138 PRINCIPLES OF ELECTRICAL DESIGN 



The added field ampere-turns may be thought of as compensat- 

 ing for two distinct effects: 



1. The loss of pressure due to armature reaction. 



2. The loss of pressure due to IR drop in armature, brush 

 contacts, and series field windings. 



The correction for (1) consists in bringing the total flux up 

 to its value on open circuit. It is the ampere-turns necessary to 

 raise the air-gap density from its average value over curve B to 

 its average value over curve A. An approximate method of mak- 

 ing this correction is to find on curve (a) of Fig. 49 (i.e., the curve 

 corresponding to point a at center of pole) the ampere-turns 

 PbP a required to produce the difference of flux density BbB a 

 when OBb = average flux density over curve B, and OB a 

 average flux density over curve A. The correction for (2) 

 consists in increasing the flux per pole to such an extent that it 

 will generate the increased voltage. This is the correction for 

 compounding; it compensates for all internal loss of pressure, 

 and must include over-compounding if this is called for. 



If E f = required full-load developed volts, and 



E = open-circuit terminal volts (being also the no-load 

 developed volts), then, 



Area of flux curve C _ E^ 

 Area of flux curve A ~~ E 



In this manner the required area of curve C is obtained. The 

 average flux density must be increased in this proportion, and 

 the necessary additional ampere-turns for this correction are 

 arrived at approximately by making PaP c (as indicated on 

 Fig. 49) such as to increase the air-gap density in the ratio of 

 E' to E. 1 



The area of the final curve C, which should be measured for 

 the purpose of checking with the required area, is that com- 

 prised between the points on the armature surface which corre- 

 spond with the position of the brushes on the commutator, 



portions of the flux curve measured below the datum line being 



i 



1 The method followed in the numerical example (p. 222) is generally 

 as here indicated except that the additional field ampere turns are calcu- 

 lated on the assumption that it is the maximum value, and not the average 

 value of the open circuit flux curve, A, that has to be increased in a given 

 proportion. The latter method is recommended as giving more nearly 

 correct results in the case of continuous current designs with high values of 

 tooth density. 





