234 



PRINCIPLES OF ELECTRICAL DESIGN 



the load (and developed voltage), but for practical purposes may 

 be assumed constant. 

 The constant losses are made up as follows: 



Windage and bearing friction = 0.018 X 75,000 = 1,350 watts. 



Brush friction (item (113)) = 324 watts. 



Iron loss in core and teeth (item 60) = 1,644 watts. 



Total = 3,318 watts. 



The full-load current in the armature is the line current plus 

 the shunt current (item (136)), and since the armature resistance 



i.o 



20 40 60 80 100 



Output _ ( Percentage of Full Load ) 



FIG. 88. Efficiency curve. 



120 



140 



is known (item (44)), the PR loss in the armature at different loads 

 can readily be calculated. 



The brush-contact PR loss is obtained by referring to Fig. 69 

 (page 179) and adding 25 per cent, to the calculated losses. 



We shall assume the "long shunt " connection, which means 

 that the series winding and diverter will, together, carry the full 

 armature current, and the series field PR losses will therefore be 

 directly proportional to the copper losses in the armature. 



Fig. 88 is the efficiency curve plotted from the figures in the 

 table. The full-load .efficiency is 0.915, and judging by the shape 



