502 



ALTERNATING CURRENTS 



140. A converter similar to that of problem 139 has six slip-rings and is 

 operated six-phase. This increases its efficiency to 92 per cent. The 

 transformers are connected, the primaries in delta and the secondaries 

 six-phase diametrical. When the converter is delivering 200 kw. and oper- 

 ating at 95 per cent, power-factor (see Fig. 140A) determine: 



(a) The voltages E,~ 2} E?- z , etc. 



(6) The voltages JS t - 8 , E,- b , ES-I. 



(c) The diametrical voltages Ei- 4 , Ey- 6 , etc. 



(d) The current in each transformer secondary. 



What is the probable rating of the converter under these new conditions, 

 and have the transformers now the proper rating? 



Primaries 



Secondaries 



FIG. 140 A. 



141. If three other transformers having the same kilovolt-ampere rating 

 as those in problem 140 be connected primaries in Y and secondaries in 

 double- Y, with connected neutrals, determine: 



(a) The rating of each primary, in kilovolt-amperes, volts, and amperes, 

 assuming 200 kw. output of the converter. 



(6) All possible voltages obtainable from the transformer secondaries. 



142. A 500-kw. synchronous converter is to be installed for supplying 

 230-volt, three-wire, direct-current service. The alternating-current supply 

 is 13,800 volts, 60-cycles. 



Make a complete diagram of connections such as would be necessary for 

 obtaining the required direct-current service. Indicate the currents and 

 voltages at each point. Obtain the efficiencies of the various parts of the 

 system from data already given. 



QUESTIONS ON CHAPTER XII 



1. Why is alternating current particularly well adapted for transmitting 

 power over considerable distances? What difficulties are encountered 

 when direct current is similarly used? 



2. State the advantages of polyphase transmission* Which of the poly- 

 phase systems is most commonly used and why? Under what conditions 

 is single-phase occasionally used? 



