TRANSFORMERS, GENERAL 



81 



22. A 10-kilovolt-ampere, single-circuit or auto-transformer is 

 designed for 220 to 110 volts and a current density of 1000 circular 

 mils per ampere; what should the areas of the two parts of the 

 winding be? Also for the following ratios of transformation: 

 220 to 160 volts and 220 to 50 volts. What are the relative weights 

 of copper for these different ratios? (8 min.) 



23. Determine the wire areas and relative weights of two- 

 circuit transformers for the same service as in problem 22. Also 

 compare the weights of single- and two-circuit transformers for 

 each case. (12 min.) 



24. A 2-kilo volt-ampere auto-transformer is wound with 

 No. 12 and No. 9 wire; for what primary and secondary voltages 

 is it available if 1000 circular mils per ampere be allowed. Use 

 the No. 9 wire as secondary. (5 min.) 



25. A 5-kilovolt-ampere transformer for changing from 220 to 

 150 volts is to be built; with 1500 circular mils per ampere, 3 feet 

 length of turns and 100 turns in the primary, how many pounds 

 of copper would be saved by using a single-circuit design? Take 

 the weight of a mil foot as 3.03 X 10~ 6 pounds and add 2% (ap- 

 proximate for large wires) for double-cotton-covered insulation. 

 (8 min.) 



26. Repeat problem 25, except with a 240-volt secondary. 

 (8 min.) 



27. In the Northwest Station of the Chicago Commonwealth 

 Edison Company, auto-transformers are used to step up the power 

 generated from 4500 to 9000 volts, the transmission voltage. 

 Using 650 amperes per square inch, what would be the cross-section 

 area of the primary and secondary parts of the winding of a 6600 

 kilovolt-ampere transformer for this purpose? (2 min.) 



28. Three standard 2200- to 220-volt transformers are con- 

 nected with their primaries in Y on a 2200-volt, 3-phase system, 

 while their secondaries are connected delta-wise; what will be the 

 e.m.f. on the secondaries? (1 min.) 



29. It is desired to put taps on three 10 to 1 transformers, each 

 having 80 turns in the secondaries so that when the primaries are 

 connected in delta on a 1100-volt system, the secondaries can 

 be connected in Y and give 110 volts between their terminals; 

 how many turns from the neutral must the taps be placed? 

 (8 min.) 



30. Two standard 2200- to 220-volt transformers, each having 



