INSTRUMENT TRANSFORMERS 



Rated full-load currents of primary and secondary, 40 and 5 

 amp., respectively 



Secondary resistance, 0.51 ohm. 

 Resistance of connected load, 0.17 ohm. 

 Inductance of connected load, 0.08 millihenry, 

 Max. flux density at 60 cycles, 290 gausses. 

 Max. flux density at 25 cycles, 700 gausses. 



x Computed Points 

 Observed Points 



1UO.U 



20 40 CO 80 



% Rated Full Load Sec. Amps. 



1UU 



x Computed Points 

 o Observed Points 



I 



1UU 



20 40 60 



% Rated Full Load Sec. Amps. 



FIG. 354. Showing agreement of computed and observed values of the 

 ratio and phase angle of a current transformer. The form of the ratio curve 

 is abnormal; the ratio usually decreases with increase of load. 



It will be noted that in order to bring the ratio to its rated 

 value a few secondary turns are left off. Attention is called to 

 the low flux densities at which the core is worked. 



Theory of Potential Transformer. In the theory of the 

 potential transformer the two most important quantities are the 

 equivalent reactance and resistance as determined by the usual 

 short-circuit test. The exciting current and the reactance and 

 i M<-<> of tin* primary windings must also be taken into ac- 

 count but. their combined influence is much less than that of the 

 equivalent impedance. 



