326 SYNCHRpNOUS MACHINES. [Exp. 



around it) the converter may be over-, under-, or flat-compounded 

 in the same manner as a D.C. generator ( I, 23, Exp. i-B). 



12. Derived Neutral. An interesting feature of a converter 

 or double-current generator is that the potentials of the neutral 

 of the A.C. system and of the D.C. system are alike. 



The neutral may be considered as a (usually fictitious) middle 

 point of the armature. Taking this neutral potential as zero, the 

 positive D.C. brush is at a constant positive potential and the 

 negative D.C. brush is at a constant negative potential, the mean 

 of these being the potential of the neutral. Referring to a single- 

 phase machine, the two A.C. brushes have alternating potentials 

 that are equal but of opposite sign, the mean of which is the 

 potential of the neutral. 



The A.C. neutral is readily obtained from a tap in the middle 

 of a transformer or choking coil across the A.C. lines of a single- 

 phase system, or opposite lines of a 2-phase or 6-phase system. 



The D.C. neutral, otherwise not easily obtained, can be readily 

 "derived" from the A.C. neutral, i. e., the neutral of the A.C. 

 side is used as the neutral for the D.C. system. This is one 

 advantage of a 2-phase or 6-phase converter with diametral con- 

 nections (27, Exp. 6-A). 



13. Direct-current generators are often constructed with A.C. 

 collector rings across which are placed choking coils for the pur- 

 pose of deriving a neutral for a 3-wire D.C. system. Any direct 

 current returned to the generator by the neutral or third wire, due 

 to an unbalanced load, passes into the middle tap of such a chok- 

 ing coil and thence differentially through the two halves of the 

 coil with no magnetizing effect upon the core. 



Special 3^wire generators are constructed with the choking coils 

 contained within the armature, the outside connections being made 

 through the commutator and one slip ring. 



