ALTERNATE CURRENT GENERATOR. 87 



23. In designing the field of an alternator, attention should be 

 given to the fact that the conductors have to carry not only the 

 exciting current but also the induced field current, which, as we have 

 seen, may at full load attain a relativelv large value. In addition, it 

 should not be forgotten that in the field magnet cores there is the 

 associated alternating flux which causes some additional heat. 



It is well known that in a case of excessive heating in the field 

 reduction of the heating is eifected by the employment of heavy closed 

 copper conductors, called dampers, embracing the field magnet poles. 



To explain this action let us consider a two-pole machine on each 

 field pole of which is a damper. 



jS'eglecting magnetic leakage and iron loss, if 'Q be the current in 

 •each damper, the magnetic flux through the armature windings is — 

 (j{nx -!- (i^t +2^) cos Mt\ 



and that through the field windings and the dampers is — 



g{vi-\- 'It, 4~ nx cos Mt\ 



so that the equations connecting x, $, and ^ are — 



rx -\- gn '— \ nx -\- {v^ -|- 2^) cos oit f = 



(I.) pf + gv ^- I i/^ + 2^ + nx cos uit r = rj 



zC+g ^^ \ vt + 2C + nx cos wt 1 = 



where z is the resistance of each damper and the other symbols have 

 the same significations as in the previous sections of this paper. 



There is no constant term in 4, and considering only the variable 

 terms (harmonics) in ^ we see at once that — 



vzC = p^ 

 from which it follows that — 



v$^2i: = v$(l + K) =:v^'say 

 where — k = ^ 



and that — 



2zc = Kpe. 



The first two of equations I. may now be written — 



rx + an -, ) nx -\- v i' cos o)t f = 0. 

 at I ) 



P J., . d ( ^ , ) 7] 



TT^J +^v- [vt'-i-nxcos.ot^ =^ 



7} being divided by 1 + k as the constant term in ^', is equal to the 

 constant term in $, that is to 17/p (^ having no constant term). 



Now, X and ^' determined from these equations will be very 

 approximately the same as x and | determined from the equations in 

 section 1 for the alternator without dampers, for a^ the given vector 

 is the same for both, as are all the t operators. The t operators differ 

 in p/1 + K being substituted for p, but in section 8, and in notes, 



