DIRECT-CURRENT MACHINERY 



165 



length of the mean flux line through it and can best be obtained 

 from the drawing; it is in this machine 30 ins. 



FIG. 131. 



The flux passing through the yoke is one half .of that in the pole 

 and for a terminal voltage of 150 volts, it is 



7,560,000 



0700000- 

 = o,7SO,000; 



the flux density in the yoke is 



B y = -- = - - = 47,250 lines per square inch. 



-Ay OU 



From the permeability curve for cast iron (Fig. 59) it is found 

 that 100 ampere turns are required to drive this flux density 

 through one inch length of the yoke, therefore the number of am- 

 pere turns required per pair of poles for the yoke is 30 X 100 = 

 3000. 



The field poles are made of sheet steel and are of rectangular 

 section, 11 ins. wide by 7.75 ins. deep along the shaft; the section is 

 11 X 7.75 = 85.25 square inches; the length of the path through 

 one pole is 9.3 ins. and per pair of poles is 18.6 ins. 



The flux density in the pole for 150 volts is 



7 ^fiO 000 

 '" 



= 88,800 



lines per square inch; from the curve for sheet steel (Fig. 59) it is 

 found that 28 ampere turns per inch are required and 18.6 X 28 

 = 520 ampere turns per pair of poles. 



The increased section of the pole face is neglected. 



The air gap is the most important section in the magnetic cir- 

 cuit because the largest part of the field magnetomotive force is 



