162 THE TRANSFORMER. 



Method II. The approximate method for rapidly 

 measuring the copper losses, if the resistance of the windings 

 has not been previously determined, has already been given 

 as Experiment XXVIII., and is very useful as a rapid 

 workshop test. 



The complete determination of the efficiency is then carried 

 out as follows : 



The transformer is supplied with current, the watts 

 taken at no-load being measured on a wattmeter connected 

 in the supply circuit. For this purpose it is usually most 

 convenient to supply the secondary instead of the primary 

 on account of the more convenient voltage. The no-load 

 current being very small, the copper losses under these con- 

 ditions are very small and the power registered on the watt- 

 meter is almost entirely due to losses in the iron. For greater 

 accuracy the small copper losses may be calculated and 

 subtracted, if thought necessary. 



The free winding of the transformer is then short circuited 

 through an ammeter and a low voltage applied to the same 

 winding as before, through the wattmeter. 



The voltage applied is so regulated as to give the full 

 range of current which the transformer will carry under 

 working conditions, and for each value of the current the 

 wattmeter reading is noted. The power registered is now 

 practically wholly due to copper losses, since the induction 

 in the core is so low that the hysteresis and eddy current 

 losses are quite small. 



From the two sets of readings taken in this way the 

 efficiency of the transformer may be obtained at any load. 



^,,v. _ Watts output 



Watts output + Iron losses and copper losses. 



Determination of the Drop at any Power-factor. By a slight 

 extension of Experiment XXVIII. for the determination of 

 the copper losses in a transformer, the terminal voltage of the 

 transformer at any load and any power-factor may be 

 approximately calculated. 



If in Experiment XXVIII. the transformer is entirely 

 without leakage, and if the small magnetising current can be 

 neglected (which is always permissible at the low induction 

 employed), the whole of the voltage, applied to the primary 

 winding would be spent in sending current through the resis- 

 tance of the windings. 



