5-A] STUDY AND OPERATION. J 37 



a primary. Measure the no-load primary current, 7 , called the 

 exciting current. Predict, and then measure, the value of / 

 when the two primary coils are in parallel and connected to a 

 55-volt supply i. e. t half the preceding voltage. Compare the 

 relative values, for the two cases, of primary turns, ampere 

 turns, volts, volts per turn and flux density. 



Measure / when the two primary coils are in series, and con- 

 nected to a 55-volt supply; and interpret the results (see Fig. 

 2, Exp. 5-B). 



Commercial transformers are commonly built with two pri- 

 maries for connection in series (for, say, 2,200 volts) or parallel 

 (for 1,100 volts) ; and two secondaries for connection in series 

 (for, say, 220 volts) or parallel (for no volts). 



16. Operation Under Load.* Join twof of the coils in series 

 to form a primary and join the other two coils in series to form 

 a secondary or make such other arrangement of coils as may 

 be desired. Connect the primary with an alternating current 

 supply say no volts, 60 cycles appropriate to the arrange- 

 ment of coils adopted. A voltmeter, ammeter and wattmeter are 

 connected:): in the primary, as in Fig. I. 



17. With the secondary on open circuit, measure the primary 

 voltage, the primary current (in this case, the no-load current, 

 7 ) and the primary power (in this case, the no-load or core 

 losses, W Q ). 



* Time should not be spent in an attempt to get very accurate results 

 in this test, particularly if it is to be followed by the more accurate test 

 by the method of losses, Exp. 5-6. 



t ( i6a). Where there is a choice of coils, select an arrangement which 

 avoids great magnetic leakage. If each coil forms one layer or section, to 

 take the first two for primary and the other two for secondary would not 

 be a good arrangement. In a commercial transformer, the primary and 

 secondary windings are so placed as to reduce magnetic leakage ; to secure 

 this end, however, all the windings should be used, that is, no coil should 

 be left idle. An arrangement of coils commonly used is as follows : low, 

 high, high, low, potential. 



$ With instruments arranged as in Fig. I, no corrections need be made. 

 (See Appendix III.) 



