82 ELECTRICAL ENGINEERING PROBLEMS 



150 secondary turns, are to be used, by means of taps, to trans- 

 form from 2200-volt, 2-phase to 220-volt, 3-phase. Where must 

 the taps be put? What would the voltages on the 3-phase system 

 be if the whole secondary were used? (8 min.) 



31. Three transformers to transform from 1000 to 100 volt, 

 3-phase are to have the primaries connected Y and the secondaries 

 connected delta. If the secondaries have 60 turns each, how many 

 turns must the primaries have? (2 min.) 



32. Three transformers connected in delta are supplying 100 

 amperes to a three-phase system. What is the current in the 

 transformers? One of these transformers burns out and is re- 

 moved; the system continuing to draw 100 amperes, what is the 

 current in the two remaining transformers? Solve graphically. 

 (10 min.) 



33. Explain how two identical transformers without taps can 

 be connected to give a 3-phase system with e.m.f. 's of 220, 311 and 

 220 volts when fed from a 1100-volt, 2-phase system. What will 

 be the secondary e.m.f. and transformation ratios of the trans- 

 formers? No changes are to be made in the windings. (3 min.) 



34. It is required to design two transformers which can be used 

 on a 6600-volt, 2-phase system to give either 2-phase or 3-phase, 

 3-wire systems at 220 volts; 2 volts per turn are to be allowed; 

 the transformers have 2 coils in the secondaries and a tap is to be 

 used. State numbers of turns in coils, location of tap and methods 

 of connecting transformers for both systems. (4 min.) 



35. Three 1000- to 100-volt transformers, A, B and C, are con- 

 nected delta, both primary and secondary. Taps are taken out 

 of the secondary as follows : (1) Between A and B ; (2) 86.6% along 

 B (from A) ; (3) middle of C; (4) 86.6% along A (from B). Show 

 graphically what the e.m.f. 's between (1) and (3) and between (2) 

 and (4) will be and what* will be their phase relation. (6 min.) 



36. Given two suitably wound transformers properly connected 

 to give 400-volt, 3-phase e.m.f. by the Scott method. Construct 

 the T-shaped diagram by the revolution of which around the 

 junction of the two lines the 3-phase e.m.f. 's set up can be pro- 

 duced. By the rotation of this diagram construct first the three 

 sine e.m.f. curves produced by the three vectors making up the 

 "T," then by the combination of these with proper signs draw 

 three 120-degree e.m.f.'s which they produce. (80 min.) 



37. The same as problem 36, except that the 120-degree e.m.f.'s 



