228 ALTERNATORS. 



For the purposes of the present experiment it was much 

 more convenient to employ motors, since the amount of lead 

 of the current could be readily adjusted by regulating the 

 motor excitation, and the amount of current taken from the 

 alternator could be easily varied by loading the motor by 

 means of a brake upon its pulley. 



In order to obtain complete curves for several values of the 

 power-factor, the same series of readings were taken as in the 

 case of the inductive characteristics. It was comparatively 

 easy to adjust the motor load and excitation to give a number 

 of readings at different power factors at a fixed current, 

 whereas it would not have been easy to take a series of readings 

 at a constant power-factor, without employing special 

 instruments, which were not available. 



A number of readings were taken with different conditions 

 of load and excitation at each of the following values of the 

 current, 10-65, 8-6, and 6-4 amps. From the readings the 

 curves comparing power-factor and voltage shown in Fig. 106 

 were plotted. The general shape of these curves is similar to 

 that of those shown in Fig. 105 ; but in this case they cross 

 each other. This is because, with a current leading only 

 very slightly, a high current produces nearly the same effect as 

 the current in a non-inductive circuit, i.e., it produces a 

 drop in voltage. The same current, when leading by a large 

 angle, however, produces a considerable rise in voltage, so 

 that the highest current produces the greatest drop at high 

 power-factor and the greatest rise at low power-factor, and 

 consequently cuts across the curve of a smaller current 

 which- is more nearly vertical. The curve corresponding 

 to zero current would, of course, be a vertical line showing 

 neither increase nor decrease in voltage in the present case 

 it would correspond to the ordinate of 100 volts. 



It is interesting in comparing the curves in Figs.. 105 and 

 106, to observe that the curves in Fig. 105 appear to show that 

 a further decrease in power-factor below about -6 does not 

 affect the voltage, while in the case of the leading current 

 curves in Fig. 106, there does not appear to be a limit to the 

 value of the power-factor which continues to change the 

 alternator voltage. 



One feature which will be noticed about the, characteristics 

 for lagging or leading currents is that the distance between 

 the curves for cos 0=1-0 and cos <t> - -9, is much greater 

 than that between the curves for cos ^ = -9 and cos </> = -8. 



