332 PRINCIPLES OF ELECTRICAL DESIGN 



determined by making a drawing of the armature coils and care- 

 fully measuring the length required. Since this design is being 

 worked out for the purpose of illustration only, we shall use the 

 formula (97) of page 261, and assume the length per turn of 

 armature winding (item (33)) to be 



(2 X 51) + (2.5 X 31.42) + (2 X 6.6) + 6 = 199.8 in. 



It will be safer to use the figure 210 in. for this mean length ; 

 because all the coils will probably be bent back and secured in 

 position by insulated clamps in order to resist the mechanical 

 forces which tend to displace or bend the coils when a short- 

 circuit occurs. 



The cross-section of the conductor (four strips in parallel) is 

 0.35 sq. in., or 445,000 circular mils. The number of turns per 

 phase is 24, and the resistance per phase at 60C. is, by formula 



210 "y 24 

 (21), page 36, 445 Q OQ = 0.01135 ohm. The IR drop per 



phase (item (35)) is 0.01135 X 700 = 7.95, or (say) 8.4 volts in 

 order to include the effect of eddy currents in the conductors. 

 The PR loss in armature winding (item 36) is 3 X 0.01135 X 

 (700) 2 = 16,700 watts, which should be increased about 25 to 30 

 per cent, (see Art. Ill, Chap. XIV) to cover sundry indeterminate 

 load losses. The total full-load armature copper loss may there- 

 fore be estimated at 21 kw., or 0.26 per cent, of the rated full- 

 load k.v.a. output; which is about what this loss usually amounts 

 to in a turbo-generator of 8,000 k.v.a. capacity. 



Items (37) and (38). The reactive voltage drop per phase due 

 to the cutting of the end flux cannot be predetermined accurately; 

 but we may use the empirical formula (99) page 266, wherein the 

 symbols have the following numerical values. 



The constant k will be fairly high in turbo-alternators, and we 

 shall assume the value k = 1.5. For the other symbols we have: 



/= 60 



p= 4 

 T< = 3 

 n, = 4 

 I c = 700 



In regard to l e and Z', the mean length per turn (item (33)) was 

 assumed to be 210 in. The length l e is therefore 210 - 2l a or, 

 1. = (210 - 102) X 2.54 = 275 cm. 



