210 PRINCIPLES OF ELECTRICAL DESIGN 



65.3 in., and by formula (21) page 36, the resistance at about 

 60C. will be 



R = - r = 0.00132 ohm. 



0.039 X 10 6 X - 



7T 



There are Z/2 or 171 turns in the armature winding, and there- 

 fore 171/4 = 42.75 turns in series in each armature circuit. 

 The value of item (43) is therefore 42.75 X 0.00132 = 0.0564 

 ohm; and of item (44), one-quarter of this amount, or 0.0141 

 ohm. The IR drop in armature winding is 0.0564 X 83.4 = 4.7 

 volts, or 2.04 per cent, of the full-load terminal voltage. This 

 compares favorably with the approximate figures given on 

 page 99. 



Item (46) : Pressure Drop in Series Winding. Refer Art. (43) 

 page 139. We may assume this voltage drop to be one-third 

 of 4.7 or, say, 1.6 volts. 



Item (48): Pressure Drop at Brushes. Refer Art. 53, page 

 179. Assume two volts. 



Items (49) and (50) : Watts Lost in Armature Windings. Total 

 PR = El = 4.7 (83.4 X 4) = 1,570 watts. Item (50) is the 

 portion of this total loss which occurs in the "active" copper 

 of the armature; its value is, 



1570 X 0.337 = 530 watts, 



wherein the factor 0.337 is item (41) of the design sheets. 



Item (51) : Flux Entering Armature at Full Load. Refer Art. 

 43, page 135. 



The volts to be developed at full load are, 



230 + 4.7 + 1.6 + 2 = 238.3 

 The full-load flux must therefore be, 



OOQ O 



6,430,000 X -- = 6,970,000 maxwells. ' 



Items (52) to (55): Flux Density in Armature Core. Internal 

 Diameter. Usual" flux densities for different frequencies are 

 given in the table in Art. 32 (page 104). A density of 14,000 

 gausses would be satisfactory; but since the losses in the teeth 

 are likely to be below the average because a 1-in. depth of 

 slot is small for a machine of this size a density of 15,000 may 



