PROCEDURE IN DESIGN OF D.C. GENERATOR 231 



demagnetization and distortion. This correction, amounting to 

 400 ampere-turns, is obtained from Fig. 82, where this number 

 of ampere-turns is seen to be necessary to raise the flux density 

 under the center of the pole from 7,260 to 7,800 gausses, as 

 explained on page 222. 



Items (133) to (137) : Shunt Field Winding. Refer Art. 58, 

 Chap. IX, and Art. 10, Chap. II. The length of winding space 

 for the shunt may be determined by dividing the total length 

 available for the windings in the proportion of the ampere- 



turns in shunt and series coils respectively. The length of the 



t\ *?nn 

 cylindrical core is 7 in., and 7 X ^o^r\ 5.53. Some allowance 



should be made for external insulation, and the net length of 

 winding space for the shunt coils might be, say, 5 in. Let us 

 assume the total thickness of winding to be 2 in. The inside 

 diameter of the winding might be 10^ i n -> making the average 

 diameter 12)4 in., and the mean length per turn, 38.5 in. We 

 shall suppose that the shunt rheostat absorbs 15 per cent, of 

 the voltage on open circuit; which leaves 187 volts across the 

 shunt winding. By formula (26) on page 42, we have: 



38.5 X 5,930 X 4 

 (m) = - ^ -- = 4,880 



Referring to the wire table on page 34, the standard size of 

 wire of cross-section nearest to this calculated value is No. 

 13 B. & S. gage. This can be used if the rheostat is arranged to 

 reduce the voltage across the winding in the proper proportion. 

 The number of turns per inch is 11.8, from which it is seen that 

 1,360 turns can be wound in the space available. 

 The resistance of all the four coils in series is 



38.5 X 1,360 X 4 X 2.328 



t 6 - 



12 X 1,000 ' 



5 930 

 The current, under open-circuit conditions, is T^ogTj = 4-36 



230 

 amp., and at full load (item (136)) it is 4.36 X 220= 4.56 am P- 



This is only 1.4 per cent, of the line current; a low value, which 

 might perhaps be increased in order to reduce the amount of 

 copper in the field coils if the temperature rise is not excessive. 

 Items (139) to (142): Series Field Coils. Refer Art. 58, page 

 192. The series turns may be placed at either end of the pole, 



