5_C] CIRCLE DIAGRAM. J 79 



EXPERIMENT 5-C. Circle Diagram for a Constant Poten- 

 tial Transformer. 



i. Introductory. It has been seen, Exp. 4-B, that when 

 the resistance is varied in a series circuit with constant reactance 

 the vector representing the current follows the arc of a circle 

 as a locus. In a similar manner, the primary current of a con- 

 stant potential transformer follows the arc of a circle as a locus 

 when the secondary resistance is varied. The same is true for 

 an induction motor when its load is varied, and use is made of 

 this fact in practical motor testing. The following experiment 

 will, accordingly, serve to make clear certain principles of the 

 induction motor as well as of the transformer; upon these prin- 

 ciples is based the method of transformer testing developed in 

 detail in Exp. 5-6. 



In Part I. the general principles governing the action of a 

 transformer will be discussed; in Part II. these principles will 

 be applied in constructing a circle diagram. The practical re- 

 sults, so far as commercial testing is concerned, are all given 

 in Exp. 5-B. The actual construction of a diagram to scale 

 gives one a definite and concrete idea of what might otherwise 

 be vague and abstract. Furthermore, the abstract diagrams 

 given here (Figs, i-u) and elsewhere are so grossly exaggerated 

 that they give very wrong ideas of real values. Even Fig. 12, 

 which is more nearly to scale, is much exaggerated. 



2. Data. The same data are required as in Exp. 5~B. See 

 25 of this experiment. 



PART I. GENERAL DISCUSSION OF THE ACTION OF A 

 TRANSFORMER. 



3. The action of a transformer will be most readily under- 

 stood by considering its action first without a load i. e., on open 

 circuit and then with a load. 



