316 PRINCIPLES OF ELECTRICAL DESIGN 



The field ampere turns necessary to produce the terminal 

 voltage OE t of the vector diagram Fig. 133 are made up of the 

 ampere turns for air gap and armature teeth, represented by the 

 maximum ordinate of the curve F of Fig. 131, together with the 

 ampere turns required to overcome the reluctance of the pole- 

 core, yoke ring, and armature core. These additional ampere 

 turns are readily calculated because the total useful flux per 

 pole is known, being represented by the area of the curve C of 

 Fig. 132. 



Having determined the total ampere turns per pole which are 

 necessary to give OE t volts (of Fig. 133) at the terminals, it is 

 easy to read the corresponding open-circuit voltage from the 

 no-load saturation curve of the machine. In this manner the 

 regulation corresponding to a known external power factor, cos 

 6, can be calculated with greater accuracy than will usually be 

 obtained by the method outlined in Art. 108, and illustrated 

 by Figs. 129 and 130. 



The meaning of the other quantities in Fig. 133 may be summed 

 up as follows: 



The angle E OE f g , or a, is the phase difference between equiva- 

 lent sine-waves representing open-circuit voltage and " apparent J> 

 developed voltage under load conditions. It is the result of 

 flux distortion due to the armature cross-magnetizing ampere- 

 turns. The vector OE g gives the r.m.s. value of the voltage per 

 phase actually developed in the armature winding by the cutting 

 of the flux linking with the "active" conductors. 



The angle E OI a or ^ is the internal power-factor angle. The 

 difference in length between OE and OE' g is the voltage drop 

 due to armature demagnetization and distortion. The point 

 Eo is shown in Fig. 133 on PE' produced, but it does not neces- 

 sarily fall on this straight line, and so indicates one important 

 difference between the construction of Fig. 133 and that of 

 Fig. 130, in which the assumptions made are not universally 

 applicable. 



The use of vectors and vector constructions, such as were first 

 described, will usually give sufficiently accurate results without 

 the expenditure of time and labor involved in the plotting of 

 flux curves and e.m.f. waves. It is in the case of abnormal 

 designs, or when the conditions are unusual, that the problem 

 of regulation may be studied most conveniently and correctly 

 by a method such as that here described, which is subject 



