ii2 ALTERNATING CURRENTS 



windings are represented by a single continuous winding, as shown 

 diagrammatically in Fig. 89. The ends of the winding AB form the 

 primary terminals, while the secondary terminals are represented by 

 one of the primary terminals (A in Fig. 89), and a terminal C in 



connection with an intermediate point of 

 the winding. We may regard the portion 



TO LOAD AC of the winding as resulting from the 

 O/ fusion of the primary and secondary. Now, 



since the primary and secondary currents 

 are nearly in phase opposition, it follows 

 that such a fusion of the two circuits will 

 give rise to a current in AC which is 

 merely the difference of the primary and 

 B secondary currents, and is thus much less 



FIG. 89. Diagram of Auto- than the current in the secondary of an 

 transformer. ordinary transformer. Hence the cross- 



section of the portion AC of the winding 



may be made much less than would be necessary with a transformer 

 of ordinary construction. 



On account of this advantage, auto-transformers would be much 

 more generally used than they are, were it not for the fact that in 

 most cases it is absolutely necessary, from considerations of safety, to 

 keep the two windings entirely distinct and heavily insulated from 

 each other, so as to prevent all risk of the dangerous high voltage of 

 the primary from reaching the secondary. 



Auto-transformers are frequently used to supply single arc lamps 

 from mains at a considerably higher p.d. than that taken by the lamp. 

 They are then sometimes termed economy coils. Their main use is, 

 however, in connection with the starting of various forms of alternating- 

 current motors. When so used they are generally known as com- 

 pensators. By means of a suitable switch, the motor is first connected 

 across the portion AC of the winding (Fig. 89), and when it has run 

 up to a certain speed, the switch is thrown over so as to connect it 

 across AB. In the first or starting position, the voltage across the 

 motor terminals is only a fraction of the normal voltage, while in the 

 second position of the switch the motor is supplied at the full line 

 voltage. 



59. Phase Transformers 



Cases sometimes arise in which it is required to pass from a two- 

 phase to a three-phase system, or vice versa. Such a transformation 

 may be effected by a suitable combination of transformers, which are 

 then termed phase transformers. Various solutions of the problem 



