102 ALTERNATING CURRENTS 



so that 



primary load current secondary turns 

 secondary current primary turns 



Now, owing to the adoption of a closed magnetic circuit the open- 

 circuit or magnetizing ampere-turns (or resultant ampere-turns of a 

 loaded transformer) form a small fraction of the total primary 

 ampere-turns at full load, so that the load ampere-turns corresponding 

 to full load may be approximately taken to be equal to the total 

 ampere-turns at full load. Hence, using the relation just established, 

 we see that, approximately, the primary and secondary currents are 

 inversely as the turns in the windings. For this reason the low- voltage 

 winding will carry a heavier current than the high-voltage winding, 

 and the cross- section of the conductor must be correspondingly greater. 

 In practice the windings are designed so as to contain, roughly, equal 

 amounts of copper. 



Combining the result for the ratio of the p.d.'s with that for the 

 ratio of the currents, we see that the product p.d. x current has approxi- 

 mately the same value for primary and secondary. In the case of a 

 non-inductive load, this conclusion is also otherwise obvious from the 

 fact that if the losses be negligible, then the primary watts must 

 equal the secondary watts. 



A transformer whose low-voltage winding is the primary, is 

 spoken of as a step-up transformer; and one whose high- voltage 

 winding is the primary, as a step-down transformer. 



51. Examples of Transformer Construction 



The most common form of construction for single-phase trans- 

 formers is that illustrated in Fig. 81. The core consists of stampings 

 arranged to form two upright cores connected by yokes at the top 

 and bottom. The stampings are held together by insulated bolts, the 

 positions of which are indicated by the small circles in the figure. 

 In order to avoid butt joints (which introduce a short air-gap into the 

 magnetic circuit and so increase its reluctance), the stampings may 

 be made to overlap alternately at the corners, a layer consisting of 

 two short core-plates and two long yoke-plates (as indicated by the 

 full lines in the figure) being succeeded by another, in which there 

 are two long core-plates and two short yoke-plates (dotted lines in 

 figure). The butt joints in any two consecutive layers thus occur at 

 different places, and the flux is free to pass from one layer to another 

 through the lap joints (of large area, and therefore low reluctance) 

 between different layers.* In order to economize copper .as much as 



* This principle of construction is not always adhered to. It has the disadvantage 

 that the dismantling of the transformer, and the subsequent building up of the core 



