228 



PRINCIPLES OF ELECTRICAL DESIGN 



formula (91), the length of winding space should be something 



greater than 



6,000 

 875 



6.85 in. Let us make the cylindrical pole 



core 7 in. long, which will determine the inside diameter of the 

 yoke ring. This will have to be about 38 in. as shown in Fig. 86. 

 Items (123) to (127): Dimensions of Yoke Ring. Assuming a 

 density of 15,000 gausses in the cast-steel yoke ring, the cross- 

 section will be 



6,970,000 X 1.2 

 2 X 15,000 X 6.45 



43.2 sq. in. 



The dimensions can now be determined, and fiie lengths of the 

 flux paths obtained from Fig. 86. 



FIG. 86. Magnetic circuit of four-pole dynamo. 



Item (128): Open-circuit Saturation Curve. Refer Chap. IX, 

 Arts. 55, 56, and 57. Also Art. 16 of Chap. III. The calcula- 

 tions of the total ampere-turns on each pole of the machine, to 

 develop on open circuit a given voltage, are shown in the accom- 

 panying table. Suitable values of terminal voltage are selected 

 to obtain points on the saturation curve. One of the values 

 should be slightly higher than the developed e.m.f. under full- 

 load conditions. It is not necessary to make the calculations 

 for very low voltages because the reluctance of the iron parts 

 of the magnetic circuit is then negligible. For each selected 

 value of the developed e.m.f., the ampere-turns for the com- 

 plete magnetic circuit are calculated exactly as explained in 



