60 ALTERNATING GENERATORS AND SYNCHRONOUS MOTORS. 
102. Calculation of Resistance.—By the use of Table 12, 
the calculation of resistances is simplified, as it is only 
necessary, after determining the length of the winding, to 
multiply the number obtained by that in column 4 in order 
to obtain the total resistance in ohms. For ‘‘ Data of Bare 
and Single Cotton Covered Magnet Wire,’’ American or 
B. & 8. Gauge, see Table 12A. 
103. Excitation Losses.—In modern generator, design where 
close regulation is necessary, it has been found advisable 
to make the ratio between the armature and field ampere 
turn about 1 to 3. This condition, combined with the per- 
missible temperature rise, leads to a limit of the excitation 
TABLE 13.—USUAL LIMITS OF EXCITATION LOSSES 
FOR DIFFERENT SIZES OF ALTERNATORS. 
Output of Alternator. Usual Limits in Per Cent. 
10 KVA 1.80 to 4.75 
Ae Wawa 135. ** 2:00 
LOO 758 100 ** 350 
200 * 15 **2 80 
300. ¢* .65 ‘* 2.50 
£00" ** 50 ** 2 20 
GOOV «5 50 ** 2 00 
AoO0r- 58 £5 ** 3 
1,000 ‘ 45 ** 2:25 
losses in an alternator. Table 13 gives the usual limits of 
excitation losses for generators of various sizes. 
The usual average values of excitation losses in terms of 
alternator capacity are indicated in Fig. 32. 
104. Exciters.—The normal output of an exciter must be some- 
what larger than the required energy for excitation, be- 
cause it has to supply the losses appearing between the 
brushes and slip rings besides the energy necessary for ex- 
citation. It is advisable to provide an exciter which is 
capable of giving an output of about 25% above that re- 
