ALTEENATOES. 193 



the triangular curve is quarter of its maximum value, or 

 three-quarters of its average value. 



Thus, unlike the case of a direct-current generator, the 

 voltage of an alternator depends not only upon the speed, 

 field strength, and number of armature conductors, 

 but also upon the distribution of field and grouping 

 of the conductors, since these affect the wave form of the 

 electromotive force generated, and consequently the virtual 

 value of the voltage. 



Voltage of an Alternator. In the armature of an alternator 

 with a winding so arranged that the conductors always 

 act in series (i.e., the conductors of one winding never 

 simultaneously have oppositely directed electromotive forces 

 induced in them) the average voltage with a closed armature 

 winding will be the same as for a direct-current dynamof i.e., 



NnZ 



av - 10 8 x 60 

 Hence for sine wave-form 



NnZ 



Where N = Number of armature conductors in series, 



7j = total number of lines passing through armature, 



from the north poles to the south poles. 

 n = revolutions per minute. 



1-11 = the ratio of the R. M.S. value to the average value 

 of the ordinates of a sine curve (see page 90). 



In a dynamo there are at least two parallel paths through 

 the armature and the number of conductors acting in series 

 is only half the total number of armature conductors. 



In an alternator no commutator is used, and the con- 

 ductors may be all connected in series, the ends of the winding 

 not being closed, but being connected to the slip rings or 

 terminals of the machine ; in this case the voltage is doubled, 

 since the number of conductors acting in series is doubled. 



For an open winding with sine wave form the formula 

 for the voltage of an alternator becomes consequently 



NnZ 



#- X 



fSee " Dynamo* and Motor Testing," page 25. Note. In the case of 

 alternators the armature conductors usually are connected in series, con- 

 sequently the formula is not affected by the number of poles if Z is the total 

 number of lines passing from all the north to south poles through the 

 armature. 



