M4 TRANSFORMERS. [Exp. 



primary and secondary currents measured when a proper voltage (a 

 few per cent, of normal primary voltage) is applied to the primary, 

 so that about the normal current flows. 



32. Circulating Current Test. As a shop test, after one standard 

 transformer has been tested, other transformers designed for the same 

 ratio may be operated from the same primary mains and tested one 

 at a time by connecting each secondary to be tested in parallel with 

 the secondary of the standard, terminals of the same polarity being 

 connected together. If an ammeter shows a circulation of current 

 through the secondaries, the two transformers are not of the same 

 ratio. 



Commercially a small difference in ratio is allowable as shown by 

 the circulating current, which, however, should never exceed one per 

 cent, of the rated full-load current. Instead of an ammeter a suit- 

 able fuse may be conveniently used, and more safely where much 

 difference in ratio may exist. 



APPENDIX II. 

 RELATION BETWEEN FLUX AND ELECTROMOTIVE FORCE. 



33. The fundamental relation between flux and electromotive 

 force is expressed by Faraday's law; that is, in a closed circuit* of 

 6" turns embracing a varying flux <, the induced electromotive force 

 is S-dcf>/dt. In a transformer, this applies alike to primary or 

 secondary. In the case of a primary coil this induced electromotive 

 force is a counter electromotive force and requires to overcome it an 

 equal and opposite impressed! electromotive force 



e = S-d<f>/dt. 



34. Sine Assumption. Assuming the wave of electromotive force 

 to be a sine wave, we have 



* = max. sin f ; 



* Not limited to a transformer. 



t (33a). The actual terminal voltage includes also resistance drop, thus 



e Ri + S-d<t>/dt. 



The resistance drop, however, is practically negligible in the primary of a 

 transformer on open circuit. 



