318 



ELECTRICAL ENGINEERING 



In Fig. 296 the part of the winding from b to c carries only the 

 primary current, while the part from a to c carries the difference 

 between the secondary and primary currents. When the ratio of 



To Line 



To Motor 



-220 Volt 



FIG. 297. Auto-transformer as induction motor starter. 



turns is high the current from a to c is much larger than the 

 primary current and this part of the winding must be made of 

 large enough section to carry it. 



The copper loss in an auto- 

 transformer is smaller than in 

 an equivalent two-coil trans- 

 former and the efficiency is 

 therefore higher but this ad- 

 vantage decreases as the ratio 

 of turns increases. With a high 

 ratio of turns it is a disadvan- 

 tage not to be able to insu- 

 late the low-voltage side of 

 the transformer from the high- 

 voltage side. 



An auto-transformer may be 

 used as a step-up transformer 



110 Volts- 



110 Volts- 



FIG. 298. Auto-transformer as a bal- 

 ance coil for a three-wire system. 



by connecting the low-voltage side to the supply. 



198. The Constant-current Transformer. The constant-cur- 

 rent transformer is shown diagrammatically in Fig. 299. The 

 primary coil P is fixed hi position and receives power at constant 

 voltage. The secondary coil S is movable and regulates for 

 constant current in the receiver circuit which it supplies irrespec- 



