176 



ELEMENTS OF ELECTRICAL ENGINEERING. 



The details of motor and generator action are of little practical 

 importance and they are described in the following paragraphs of 

 this article, but the magnetizing action and heating effects of the 

 armature current are important, inasmuch as they determine the 

 power rating of the machine. 



Motor and generator action in the synchronous converter.* Let a and /$, Fig. 

 149, be the direct-current brushes of a synchronous converter. Let the negative brush 



b be chosen as the zero or refer- 

 ence point of potential so that the 

 electromotive force between b 

 and any other given point may be 

 spoken of as the potential at the 

 given point. 



Let the half-circle represent 

 one side of the armature, and let 

 r be the position at a given in- 

 stant of the point of attachment 

 of one of the collector rings. At 

 the given instant a definite current 

 i is entering the armature at r 

 from the alternator, which is driv- 

 ing the converter ; this current is 

 entering at a potential which is 

 less than the potential of the brush 



Fi 149 a, a portion B of the current 



i flows down hill (against the 



induced electromotive force in the windings, which electromotive force is repre- 

 sented by the fine line arrow) to brush b and delivers power to the portion rb of 

 the armature in the form of motor action, and the remainder A of the current i 

 flows up hill (with the induced electromotive force) to brush a and receives power 

 from the portion ra of the armature in the form of generator action. That is to say 

 at each instant certain portions of the armature of a synchronous converter have motor 

 action and develop torque in the direction of the motion of the armature, and other 

 portions of the armature have generator action and develop torque tending to oppose 

 the motion of the armature. In the two-ring converter the motor action at times 

 greatly exceeds the generator action, at other times the generator action greatly 

 exceeds the motor action, and the armature is alternately accelerated and retarded. 

 This must be so because the single-phase alternating current delivers power to a two- 

 ring converter in pulses. In a polyphase converter, however, the motor action is 

 at all times very nearly equal to the generator action, being just enough in excess to 

 supply the mechanical and magnetic friction losses in the motor. 



* See a paper on "The energy transformations in the synchronous converter," by W. 

 S. Franklin, Trans. Am. Inst. Elec. Eng., Vol. 22, pages 17-33, I 93- 



