FLUX DISTRIBUTION 



139 



considered negative, and deducted from the area measured 

 above the datum line. 



The amount by which the area of the full-load flux curve C 

 must exceed the area of the open-circuit flux curve A may be 

 determined approximately by estimating the probable voltage 

 drop in the series winding (if any) and at the brush-contact 

 surfaces. In the case of compound-wound machines, it will 

 be known at the outset whether the dynamo is to be flat-com- 

 pounded or over-compounded. Over-compounding is resorted 

 to when the drop in the circuit fed by the machine is likely to 

 be high. The terminal voltage may then be 5 per cent., or even 

 10 per cent., higher at full load than on open circuit. The balance 

 of the e.m.f. to be developed at full load consists of: 



1. The IR drop in armature winding. 



2. The IR drop in series field (if any). 



3. The IR drop in interpole winding (if any). 



4. The IR drop at brush-contact surface. 



Item (1) can readily be calculated since the armature winding 

 has been designed. Item (2) may be estimated at from one- 

 fourth to one-half the armature drop. Item (3) may be esti- 

 mated at from one-fourth to one-half the armature drop. Item 

 (4) will be discussed later, but it may be estimated at 2 volts, 

 and is practically constant for machines of widely different 

 voltages and outputs. 



By totalling these items of internal loss of pressure, and 

 adding thereto the required difference between the terminal 

 volts at full load and at no load, the full-load developed voltage 

 E r is obtained, and the required area of curve C is therefore: 



E' 



Area of open-circuit flux curve A X 



E 



where E is the open-circuit terminal voltage as previously 

 defined. 



The ampere-turns necessary to produce the curve C of this 

 particular area will not be far short of the total ampere-turns 

 on the field coils at full load, because the air gap, teeth and slots 

 have considerably greater reluctance than the remainder of the 

 magnetic circuit. The extra ampere-turns required to over- 

 come the reluctance of the armature core, magnet limbs and 

 frame will be considered later when dealing with the magnetic 

 circuit as a whole and the field-magnet windings. 



