H DIRECT CURRENT GENERATORS. [Exp. 



4. In a compound generator, the necessary increase of field 

 excitation with load is simply and effectively obtained by means 

 of an auxiliary series winding. Since the current in the series 

 winding is the load current, the magnetizing action of the series 

 winding (that is its ampere-turns or magnetomotive force) in- 

 creases in direct proportion to the load. This increases the mag- 

 netic flux and hence the generated voltage by an amount depend- 

 ent upon the degree of saturation of the iron. 



Looked at in another way, a shunt winding (which alone gives 

 a falling characteristic) and a series winding (which alone gives 

 a rising characteristic) are combined so as to give the desired 

 flat compounding or a certain degree of over-compounding. As 

 the shunt winding alone gives very nearly the desired charac- 

 teristic, the shunt is the principal winding, the series winding 

 being supplementary and of relatively few ampere-turns. 



The characteristic curves for a shunt or compound generator 

 may be classed as no-load characteristics, and load characteristics. 



PART I. NO-LOAD CHARACTERISTIC. 



5. There is one no-load characteristic, the saturation curve, 

 which shows the saturation of the iron for different field exci- 

 tations; for this the generator is usually self -excited but may be 

 separately excited when so desired. 



6. (a) No-load Saturation Curve.* This curve shows the 

 terminal voltage for different values of field current. 



7. Data. The machine is connected as a self -excited shunt 



voltage to rise; (2) when the voltage is too high, it momentarily removes 

 the short circuit, causing the voltage to fall. The voltage would be much 

 too high or too low, if the short circuit were permanently made or broken. 

 The short circuit is, however, rapidly made and broken and of a varying 

 duration, a nearly constant voltage being thus secured. It may be applied 

 directly to a generator (D.C. or A.C.) or to its exciter. It may be used 

 advantageously in connection with a compound winding, and may be 

 arranged so as to cause the voltage to rise with load in the same manner 

 as in an over compounded generator. 

 * Also called excitation characteristic, or internal shunt characteristic. 



