190 



TRANSFORMERS. 



[Exp. 



PART II. THE CIRCLE DIAGRAM AND ITS CONSTRUCTION. 



24. The circle diagram for a transformer shows the varia- 

 tion in the primary current for different values of load resist- 

 ance with constant impressed voltage. In Fig. n, the primary 



^** Circle Locus of 

 Primary Current 



Short circuit\p" 



1 



JP< 



FIG. ii. Simplified diagram; R and X all in secondary. Corresponds to Fig. 9. 



current is OP, being composed of the no-load current OA and the 

 load current AP. As the load resistance is decreased from in- 

 finity to zero, the point P will trace the arc of a circle, and will 

 take the position P" on short circuit.* If it were possible to elimi- 

 nate the resistance of the transformer windings, the point P 



* (243). If a transformer is constructed so as to have a large leakage 

 reactance (or if a reactance is included in the circuit external to the 

 transformer), the short-circuit current and the diameter, E t H- X, are 

 reduced. The transformer may then be operated at or near short circuit, 

 in which case the current will be nearly constant. This method is used 

 for obtaining constant current from a constant potential line. (See 43, 

 Exp. 5-A.) Large reactance or magnetic leakage in any apparatus tends 

 towards constant current operation. See 8, Exp. 3-A, 27, 2/n, Kxp. 

 3-B, and 14, Exp. 4-6. 



