S _B] TEST BY LOSSES. I 5 I 



In a constant potential transformer the magnetization and 

 hence the core loss and exciting current are (substantially) the 

 same at all loads, being dependent upon voltage and not upon 

 current. The copper and load losses, on the other hand, depend 

 upon current and vary with the load. (In a constant current 

 transformer, the conditions are reversed; copper losses are con- 

 stant and core loss varies with the load.) 



PART I. OPEN-CIRCUIT TEST. 



3. With the secondary open, measurements are made on the 

 primary with ammeter, voltmeter and wattmeter, one method 

 for making the connec- ^^^ 



tions* being shown in / A j 



Fig. i. Although any | 

 coil or combination of |i 



> s- 



coils could be used as a <f 



111 



"c 



II 



4 



primary in this test, it pia x> O ne method of connection for open- 



is most convenient to circuit test for core loss and exciting current. 



Use a low potential COil See & for method of connecting instru- 



ments requiring no corrections. 



(50, loo or 200 volts) 



as primary to suit the instruments and supply voltage available; 



furthermore, there is less danger in working on the low potential 



side. 



For the same degree of magnetization, the exciting current 

 in a loo-volt coil is ten times as large as in a i,ooovolt coil, the 

 ampere turns being the same. It is, accordingly, a simple matter 



*(3a). For selection of instruments, see 44. The arrangement of 

 instruments shown in Fig. I should be followed when the highest accuracy 

 is desired; the wattmeter reading is to be corrected by subtracting* 

 E 2 (i/R w -\-i/R v ), which is the power consumed in the potential coils of 

 the wattmeter and voltmeter. 



It is, however, much simpler and in many cases sufficiently accurate to 

 arrange the instruments as in Fig. I of Exp. 5-A, and to make no correc- 

 tion. See Appendix III., Exp. 5-A. 



