Till-: TRANSFORMER 



189 



If the secondary current and voltage are to be used, the second- 

 ary equivalent constants, Z 2, Rot, and Xos, must also be used. 

 This will be demonstrated in the methods of transformer testing 

 which follow. 



83. Open-circuit Test. Figure 177 shows a transformer 

 having the low side connected to an alternating source of supply 

 and the high side open-circuited. Either an auto-transformer 

 or a drop wire is shown as a means of varying the voltage sup- 

 plied to the low side of the transformer. A voltmeter, an am- 

 meter, and a wattmeter are connected in the primary circuit. 

 The voltmeter reads the voltage across the primary terminals, 

 the ammeter reads the no-load current, and the wattmeter reads 

 the power taken by the transformer under these conditions. 



High 



FIG. 177. Connections for open-Hrniit teat. 



This power goes to supply the primary / // k>68 and the core loss 

 of the transformer. As the exciting current is very small, the 

 primary /'-'A' loss due to it may be neglected. Therefore, the 

 wattmeter reads the transformer core loss. If the primary volt- 

 varied and the core loss be determined for different values 

 of voltage, a curve is obtained showing the relation of core loss 

 \t no load the flux is practically proportional to 

 rminal voltage, as the primary impedance drop due in the 



ll'nload current i> ne'Jiiiible. (See equation 1<>. pa ire 171.1 

 The -dd\ '-current loss varied as the square of the voltage and 

 tin- 1; i loss as the l.C, pmver of the voltage. Tin- 



loss will increase, therefore, nearly as the square of t he voltage, 

 own in I-'iir. 178 (a). 



Transformer! are usually so designed that the rood economical 

 f materials is obtained Th> is operated at 



