300 



ELECTRICAL MACHINERY 



so as to make the voltage of d-e greater than that of a-6, 

 it shows that it is assisting the voltage of a-b; when the 



voltage of d-e is less than that 

 of a-b the secondary e.m.f. is 

 opposing the primary e.m.f. The 

 secondary e.m.f. in the two cases 

 would be 1210 and 990 volts 

 respectively. 



The Autotransformer May In- 

 crease or Decrease Voltage. The 

 autotransformer is represented 

 as having but one coil with proper 



taps; Fig. 191 shows the connection for the 1100-1200 

 volt connection, and Fig. 192 shows the connection for 

 the 1100-1000 volt connection. 



Neutralization of Current in Part of the Coil. In that 

 part of the coil common to both circuits, the primary and 

 secondary currents tend to neutralize one another. Suppose 

 the load in Fig. 192 is 10 amperes in the secondary circuit 



FIG. 190. An ordinary 

 Transformer Connected as an 

 Auto-transformer. 



FIG. 191. An Auto-transformer 

 with Ratio Greater than Unity. 



FIG. 192. An Auto-transformer 

 with Ratio Less than Unity. 



of 1000 volts; evidently 10 kv-a. must be supplied by the 

 primary coil to supply this load and 10 kv-a. at 1100 volts 



10000 



would require =9.1 amps. Therefore, the part of 



1 luu 



the coil AB (Fig. 192) carries 10 9.1 = .9 ampere, neglect- 

 ing the exciting current. The part B-C, however, carries, 

 the full 9.1 amperes. 



