540 



ELECTRICAL MEASUREMENTS 



It is convenient to calculate and plot once for all a curve such 

 as is shown in Fig. 323. 



The ordinates of the curve are power factors or values of cos 6; 

 the abscissae are values of the ratio 

 Smaller reading 

 Larger reading 



cos (6 + 30) _ 



\f\o\ -K . 



cos (6 - 30) 

 The relation between the power factor ano the ratio R is 



Power factor 



- 1\ 2 



9 



1.0 



J.6 

 I .5 



I; 



-1.0-9 -.8 -7 -.6 -.5 -.4 -.3 -.2 -.1 -f.l 4-.2 +.3 +.4 +.5 +.6 -K7 +.8 +.9+1.0 

 Smaller Reading = COB ( fl + 30) 

 Larger Beading Cos (0-30) 



FIG. 323. Power-factor chart, two-wattmeter method, balanced three- 

 phase load. 



Synchroscopes, Synchronizers. When alternators are operated 

 in parallel, it is necessary in putting a machine on the system 

 that the voltage of the incoming machine be equal in magnitude 

 and opposite in phase to the voltage of the busses. To avoid 

 accidents and injurious stresses in the machines, it is necessary to 

 have some instrument which will show when the proper phase 

 relation has been attained and in the case of large machines, 

 whether the speed of the incoming machine must be increased 

 or diminished before the main switches are closed. 



The Lincoln Synchroscope. The Lincoln and kindred forms 

 of synchroscopes furnish the switchboard attendant with the 

 desired information. This instrument is in principle a Tuma 

 phase meter with the slight modification that the currents are 

 carried to the crossed coils by brushes and slip rings so that the 



