170 THE MAGNETIC CIRCUIT [ART. 53 



that is, the flux corresponding to the excitation O a at no-load is 

 the same as the flux corresponding to the excitation O a ' when tlu> 

 machine is loaded. The problem is then, knowing the point a and 

 the distance cd =c'd' to find the point a' such that equation (96) 

 is satisfied. The two areas have the common part c'g'bfd, and 

 the parts cee'c' and dff'd' are equal to each other. Therefore, 

 eq. (96) is satisfied if the 



area be'g' = area bf'h' (97) 



The position of the point a' is found either by trials, or as the 

 intersection of the curves rr r and ss' ; the ordinates of the curve rr' 

 represent the area be'g( for various assumed positions of the point 

 a', and the ordinates of the curve ss' represent the corresponding 

 values of the area bf'h'. 



Thus, the total field ampere-turns required for a direct-current 

 generator under load are found as follows: (a) To the specified 

 terminal voltage add the voltage drop in the armature, under the 

 brushes, and in the windings (if any) which are in series with the 

 armature, viz. ; the series field winding, the compensating winding, 

 and the interpole winding. This will give the induced voltage E. 

 (b) From the no-load saturation curve find the excitation corre- 

 sponding to E. (c) Estimate the amount of the brush shift (if 

 any) and calculate the corresponding demagnetizing ampere-turns 

 according to eq. (94) . (d) Determine the ampere-turns aa' (Fig. 43) 

 required for the compensation of the armature distortion, (e) 

 Add the ampere-turns calculated under (b), (c) and (d). In the 

 case of a motor subtract the voltage drop in the machine from 

 the terminal voltage, to find the induced e.m.f. E, but otherwise 

 proceed as before. 



If the machine is shunt-wound or series-wound, the field wind- 

 ing is designed so as to provide the necessary maximum number of 

 ampere-turns at a required margin in the field rheostat for the 

 specified voltage or speed variations. When the machine is com- 

 pound wound, the shunt winding alone must supply the required 

 number of ampere-turns at no-load. The series ampere-turns is, 

 then, the difference between the total m.m.f. required at full-load 

 and that supplied by the shunt-field. When a generator is over- 

 compounded, the shunt excitation is larger at full load than it is at 

 no load, and allowance must be made for this fact. 



