238 ALTERNATORS. 



The readings obtained in the case of a 6 kw. 3-phase 

 generator with rotating field when loaded on one 

 phase only are plotted in Fig. 113. The voltage was 

 kept constant at 95, and the speed regulated to give 

 a periodicity of 40 </>. The machine employed was the 

 same as that for which the magnetisation curves, Figs. 92 

 and 94, and the characteristic; Fig. 100, were drawn. On 

 referring to the magnetisation curves, it will be seen that the 

 lowest excitation employed in the present experiment, viz., 

 1-33 amps., is well above the knee of the magnetisation curve, 

 and that the ratio of increase in voltage to increase of excita- 

 tion is fairly constant. Consequently, if the characteristic 

 were straight, indicating a uniform drop of voltage as the 

 load increased, the curve in Fig. 1 13 would also be practically 

 a straight line. The characteristic in Fig. 100 begins, however, 

 to bend downwards at about 20 amps., and consequently 

 with currents above this value the excitation must increase 

 more rapidly in order to keep up the voltage to its original 

 value. This is indicated by the upward bend in the excita- 

 tion curve, Fig. 100, occurring at 20 amps. 



A curve of the kind just described shows the range which a 

 field-regulating resistance must have in order to maintain 

 constant voltage, and also shows what variation of resistance 

 will be required for any given variation of load which may be 

 contemplated. 



In America alternators have frequently been arranged 

 with a compound winding in the same way as direct-current 

 dynamos In this case usually a portion of the current is 

 rectified -and passed through a series winding which adds its 

 magnetising effects to those due to the ordinary separately- 

 excited field windings. In such cases, the curve under 

 consideration would give the required information for deter- 

 mining the number of series windings necessary to maintain 

 an approximately uniform voltage. For such a purpose 

 the number of amperes excitation obtained from the curve 

 must be multiplied by the number of turns through which 

 it flows in order to give the total number of ampere-turns 

 required. The difference between this total number and 

 the number supplied by the separately excited winding 

 gives at once the ampere-turns to be provided by the series 

 winding. 



In making use of the curves such as that shown in Fig. 113, 

 it must be remembered that a different curve showing a 



