336 THE ROTARY CONVERTER. 



current supplied to the armature. The voltage and 

 frequency of the alternating supply should be kept 

 constant. 



The direct-current load circuit should then be closed,, 

 and a similar series of readings taken for one or two 

 values of the load current, which should be kept constant 

 during each complete series of readings. 



The results should be entered in tabular form, as 

 shown in the case of Experiment XLYL, page 322. 



Curves should be plotted showing excitation measured 

 horizontally, and primary voltage, primary current, 

 primary watts, and power factor vertically. 



Figs. 167, 168, and 169 give some curves obtained in 

 the manner just described on the same converter as that 

 already experimented upon. 



In Fig. 167 are shown the curves of alternating current 

 supplied, and power factor for no load and for a load of 

 4 amperes, equal to about a quarter full load. The smaller 

 the load on the converter, the greater is the current varia- 

 tion produced by a change in excitation, and the more 

 steep and pointed at the bottom does the curve become. 

 At heavy loads the current curve is comparatively Hat 

 and rounded, showing that the permissible variation of 

 excitation is then much greater. 



The power-factor curves show somewhat similar 

 features, although the curves are inverted, since an 

 increase of primary current corresponds to a decrease of 

 power factor when the load is maintained constant. The 

 no-load curve is very steep and pointed, whereas with 

 heavy loads the curves become flat or well rounded at the 

 top. When the converter is loaded, the power-factor 

 curves will generally rise to 1, as shown in Curve II. At 

 no load, on the other hand, it is not generally possible to 

 obtain a point when the machine works non-inductively. 

 This is due to the influence of harmonic variations in the 

 wave form of the current which occur under these 

 conditions. 



The similarity of the curves in Fig. 167 to those 

 already obtained with a synchronous motor is at once 

 apparent. As explained in connection with the motor 

 curves, the conditions to the left of the point of maximum 

 power factor correspond to -a lagging current, while points 

 to the right indicate leading currents in the supply 



