.LVD PROBLEMS 4S1 



2. Show by a simple sketch that the inductors of an alternator armature 

 have inductance. Compare the relative inductances, other conditions 

 being equal, of (a) a smooth-core armature; (6) an iron-clad armature in 

 which the slots are deep and narrow; (c) an iron-clad armature of the same 

 num her of slots but in which the slots are shallow and broad; (d) an iron- 

 clad armature having semi-closed slots. 



3. What is the effect of the numl>er of conductors per slot upon the arma- 

 ture inductance? How does the reactance of a 25-cycle armature compare 

 with that of a 60-cycle armature, other conditions being equal? 



4. (live two reasons why the resistance of an alternator armature to 

 alternating current is greater than its resistance to direct current. What 

 is the order of magnitude of this increased resistance? How may this 

 effective resistance be determined? 



5. What is the effect of the current in an alternator coil upon the main 

 field. (u> When the current is in phase with the induced electromotive 

 force? (6) When the armature current lags the induced electromotive 

 force by 90? (c) When the current leads the induced electromotive force 

 by 90? (d) When the current lags and leads the induced electromotive 

 force by an angle d? 



6. Compare the effects of (5) with corresponding effects in direct-currant 

 machines. Under what conditions does the armature magnetomotive force 

 for a given armature current have its greatest effect upon the main field of 

 salient-pole alternators? 



7. >how by a vector diagram how the induced electromotive force in an 

 alternator armature may be calculated knowing the terminal voltage, the 

 armature resistance drop and the armature reactance drop, (a) When the 

 power-factor of the load is unity. (6) When the run-out lags the terminal 

 voltage by an angle 0. (c) When the current leads the terminal voltage by 

 an angle 0. Give the trigonometric solution of the diagram in every case. 



8. Why is tho induced electromotive force in an alternator armature, when 

 loaded, not equal to the no-load voltage? Why are open-circuit and 

 short-circuit tests used in obtaining data for calculating alternator regu- 

 lation rather than actually loading the machine? 



9. How is the armature reaction taken into consideration in the synchro- 

 im|K'dance method of determining regulation? 



10. Show that when a coil has moved '.HI electric:; Liroes from 

 mt where the flux linking it is a maximum the induced electromotive 



force becomes a maximum. Distinguish between a space diagram and a 

 Wwi diagram. When can the two be combined ? 



11. \\ hy i- it rational to combine the -pace magnetomotive force diagram 



of an alternator with the time-voltage ami tin "f the same 



M? What is the phase relation exi-ntm 1 



current and the armature maunetomot r ho. result an' li- 



the induced ele< -. force; (< the impressed field and the no-load 



12. Show leading ' 



by 90, and m pha.-e \\ith tin- voltage which balance* the am 



