166 



ELECTRICAL ENGINEERING 



consumed in driving the flux across it, and its section and length 

 must be calculated very carefully. 



The section of the air gap can be taken as the area of the pole 

 face; from the drawing it is found to be 100 sq. ins. 



If the length of the air gap is taken as the radial length from 

 pole face to armature a large error will result because the average 

 length of the lines crossing the gap from pole to armature is greater 

 than the radial length as seen in Fig. 132. 



Pole 



Armature 

 FIG. 132. Flux in the air gap. 



The radial length must be multiplied by a constant greater than 

 unity in order to give the correct length. Carter has derived 

 values for this constant depending on the ratios of tooth width 

 to slot width and slot width to gap length. For machines of or- 

 dinary design the constant ranges from 1.1 to 1.2 and a value of 

 1.17 has been taken in the present case. Therefore the corrected 

 length of the air gap is 0.188 X 1.17 = 0.2014 in. under each pole 

 and for a pair of poles is 0.4028 in. 



The flux crossing the gap corresponding to 150 volts is 6,300,000 



6 300 000 



lines and the flux density is - - = 63,000 lines per square inch. 



J.UU 



The number of ampere turns required to drive a flux density 

 of B lines per square inch through a length of one inch in air is 

 found as follows: 



If 9& = lines per square centimeter, I cm. = the length of the 

 path and nl = the ampere turns required, then 



but 

 therefore, 



B 



(2.54)' 



/ 



I = 1 in. = 2.54 cm. and fi = 1; 



B 



(2.54) 



0.47m/ 

 2.54 



