QUESTIONS AND PROBLEMS 501 



33. Give two methods by which the direct -mm-nt polarity may be re- 

 versed, if necessary. 



34. Describe how the SJM -| of rotation in space of the field prodm -d l,\ 

 the armature currents Incomes less and less, during starting, as the armature 

 speed, approaches synchronism. How does this affect commutation? 

 Describe the behavior of a direct-current voltmeter connected across the 

 brushes during the starting period. When should the field switch l>e closed? 



35. How may the armature be induced to build up the field poles to the 

 right polarity and so insure the correct direct-current polarity at the 

 brushes ? 



36. Describe briefly the procedure of starting a synchronous con 

 by means of an auxiliary machine. 



37. Give the connections of both the shunt- and the series-field circuits 

 of a synchronous converter when it is started from the direct-current 

 side. Why should the switch between the transformer secondaries and the 

 slij>-rings be opened during the starting period? What difficulty is en- 

 countered in synchronizing? 



38. Discuss the operation of synchronous converters in parallel. How 

 many equalizers may be required? How are the loads between machines 

 adjusted? Why is it preferable that each converter have its own trans- 

 former bank? 



39. Why may synchronous converters operating in parallel show a 

 tendency to run away under some circumstances? Describe methods which 

 are used to prevent synchronous converters from thus running away. 



40. What is the principle by which a neutral is obtained in the three-wire 

 generator? Why is it undesirable to use three single secondaries connected 

 in V when obtaining a neutral? How may a Y-connection be used and at the 

 same time direct-current magnet j/at ion of the con- !> prevented'.' 



41. Sketch the complete connections of a three-wire, six-pha- 



nous converter having two series fields, where the transformer secondaries 

 are connected 6-phase star. 



PROBLEMS ON CHAPTER XI 



139. It is desired to secure a 200-kw. synchronous converter and its 

 formers for changing three-phase, 6,600-volt power into 11.1 volts 

 current. The converter has three slip-rings and the transfm mere are 

 connected, primaries in delta and secondaries in V. T! 

 an efficiency o f 90 per cent, and the transformer bank 

 97.8 per cent. When the converter oj>erate8 at 95 per cent, power-factor 

 determine: 



rating of ' 



lip-ring. 



(c) The rating in kilo volt-amperes, amperes, and volts of the transformer 

 secondaries. 



(d) Tho power input, tl " voltage of each transformer 

 primary when the con verier is delivering rated out put . 



