i-B] COMPOUND GENERATOR. 25 



for full load (200 amperes) there are needed 2200 more ampere- 

 turns excitation than at no load. The series winding will be 

 traversed by the current of 200 amperes, and must accordingly 

 have ii turns in order to make the required 2,200 ampere-turns. 



If the armature characteristic were a straight line, the series 

 turns calculated as above would be the same for all loads 

 and the generator could be compounded so as to have perfect 

 regulation and give an exactly constant voltage at all loads. But 

 the armature characteristic always curves, bending more after 

 saturation is reached. The series turns are, therefore, calcu- 

 lated for one definite load (full load) ; for other loads the com- 

 pounding will be only approximately correct (21). 



The armature characteristic and hence the proper number of 

 series turns for correct compounding, will differ for different 

 speeds and terminal voltage, an interesting subject for further 

 investigation. 



APPENDIX I. 



MISCELLANEOUS NOTES. 



29. Determining the Number of Shunt Turns. The number of 

 shunt turns on a generator can be more or less accurately deter-, 

 mined, if the machine has a series winding or a temporary auxiliary 

 winding with a known number of turns. 



With the machine separately excited, take an ascending no-load 

 saturation curve, using the shunt field winding of unknown turns; 

 take a second similar curve, using the series or auxiliary field wind- 

 ing of known turns. A comparison of the two curves shows that 

 the shunt winding requires a much smaller current than does the 

 auxiliary winding to give the same generated armature voltage. De- 

 termine this ratio of currents for equal terminal voltage (found for 

 several voltages and averaged) and suppose it to be 1 : 40. The 

 shunt turns are then 40 times as many as the auxiliary turns, the 

 ampere-turns for equal terminal voltage being the same. If for 

 example the auxiliary turns are 10, the shunt turns are accordingly 

 400. 



30. The number of turns in two field windings can be compared 



