174 PRINCIPLES OF ELECTRICAL DESIGN 



which is simply formula (85) with the armature flux per centi- 

 meter of length expressed as BW a instead of <J> . The axial 

 length lp of the interpole face can, therefore, be determined if a 

 suitable value for the average air-gap density under full-load con- 

 ditions is assumed. Let B p stand for this value; then 



B p l p W a = $ e + * M la + BW a (l a - l p ) 



whence 



j _ $e + *Ja + BWglg ( . 



W a (B p + B) 



If the flux densities are expressed in gausses, the dimensions must 

 be in centimeters, and if B p is taken as 4,500 gausses, the length 

 IP is found, by formula (86), to be 12.05 cm. or, say, 5 in. 

 The total flux in the interpole air gap at full load is then, 



$ c = 64,800 + 47,000 + 1,400 X 6.45 X 1.375(11 - 5) 

 = 186,400 maxwells. 



Assuming a leakage factor of 1.9 and a cross-section under inter- 

 pole winding of (5 X 1H) S Q- m -> the full-load density in the 

 core of the interpole would be 7,300 gausses. 



Calculation of Ampere-turns Required on Interpole. Referring 

 to Fig. 65, it will be seen that when the coil A is about to be short- 

 circuited, the interpole flux enters the armature through the 

 teeth 1 and 2. At the end of commutation this flux enters the 

 teeth 3 and 4. The permeance of the air gap of length 6 may 

 vary slightly with the change in the position of the armature 

 teeth; it may be calculated by any of the approximate methods. 1 

 Assuming an actual air gap Y in. long, the equivalent air gap 

 might be 0.3 in. The full-load ampere-turns required to over- 

 come air-gap reluctance will therefore be 

 0.3 X 2.54 X 4,500 



0.47T 2 ' 7 " 



To this must be added the ampere-turns to oppose the armature 

 m.m.f. If the reduction of current in the short-circuited coils is 

 neglected, the armature ampere-turns per pole are 



IZL = (120 X 8) X 76 

 2 p 2X6 



= 6,080 



If we neglect the very small m.m.f. required to overcome the re- 

 luctance of the interpole core, the total ampere-turns on each 

 1 See Art. 36, p. 115. 



