194 ALTERNATORS. 



The usual plan of stating the voltage of an alternator is 

 to put it in the form 



NnZ 



E - lonreo x * 



k being a constant depending upon the nature of the wave 

 form of the machine. 



In a machine with uniformly distributed and closed 

 winding, like a direct-current armature, the alternating 

 virtual voltage is only -707 of the continuous voltage which 

 the same armature would give if provided with a commu- 

 tator. This is due to the fact that only twice during each 

 period will the alternating voltage reach its maximum value, 

 and this voltage is the same as that continuously delivered 

 to the brushes situated on the commutator, and placed in such 

 a position as always to receive the maximum armature 

 voltage when the machine is acting as a direct-current 

 generator. 



Thus the alternating voltage (which in this case is 

 practically of sine wave form) is -707 of the maximum 

 voltage i.e., -707 of the direct-current voltage. 



The ratio of the R.M.S. value to the mean value is usually 

 known as the form factor. The form factor varies from about 

 1-1 to 1-4 for different types of machines. 



Periodicity of an Alternator. In most types of alternators 

 the magnets are alternately north and south when counted 

 in order round the armature. Thus each conductor passes 

 under a north and south pole alternately, and, in doing so, 

 experiences a complete cycle of changes in the electromotive 

 force induced in it. In such a machine the periodicity of 

 the current will be equal to the number of revolutions of the 

 armature (or field) per second x the number of pairs of 

 magnet poles or 



. ,. ., n p 



periodicity g( J 



n = revolutions per minute. < 



p = number of pairs of poles. 



In certain special machines the poles on each side of the 

 armature are all of the same polarity, and the electromotive 

 force induced in the conductors is due to thp passage of the 

 conductors through alternately weak and strong fields in 

 the same direction. In this" case the periodicity equals the 

 number of revolutions per second x the number of poles. 



