208 ALTERNATING cr /{RENTS 



(b). The current I c b will now be the algebraic difference of the 

 original primary current he and the secondary current / C V ', or 10 

 amp. as shown. Therefore, instead of having two windings, one 

 of which carries 10 amp. and the other 20 amp., a single winding 

 only is necessary and its rating need not be greater than 10 amp. 

 The copper represented by the 20-amp. secondary, Fig. 197 (a), 

 may in this case be eliminated and yet there is sufficient copper 

 to transfer the same power from one circuit to the other. Such 

 a transformer is called an auto-transformer or compensator. 



The primary voltage is E ac and the winding ac receives the 

 power; the secondary voltage is Eb c ', the ratio of transformation 

 is Ebc/E ac ; the magnetizing current flows through winding ac. 

 Therefore, the winding ac could properly be considered as the 

 primary and the winding be as the secondary. When discussing 

 the current and power relations within the transformer itself, 

 the treatment is simplified by considering the winding ab as the 

 primary and the winding be as the secondary, the magnetizing 

 current being neglected. 



The coil be supplies power to the load and is the secondary of a 

 transformer of which ab is the primary. Neglecting losses and 

 magnetizing current, both of which are small: 



The power delivered to the load is 50 X 20 = 1,000 watts. 



The power in the primary ab is 50 X 10 = 500 watts. 



The power in the secondary be is 50 X 10 = 500 watts. 



Only 500 watts are transformed, but 1,000 watts pass to the 

 load. 



The extra 500 watts are not transformed, but merely flow con- 

 ductively from the line a' a to the line bd. In this case but half 

 the total power is transformed. 



In the drop wire, the current flowing from a to b, Fig. 196, 

 undergoes a drop in potential. The power, represented by the 

 product of this drop in potential and the current, goes to heat 

 the wire ab. In the auto-transformer, however, the power repre- 

 sented by the current undergoing a drop in potential from a to b, 

 Fig. 197 (b), is not wasted, but is transferred to the magnetic field. 

 This power, transferred to the magnetic field, appears in the 

 winding be where a current of 10 amp. is raised 50 volts in poten- 

 tial. That is, by transformer action, power is transferred from 

 winding ab to winding be. 



