CHAPTER IX. 



THE CONVERSION OF ALTERNATING CURRENT INTO 

 DIRECT CURRENT. 



84, The conversion of alternating current into direct current. 



It is frequently necessary to utilize in the form of direct current 

 the power which is delivered by an alternator, or, in other words, 

 to convert alternating current into direct current. Thus, if the 

 field of an alternator is to be compounded, it is necessary to 

 rectify the alternating current in the series field winding of the 

 machine, as explained in Art. 2. If the power delivered by an 

 alternator is to be used for charging storage batteries it must be 

 used in the form of direct current. If power, which is trans- 

 mitted to a distance in the form of alternating current, is to be 

 used to drive direct-current motors, such as are extensively em- 

 ployed in electric railway operation, it must be converted from 

 alternating current to direct current. This conversion from alter- 

 nating current to direct current is accomplished in practice in 

 four ways ; namely, (a) by the rectifying commutator, (ft) by the 

 motor-generator, (c) by the synchronous converter and (d) by 

 the mercury -arc rectifier. 



(a) The rectifying commutator. The ordinary direct-current 

 generator is essentially an alternator with a commutator arranged 

 to properly reverse the connections between the armature wind- 

 ings and the receiving circuit so that, although the current in a 

 given armature conductor is an alternating current, the current 

 in the receiving circuit is always in one direction. The commu- 

 tator of a direct-current generator, however, reverses the connec- 

 tions of successive small portions of the armature windings, 

 whereas the rectifying commutator, strictly so-called, which is 

 described in Art. 2, reverses the connections of the armature 

 winding of an alternator as a whole. The rectifying commutator 



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