TRANSFORMERS, GENERAL 



79 



resistivity 12; with the coils in parallel the transformer is to give 

 110 volts at full load. Using 1200 circular mils per ampere, 

 required the current that the transformer will give at this pressure ; 

 also the current and e.m.f. with the coils in series; the primary 

 winding is made up of 2 coils each having 240 turns of the same 

 mean length and using the same current density; required the 

 currents and e.m.f. 's in the primary with the coils both in series 

 and in parallel; required also the secondary and primary resist- 

 ances, the primary and secondary drops and the regulation (neglect 

 magnetizing current). (10 min.) 



8. A 4-kilowatt, 10 to 1 transformer has 100 square centimeters 

 cross-section of iron and with 600 turns in the primary has a density 

 at 1000 volts of 6500; what would the density become if used on 

 a 1200-volt system? If the frequency were decreased 10%, what 

 would it be? Also if the area were made 125. Also if the turns 

 were changed to 500. (5 min.) 



9. Two transformers have the same iron volume but different 

 windings, so that B is equal to 4000 and 6000 respectively. Re- 

 quired the ratio, (a) of the hysteresis losses and (b) the eddy 

 current losses. (4 min.) 



10. The cross-section and hence the volume of the iron in a 

 transformer is increased 20%; with the same winding, what 

 changes will result in the hysteresis loss and in the eddy cur- 

 rent loss. What changes would then have to be made in the 

 winding to bring each of these losses back to its former value? 

 (10 min.) 



11. Without changing the iron and e.m.f. of a transformer the 

 number of turns is increased 10%. What changes in the hysteresis 

 and eddy current losses will result? If in this transformer the 

 copper and iron losses are equal, what will be the percentage change 

 in the total loss (neglect the eddy loss). (8 min.) 



12. A 50-kilovolt-ampere transformer has 210 watts hysteresis 

 and 32 watts eddy current loss. If the magnetic density in this 

 transformer is 9000, what would be the effect on the hysteresis, 

 eddy and total iron loss of increasing it to 11,000? Of reducing 

 it to 7000? Use 1.7 for the hysteresis exponent. (7 min.) 



13. In problem 12 if there were no change in density, but a 

 change from 60 to 25 in the frequency, what would the losses 

 become? (4 min.) 



14. If in problem 13 the impressed e.m.f. were kept constant, 



