24 DIRECT CURRENT GENERATORS. [Exp. 



(amperes or ampere-turns*) necessary at different loads to main- 

 tain a constant voltage at the terminals of a shunt generator 

 driven at constant speed.f The connections are shown in Fig. 3, 

 readings being taken of field current, line current, terminal 

 voltage and speed. Separate excitation may be used when a 

 higher excitation is wanted than can be obtained by self-excita- 

 tion. The load current is increased from no load to about 25 

 per cent, overload. At each load, before readings are taken, the 

 voltage is brought to the desired constant^ value by adjusting 

 the field rheostat. 



27. The rise in the armature characteristic shows the in- 

 crease in ampere-turns of excitation needed to compensate for 

 loss in voltage due to resistance drop, armature reactions, etc. 

 (16). 



If in service the machine is to be operated as a shunt gene- 

 rator, this increase in excitation can be obtained by adjusting 

 the field rheostat as was done in obtaining this curve. 



If, however, the machine is to be operated as a compound gen- 

 erator, this increase in excitation is to be obtained by the ampere- 

 turns of the series winding. 



28. Determination of Proper Number of Series Turns. We 

 know from the armature characteristic the additional ampere- 

 turns of excitation which must be provided at full load to pro- 

 duce the desired terminal voltage. We know also the amperes 

 (load current) which will flow through these turns at full load. 

 The necessary number of turns is accordingly readily found by 

 dividing ampere-turns by amperes. Thus in Fig. 7, we note that 



*To plot in ampere-turns, the number of turns in the shunt field must 

 be known; see Appendix I. The number of turns multiplied by field 

 current gives the number of field ampere-turns. 



t (26a). In case the generator is to be normally driven by an induc- 

 tion motor, with speed decreasing with load, it should be so operated in 

 taking the armature characteristic. (See i6a, 2ob.) 



$ (26b). The curve may be taken for a voltage which increases with 

 load; such a curve would show the series ampere-turns to be added for 

 over-compounding. 



