266 PRINCIPLES OF ELECTRICAL DESIGN 



(98), the voltage component developed per phase winding by the 

 cutting of the end flux is 



E. = (2.22k)fpT,H e 75 logic (12n/) X L X 10~ 8 (99) 



This quantity is usually referred to as the reactive voltage 

 drop per phase due to the inductance of the end connections; 

 it appears as the vector PE g in Figs. 99 and 100. If the mul- 

 tiplier (2.22k) be taken as 2.4, the formula agrees well with 

 the average of tests on machines of normal design. 



88. Total Losses to be Radiated from Armature Core. The 

 losses in the iron stampings teeth and core are calculated as 

 explained in Chap. VI (Art. 31). The flux to be carried at full 

 load by the core below the teeth is that which will develop the 

 necessary e.m.f. as obtained from the vector construction of 

 Fig. 100. The radial depth of the armature stampings is cal- 

 culated by assuming a reasonable flux density in the iron. This 

 will usually be between 7,000 and 8,500 gausses in 60-cycle 

 machines, increasing to 10,000 or even 11,000 in 25-cycle 

 generators. 



The permissible density in the teeth, as previously mentioned, 

 rarely exceeds 16,000 gausses at 60 cycles and 18,000 gausses at 

 25 cycles. Higher densities may have to be used occasionally, 

 but special attention must then be paid to the methods of cool- 

 ing, in order to avoid excessive temperatures. The tooth density 

 being appreciably lower than in D.C. machines, the apparent 

 flux density at the middle of the tooth may be used for estimating 

 the watts lost per pound. The maximum value of the tooth 

 density will depend upon the maximum value of the air-gap 

 density, and this, in turn, is modified by armature distortion and 

 slot leakage. The flux that must enter the core and be cut by 

 the armature inductors is known, but the amount of flux enter- 

 ing the teeth under each pole face is greater, since it includes the 

 slot leakage flux in the neutral zone, the amount of which de- 

 pends not only upon the current in the armature, but also upon 

 its phase displacement, i.e., upon the power factor of the load. 

 Then, again, the maximum value of the air-gap flux density de- 

 pends not only upon the average density, but also on the shape 

 of the flux distribution over the pole pitch. It will not be 

 necessary to go into details of this nature for the purpose of 

 estimating the temperature rise of the armature, and a sinusoidal 



