T7O\**). 230 
20 60 **° 90 90.-<%-- 260 150 ** 210 
(24 50 ** <95 Te © = 325 2 Oy ETO 
Generally speaking, the bipolar type is used for high 
speeds, such as 7500 revs. per miu. or more. <A further: 
reason for selecting the bipolar type for very high speeds 
is the fact that machines running at 1,5 0 revolutions, or 
more, are usually quite small. For speeds between 400 
and 900 revs. per min. the four-pole construction is 
especially suitable, giving frequencies from 154 to 30 
cycles. This includes nearly ull belted dynamos and mo- 
