408 ELEMENTS OF ELECTRICAL ENGINEERING. 



as to keep the terminal voltage of the machine constant ; and let 

 corresponding values of armature current I a and of shunt field 

 current / be observed. The field excitation corresponding to any 

 particular value of / is NI t ampere-turns, where A 7 " is the number 

 of turns of wire in the shunt field winding. A curve may now be 

 plotted using observed values of / a as abscissas and corresponding 

 values of NI 8 as ordinates as shown in Fig. 42. This curve is 

 called the armature characteristic of the given generator. 



From this armature characteristic the number of turns of wire 

 required in the series field winding for flat-compounding may be 

 determined as follows : Consider the full load armature current 

 represented by Oc, Fig. 42, for which a total field excitation 

 ac is required. Of this total field excitation the shunt field 

 winding supplies the amount be, inasmuch as the terminal volt- 

 age of the generator is constant. Therefore the series field wind- 

 ing must supply the portion ab of the field excitation, and the 

 required number of series field turns may be found by dividing 

 the ampere- turns represented by ab by the current represented 

 by Oc (for the long-shunt compound generator). 



(b) Over-compounding. The following numerical example illus- 

 trates sufficiently the calculation of over-compounding. The shunt 

 field winding of a certain 12.1 kw. bipolar generator supplies the 

 4,000 ampere-turns of field excitation required to give a terminal 

 electromotive force of no volts at zero current output. When 

 the current output is 100 amperes, a field excitation of 5,800 

 ampere turns is required to give 121 volts between generator 

 terminals (10 per cent, over-compounding). On account of the 

 increase of terminal voltage from no-load to full load, the shunt 

 field winding supplies \\ -J- x 4,000 = 4,400 ampere turns at full 

 load. Therefore the series field winding must supply 1,400 am- 

 pere-turns, and the series winding must consist of 14 turns of wire. 



CHARACTERISTIC CURVES OF MOTORS. 



35, Experimental determination of speed-torque characteristic 

 of a motor. The characteristic curve of a generator is definite 



