312 



ELECTRICAL ENGINEERING 



With water at 25 C. about J gallon per minute per kilowatt lost is 

 required for 40 C. rise. If too much water is used the transformer 

 will be cooled below the temperature of the air and moisture may 

 collect on it and cause a breakdown of the insulation. The cooling 

 coils should have a surface in contact with the oil of about 0.9 

 sq. in. per watt lost. 



192. Transformer Connections. If the primary and sec- 

 ondary windings are each divided into a number of coils, which 



Ratio 2200:220=10:1 Ratio 2200:110=20:1 Ratio 1100:220=5:1 Ratio 1100:110= 10:1 



FIG. 287. Transformer connections. 



can be connected either in series or parallel, a number of differ- 

 ent ratios of transformation can be obtained. 



Take for example a standard light- 

 ing transformer with two coils for 

 110 volts on the low-voltage side 

 and two coils for 1100 volts on the 

 high-voltage side. The four possi- 

 ble connections are shown in Fig. 

 287. 



If small percentage changes of 

 ratio are required for line regula- 

 tion a number of taps are brought 

 out from the primary or secondary 

 winding so that the number of turns 



in use may be changed by the re- 

 FIG. 288. Secondary with taps. quiredamount (pig> } 



193. Single-phase Transformers on Polyphase Circuits. Sin- 

 gle-phase transformers are used in groups on polyphase circuits. 



On three-phase circuits the transformers may be grouped in 

 four ways as shown in Fig. 289. Both the primaries and 

 secondaries may be connected in "Y" or "A". 



The A A arrangement is used where the voltages are not ex- 

 tremely high and where a neutral point is not necessary. It 



