72 PRINCIPLES OF ELECTRICAL DESIGN 



mechanically at a speed of N revolutions per minute through the 

 flux produced by the field poles. If $ stands for the amount of 

 flux entering or leaving the armature surface per pole, we may 

 write, 



<&pN 

 volts generated per conductor = -ins \/ AQ 



It is not at present necessary to discuss the different methods of 

 winding armatures, but let there be a total of Z conductors 

 counted on the face of the armature. Then, if the connections 

 of the individual coils are so made that there are p\ electrical 

 circuits in parallel in the armature, the generated volts will be 



_ *pNZ 

 = 60 X pi X 10 8 



This is the fundamental voltage equation for the dynamo; it 

 gives the average value of the e.m.f. developed in the armature 



conductors, and since the virtual and 

 average values are the same in the 

 case of continuous currents, the 

 formula gives the actual potential 

 difference as measured by a volt- 

 meter across the terminals when no 

 current is taken out of the armature. 

 Under loaded conditions, the e.m.f. 

 as calculated by formula (38) is the 

 terminal voltage plus the internal IR 

 FIG. 23. pressure drop. 



The expression "face conductors" 



may be used to define the conductors the number of which is 

 represented by Z in the voltage formula. It is evident that this 

 number includes not only the top conductors, but also those that 

 may be buried in the armature slots. The word " inductor" is 

 sometimes used in the place of "face conductor," and where 

 either word is used in the following pages it must be under- 

 stood to refer to the so-called "active" conductor lying parallel 

 to the axis of rotation whether on a smooth core or slotted 

 armature. 



19. The Output Formula. The part of the dynamo to be 

 designed first is the armature. After the preliminary dimensions 

 of the armature have been determined, it is a comparatively 



