TRANSFORMERS 



317 



secondary, often reaching thousands of volts in transformers with 

 a large ratio of turns. 



When the secondary is closed a current flows in it and produces 

 a m.m.f. opposing the primary m.m.f. and reducing the flux to a 

 very small value. 



The primary e.m.f. is the drop of voltage in the transformer due 

 to its impedance and is very small, since the secondary is short 

 circuited. 



The iron core is only designed to carry the flux required with a 

 closed secondary and it would be very highly saturated under open- 

 circuit conditions and would become hot due to the excessive iron 

 losses. 



197. Auto-Transformer. An auto-transformer has only one 

 winding; the primary includes all the turns while the secondary 

 includes only a part of them. 



n 



The secondary voltage is usu- 

 ally made variable by bring- 

 ing out a number of tops. 



Auto-transformers are used 

 very extensively to obtain a 

 variable voltage for starting 

 induction motors, synchro- 

 nous motors, single-phase 

 series motors, etc., and as 

 balance coils on three-wire 

 distributing circuits. 



Fig. 296 shows an auto- 

 transformer for starting and 

 operating a single-phase series 

 motor from a high-voltage 

 trolley in electric railway ser- 

 vice. 



The transformer in Fig. 297 

 is suitable as an induction 

 motor starter. For three-phase motors three auto-transformers 

 connected "star" may be used or two connected in "open delta." 

 The unbalance of voltage in the open delta is not objectionable, 

 since the transformers are only employed for starting. 



Fig. 298 shows an auto-transformer used as a balance coil on a 

 220-volt, three-wire system. 



FIG. 296. Auto-transformer for single- 

 phase electric railway motors. 



