180 ELEMENTS OF ELECTRICAL ENGINEERING. 



89. Armature reaction of the synchronous converter. The 



magnetizing action of the current in the armature of a dynamo is 

 called armature reaction, and armature reaction has in general 

 two distinct effects, namely, (a] It crowds the flux to one side or 

 the other of the pole pieces of the field magnet, and (&) it tends 

 to weaken (or strengthen) the field. The first is called distortion 

 and the second is called demagnetizing action. The armature 

 reaction of a synchronous converter is generally quite small and 

 therefore of but little practical importance. 



In discussing the armature reaction of a synchronous converter it is permissible to 

 consider that the alternating current which is delivered to the machine and the direct 

 current which is delivered by the machine each flows through the armature windings 

 as if the other did not exist. 



Distortion. The crowding of the flux to one side or the other of the pole pieces 

 of a dynamo accompanies and is in a certain sense the cause of the torque with which 

 the field magnet acts upon the armature. When a synchronous motor is running 

 steadily there is only sufficient torque action to overcome mechanical and magnetic 

 friction and the distortion of the field is negligibly small. When a synchronous con- 

 verter hunts its speed pulsates and a very considerable torque acts upon the armature, 

 first in one direction and then in the other, and the flux is crowded to one side and 

 then to the other side of the pole pieces. This action is discussed in Art. 72. 



Demagnetizing action. Since there is no perceptible field distortion in a syn- 

 chronous converter the axis of direct-current commutation is approximately at right 

 angles to the axis of the field, so that the direct current in the armature has no per- 

 ceptible demagnetizing action, positive or negative. See pages 151 to 161 of the first 

 volume of this text. 



The demagnetizing action of the alternating current in the armature of a synchron- 

 ous converter depends upon the phase difference between B and /, these letters being 

 used in the sense in which they are used in Chapter VIII. When B and / are 

 exactly opposite to each other in phase, that is when the angle (BI] is equal to 

 180, there is no perceptible demagnetizing action, positive* or negative, due to the 

 alternating current in the converter armature. When the angle (BI] differs from 

 1 80 the demagnetizing action of the alternating current in the armature is propor- 

 tional to the sine of this angle as explained in Art. 65. 



90. Electromotive force relations of the synchronous converter. 

 There is a fairly definite relation between the value of the electro- 

 motive force between the direct-current brushes of a synchronous 

 converter and the effective value of the alternating electromotive 

 force between collector rings. The following discussion of these 

 electromotive force relations is based on the assumption that the 

 alternating electromotive force of the machine is harmonic and 



