MM) 



ALTERNA Tl.\(,' ( ' I ' 



as high a flux density as the allowable core loss will permit. A 

 study of Fig. 178 (a) shows that a slight increase of voltage, 

 above rated voltage, produces a very large percentage increase 

 in core loss. As transformers are rated by their maximum safe 

 operating temperatures, this increased core loss may cause over- 

 heating of the transformer. Therefore, the effect of operating 

 transformers at over-voltage is to produce a large increase in 

 temperature. 



If the magnetizing current be plotted as abscissas, and the 

 voltage as ordinates, a saturation curve similar to that of Fig. 

 178 (6) is obtained. The point marked " rated voltage" is the 

 point on the saturation curve at which transformers are generally 

 operated, and is well beyond the knee of the curve. Outside 

 the question of increased core loss, the usual transformer cannot 



Voltage I m (Magnetizing Current; 



(a) (b) 



FIG. 178 (a). Relation of core loss to voltage in a transformer. 

 FIG. 178 (6). Relation of magnetizing current to voltage in a transformer. 



be operated at a voltage very much in excess of its rated voltage, 

 for the exciting current increases very rapidly with small increase 

 in voltage, as indicated in Fig. 178 (6). 



The flux density in the core is determined primarily by the per- 

 missible core loss. Open-hearth annealed sheet steel, such as is 

 used in dynamos, can be used for transformer cores. For a given 

 flux density and frequency, however, silicon steel has much less 

 core loss per unit volume than open-hearth steel, the effect of 

 the silicon being to increase the electrical resistance, and hence 

 reduce the eddy-current loss. Because of its small core loss, 

 silicon steel may be operated safely at very high flux densities. 

 The greater cost of silicon steel is more than offset by the saving 

 in iron and in copper, and in the general reduction of the 

 transformer dimensions. 



