232 PRINCIPLES OF ELECTRICAL DESIGN 



preferably near the pole shoe. The space available in a radial 

 direction is about 7 5K = l/^ in. The number of turns 



per pole is -ss^- = 5.06. Let us put 5J^ turns on each pole, and 



make the final adjustment by means of a diverter. The current 



through the series winding will therefore be 1 , = 300 amp. 



o.o 



The total depth of winding might be about the same as for the 

 shunt coils, i.e., 2 in. The mean length per turn would then be 

 38.5 in., and the total length, 4 X 5.5 X 38.5 = 847 or, with an 

 allowance for connections, say, 890 in. Assuming a current 

 density of 1,200 amp. per square inch, the cross-section would be 



300 



0.25 sq. in. This winding may consist of flat copper 



strip wound on edge, or of any other shape of conductor of this 

 cross-section. If preferred, two or more conductors of some stock 

 size can be connected in parallel to make up a total cross-section 

 of about 0.25 sq. in. The space available is more than sufficient, 

 and we shall assume for the present that the cross-section is 

 exactly 0.25 sq. in., or 318,000 circular mils. The resistance, at 



890 

 60C., will then be 01Q ^^ = 0.0028 ohm. The drop in volts in 



the series winding is therefore 0.0028 X 300 = 0.84, which, being 

 very small, may be increased if it is found that the temperature 

 rise is appreciably below the specified limit. 



Items (143) and (144): Temperature Rise of Field Coils. Refer 

 Art. 59, Chap. IX. The area of the two cylindrical surfaces is 

 approximately 7 X ?r(10 + 14) = 528 sq. in. The area of the 



,jP _ 2 _ 2 



two ends is 2 X T (14 10 ) = 351 sq. in. The total cooling 



surface of all the field windings is therefore 679 X 4 = 2,720 sq. 

 in., approximately. 



The PR loss at full load in the shunt winding is (4.56) 2 X 40.5 

 = 843 watts; and the I 2 R loss in the series coils is (300) 2 X 

 0.0028 = 252 watts, making a total loss of 1,095 watts. 



The cooling coefficient, as given by curve A of Fig. 76 (page 

 194), is 0.009, and the temperature rise will therefore be 



1,095 



0.009 X 2,720 



= 44.8C. 



This is a little higher than the specified limit of 40, and if the 

 cooling coefficient could be relied upon for the accurate prede- 



