352 PRINCIPLES OF ELECTRICAL DESIGN 



made, we may provide a number of spaces between the stator 

 iron and the casing to allow of air being passed over the outside of 

 the armature core. Let us suppose that there are twelve such 

 ducts, each 10 in. wide by 1 in. deep; the total cross-section of 

 the air ducts is then made up as follows: 



Outside stator stampings 12 X 10 XI = 120 sq. in 



Holes punched in stator stampings (Fig. 136) = 590 



Spaces above wedge in stator slots (Fig. 135) 



48 X 1.5 X 1 = 72 



Clearance between stator and rotor % X v X 39> = 108 

 Spaces in rotor forging below slots 32 X 1.94 = 62 



Total. . = 952 



sq. in. 



or 6.6 sq. ft. 



The total losses in the machine, without including windage and 

 sundry small losses, are 



Total core loss (item (32)) 120 kw. 



Stator I 2 R loss (item (36)) 21 kw. 



Rotor PR loss 66 kw. 



Total 207 kw. 



If we allow 100 cu. ft. of air per minute for each kilowatt dissi- 

 pated (see Art. 33, Chap. VI), it will be necessary to pass 20,700 

 cu. ft. of air through the machine per minute. This makes the 



20 700 

 velocity in the vent ducts ^ ^ = 3,140 ft. per minute, which 



is well below the permissible limit. 



Item (70). With varying degrees of tooth saturation espe- 

 cially when, as in this design, all the teeth are not of the same 

 cross-section the only correct method of predetermining the 

 open-circuit saturation curve (similar to Fig. 124, Art. 104), is to 

 plot the flux distribution for different values of the exciting 

 ampere-turns, and calculate the e.m.f. developed in each case. 

 It is not necessary to calculate a large number of values in this 

 manner; two or three points taken with fairly high values of the 

 exciting current will show how the tooth saturation affects the 

 resulting flux; and a curve can be drawn connecting the known 

 straight part of the saturation curve with these ascertained values 

 for the higher densities. 



The saturation curve for zero power factor can be drawn as 

 explained in Art. 106 (Fig. 125), and the construction of Figs. 



