482 ALTERNATING CURRENTS 



reactance drop, can be substituted for the effect of armature reaction, and 

 the no-load electromotive force therefore be determined. What armature 

 constant may be increased to include this fictitious voltage, and what as- 

 sumption is made in doing this? 



13. What is meant by synchronous reactance? Synchronous impedance? 

 Describe carefully the method usually employed to determine these 

 quantities. 



14. W T hat error occurs in the value of the synchronous reactance when it is 

 determined under short-circuit conditions? How does this affect the 

 regulation determined by using this value of synchronous reactance? 



16. Why does the synchronous impedance method of determining 

 regulation give unsatisfactory results with single-phase machines? Why 

 are results obtained with polyphase machines more in accord with the 

 actual performance of the machine? 



16. Describe the open-circuit test, giving the connections used. Repeat 

 for the short-circuit test, giving two methods of connecting the ammeters. 

 Compare the ammeter readings in each case with the line current and the 

 coil current of a delta-connected machine. 



17. How is the regulation of a Y-connected machine calculated? Of 

 a delta-connected machine? How do the respective coil resistances and 

 reactances compare in the two cases for the same machine? What care 

 should be taken when either method is used? 



18. In what fundamental way does the magnetomotive-force method 

 differ from the synchronous impedance method ? Show by a vector diagram 

 the various voltages and the magnetomotive forces which are substituted 

 for voltages in this method. How is the resultant field obtained? The 

 no-load electromotive force? 



19. How do results obtained by the synchronous impedance method 

 compare with those obtained by the magnetomotive force method? Why 

 do the two methods give different results? Which should be used? 



20. Fundamentally, how does the A. I. E. E. method of determining 

 regulation differ from the synchronous impedance and the magnetomotive 

 force methods? What difficulty is encountered in this method which is 

 not encountered in the other two methods? How do the results which it 

 gives compare with the actual performance of the machine? 



21. Compare the construction of the Tirrill regulator as applied to 

 alternators with the regulator as used on direct-current machines. (See 

 Vol. I, page 307, Fig. 280.) What is the function of the "main contacts" 

 and how are they operated? What is the function of the relay contacts 

 and how are they operated? Explain the operation of the entire regulator 

 from the time that the exciter voltage commences building up until the 

 machine has reached rated voltage, and load then applied. 



22. Explain why the prime mover characteristics alone determine the 

 kilowatt division of load between alternators. Why is this true of alternators 

 and not true of direct-current generators? Why is it undesirable that the 

 prime movers have flat speed-load characteristics? 



23. What effect has a temporary change of speed of one prime mover 



