I3 S TRANSFORMERS. fExp. 



18. Load the secondary by means of suitable non-inductive 

 resistance. Change this resistance by steps so as to vary the 

 secondary current between no load and 25 per cent, overload. 

 At each step measure the primary voltage E 19 current I 19 and 

 power, W\ also the secondary* voltage E 2 , and secondary cur- 

 rent I 2 . The product of the secondary voltage and current will 

 give the secondary power W 2 , the secondary load being non- 

 inductive. In practice, a load of incandescent lamps is non- 

 inductive, but not so a motor load. 



19. Measure the resistance of primary and secondary. (See 

 15, Exp. s-B.) 



20. For each load, compute the power factor (W^-^EJ^) ; 

 also the angle 6 by which the primary current lags behind the 

 electromotive force. (Power f actor = cos 0.) 



Plot /!, IV lt power factor, 6, E 2 and W z for different values 

 of I 2 , as in Fig. 4. Plot, also, the copper loss for primary 

 (RJj 2 ) and for secondary (R 2 I 2 2 ) and the core loss W Q (the 

 value of W \ on open circuit) which is constant at all loads, as 

 in Fig. 8, Exp. 5-B. 



Note that E 2 decreases with load. Determine the per cent, 

 regulation the per cent, increase in E 2 in going from full load 

 to no load. 



Note the current ratio for different loads. It will be seen that 

 as the transformer becomes loaded (by decreasing resistance in 

 the secondary) the secondary current becomes more nearly equal 

 to the primary current (multiplied by .S^-^-S,,). In a loaded 

 transformer, primary and secondary ampere-turns are practically 

 (but not exactly) equal. 



It is seen that in a transformer there is a loss in volts, a loss 

 in amperes and a loss in watts, this last determining the efficiency. 

 While best for illustrating the operation of a transformer, the 



* By means of suitable transfer switches one voltmeter and one ammeter 

 may be used for both primary and secondary. 



