498 



ALTERNATING CURRENTS 



135. A 500-kw., 2,300-volt, three-phase load has an average power-factor 

 of 0.5, lagging current. How many leading quadrature kv-a. are necessary 

 to raise its power-factor to 0.6? 0.8? 0.9? 1.0? Plot a curve with power- 

 factor as abscissas and kv-a. as ordinates. 



136. What size synchronous motor would be necessary to raise the power- 

 factor of problem 135 to unity and at the same time take 500 kw. from the 

 line in order to carry its mechanical load? 



137. Repeat problem 136 except that the power-factor is raised only to 

 0.9. 



20 



10 20 30 40 50 60 70 80 90 100 



Field Current 

 FIG. 131A. 



1S8. A 48-pole, 60-cycle, synchronous motor shows a tendency to hunt. 

 How many space-degrees either side of its true position may its rotor swing 

 if its back electromotive force may have a phase displacement of not more 

 than 10 time-degrees? 



QUESTIONS ON CHAPTER XI 



1. Give several uses of electrical power in which it is impossible to employ 

 alternating current. 



2. Make a sketch showing the method of operation of the rectifying 

 commutator. What are the disadvantages of this type of rectifier? For 

 how large capacities is it possible to operate this type of rectifier? 



3. Upon what principle does the mercury-arc rectifier operate? Why are 

 two anodes usually employed? Why is it necessary to have reactance in 

 circuit? Sketch the diagram of connections which would be used for 

 charging a low-voltage storage battery and trace the current flow, explaining 

 carefully the operation of the auto-transformer. Why is a starting anode 

 desirable? 



4. What is the underlying principle of the tungar rectifier? Why are 

 the electrons repelled from the incandescent filament during one half- 



