500 ALTERNATING CURRENTS 



19. Compare commutation in a converter when operating as such and 

 when operating as a direct-current generator carrying the same load. 

 Why does the very materially increased armature current resulting from 

 low power-factors have little distorting effect on the main fields? What is 

 its effect on commutation? 



20. Why are commutating poles desirable in synchronous converters 

 even although the main field is not distorted to any considerable extent 

 by armature reaction? 



21. Why are the voltage ratios in a converter almost constant under 

 operating conditions? Why is it possible to modify the ratio of the direct 

 to the alternating voltage a small amount by changing the excitation? 



22. Explain how a series reactance may be used to control the direct- 

 current voltage. When may a separate reactance be omitted? State the 

 disadvantages of this method of voltage control. 



23. Explain the use of the induction regulator as a means of controlling 

 the direct-current voltage. What is fhe objection to the use of the regulator ? 



24. Explain the operation of the series booster. What are its advantages 

 and its disadvantages? 



25. Why is it impracticable to control the direct-current voltage by 

 changing to different transformer taps when the converter is in operation ? 



26. Explain the underlying principle of the split-pole method of voltage 

 control. 



27. Sketch a diagram of connections, including all instruments, which 

 would be used in determining the various characteristics of the converter. 

 What characteristics is it instructive to determine? How should they be 

 plotted? 



28. Why are transformers almost always necessary with synchronous 

 converters? Sketch the connections of the double-Y, six-phase secondary 

 connection, showing the primaries in either Y or delta. Indicate the 

 voltage at each point, assuming 220 volts between the three-phase lines on 

 the primary side. What is the advantage of this system ? 



29. Repeat (28) for the double-delta connection of secondaries. 



30. How does the rating of a synchronous converter, when operating 

 inverted, compare with its rating when operating direct? Why? How do 

 the speed relations in the two cases compare? Show by careful analysis 

 the sequence of reactions which may cause an inverted converter to race. 

 What means are used to prevent racing? 



31. By what reactions does a synchronous converter armature start ro- 

 tating when polyphase currents are supplied to its slip-rings? What is a 

 sectionalizing switch, and what should be its position when starting the 

 converter from the alternating-current side? Why is it necessary to open 

 the series-field shunt, etc.? When starting, why does sparking take place: 

 under the brushes even with no direct-current load? Why are brush-lifting 

 devices necessary? 



32. How does the armature pull into synchronism ? What effects occur 

 if the shunt-field current opposes the field built up by armature reaction? 

 How may the continual "slipping of a pole" be stopped? 



PROPERTY OF ELECT -.TORY, | 



FACULTY OF APPLIED SCIENCE, -i 



