0r/->77o.Y.< AM) ritOHLKMS 487 



22. What advantage is claimed for the Type-H transformer of the General 



Electric Co'.' What provisions are made for keeping this type of transformer 



cool? 



23. Do-cribe ono method of keeping transformers cool. Outside of its 

 cooling properties, what other advantage is obtained by using oil? 



24. Describe two other methods used to dissipate the heat from self- 

 transformors when the surface of the case itself becomes inadequate? 



26. What are the advantages and disadvantages of air-cooled trans- 

 formers? How is the circulation of air maintained? 



26. Describe the method ordinarily used for artificially cooling oil-filled 

 transformers. What care chould be taken when this method is used? 



27. Kxplain the principle upon which three-phase transformers operate. 

 What are the advantages and the disadvantages of this type of trans- 

 former? From the operating standpoint, in what ways do the shell type 

 and the core type of three-phase transformer differ'.' 



28. In what way does the auto-transformer differ from a resistance 

 drop wire? From an ordinary transformer'.' 



29. I'ndor what condition- is it advantageous to use an auto-trans- 

 former'.' ruder what conditions should an auto-transformer not be used? 

 How may an ordinary transformer be connected to operate as an auto- 

 transformer? 



30. Indicate the different connections that can be used for three-phase 

 transformer banks. State the conditions for which each connection is best 

 adapted. 



31. What is meant by a "floating neutral" and by what connection is 

 it produced? How may it be eliminated? 



32. Under what conditions cannot three-phase transformer banks be oper- 

 ated in parallel, even although the ratios between line voltages are alike 

 for the several banks? 



33. Give the reasons why, at no load, the throe throo-ph:ise voltages 

 existing across delta-delta-connected transformer secondaries are not 

 in any way di-turbed by the removal of one of the transformers, if the 

 voltages are balanced? 



34. What is the ratio of the kv-a. rapacity of the delta-connected bank 

 to the V-connected bank? Under what conditions is the V-connection 

 used'.' 



36. Make a diagram of the T-comu < (ion when used for transforming 

 three-phase to three-phase. How does the total three-phase kv-a. capacity 

 of the T-bank compare with the sum of the kv-a. capacities of the individual 

 transform en? 



36. Show ho I >e used for obtaining a two-phase, 



vstem. What ei.n.iect ion is necessary if the three-wire system 

 is to have e|nal voltages ,,,, !,(), legs? 



37. I a two-phase (or four-phase) four- or five-wire system be 



,.-d fp.m ' 'lie neutral of the "T"T 



38. How does the const met onstant-curn-nt transformer differ 

 from that of a constant-pot, i,' il transformer? Assuming a change of load, 



