A\D /'/,'o/>'/./-:.VN 4 S3 



on the phase relation of the electromotive force induced in the two machines 

 which they drive? What effect does the resultant voltage produce? Why 

 is tlu- resultant current called the synchronizing current'.' Show that the 

 action of this current is such as make the parallel operation of alternators 

 a condition of stable equilibrium. 



24. If the field of one of two alternators operating in parallel is strength- 

 cue, 1. in what two ways is its internal electromotive force affected? Its 

 current? Why? How is the electromotive force and the current of the 

 other machine affected at this same time? Show that the reactions result- 

 rn changing these field excitations cannot change the kilowatt division 

 of load between the machines. What is the objection to having two 

 alternators in parallel, one operating with a leading current and the other 

 with a lagging current? 



26. Sketch the connections of a simple method which may be used to 

 show the proper time for switching alternators in parallel. How should 

 the voltage rating of the synchronizing lamps compare with that of the 

 ? How do such lamps indicate the relative phase relation of the 

 incoming machine and the bus-bars? When should the line switch be 

 doted?* 



26. State two disadvantages of the "three-dark" method of synchroniz- 

 ing. How may these disadvantages be in part eliminated by a different 

 grouping of the lamps? Why is the use of a synchronism indicator superior 

 to t! <>thods, especially with certain type> of alternators? 



27. What types of prime mover have pulsating torques? How may the 

 effect of these torque pulsations he magnified several times by direet-con- 



: alternators? Why is it undesirable that pulsations of frequency be 

 communicated to th- " State the general remedies which may be 



used to reduce "hunting" and the reason for the us- >f these. 



PROBLEMS ON CHAPTER VI 



81. A 5")0-volt. 50-kv-a. alternator has an effective armature re-i-tancc 

 of .is ohm :.nd an armature -if n.7.~> ohm. What i* the induced 

 electromotive force of this armature when the machine deliver- it 

 current at rated voltage and at unity power-factor? 



82. Repeat problem 81 for 0.8 p>v, lagging current; leading 

 cum 



83. The core loss friction and windage of the alternator of prob! 

 is 1,700 watte. The field nip. at 1 1.', volts. What i-. the ell 

 of this alternator at it- rat.-.l In 



84. The -vnchroii" >f the alternator of problem 8 



to the armature leakage i 'ig full non-inductive 



load lit rated vi.ltage and at unit (ind the QO-tOftd 



of the machine when this |oa<l i- thrown ofT. 



85. Find the no-load voltage- in problem X2. when t hew loads 

 off the a! 



86. I fcfofl f tl.. lit ions 



1 65? 



