404 ELEMENTS OF ELECTRICAL ENGINEERING. 



cycles, two-phase, to no volts three-phase. The cross-section 

 of each iron core is 75 square centimeters. Find (a) the number 

 of turns of wire in each primary coil, and (b) the number of turns 

 of wire in each secondary coil, allowing a maximum flux density 

 of 3,500 lines per square centimeter. Ans. (a) N r = 1,429 

 turns, (ft) number of turns in coils a, b and c, Fig. 198 are 79, 

 79) l $6> respectively. 



123. A three-ring converter is to supply direct current at 500 

 volts to a street-car line. At what voltage must the alternating 

 currents be delivered to the machine ? The step-down transfor- 

 mation is accomplished by three similar transformers with theh 

 primaries delta-connected to the high voltage mains and their 

 secondaries Y-connected to the three rings of the converter, 

 What is the ratio of transformation of each transformer, the line 

 voltage being 1 0,000 volts? Ans. 306 volts. N"/N'= 0.0177. 



CHAPTER XL GENERAL THEORY OF THE TRANSFORMER. 



Specification of a shell-type transformer. Following are the 

 data for a shell-type transformer (see Fig. 207, 208, and 209) : 



The fine wire coil consists of 560 turns of number 17 B. & S. 

 copper wire. Mean length of turn 29 \ inches. 



The coarse wire coil consists of 28 turns of two number seven 

 (B. & S.) wires in parallel. Mean length of turn 29! inches. 



/= 1 1 inches, g = \ inch, Y= i\ inches (coarse wire coil), 

 X= 1 1 inches (fine wire coil), and X/2 = loj inches. 



At each end of the core 4^ inches of length of coils are ex- 

 posed. Section of magnetic circuit = loj x lyf square inches, 

 and -j 8 Q- of this is iron. Volume of iron = 24! x 10^ cubic 

 inches x -$. Thickness of laminations 14 mils. 



Specification of a core-type transformer. Following are the 

 data for a core-type transformer (see Fig. 212): 



Each fine wire coil, P, Fig. 212, consists of 1,000 turns of 

 number 17 B. & S. copper wire. Length of mean turn = n| 

 inches. 



