* 

 SYNCHRONOUS MOTORS. 275 



by the current remains constant in direction rela- 

 tive to the field magnets. This condition means rather 

 more in the case of an alternating motor than 

 it does with a direct-current machine. Its meaning 

 in this case is that the armature current must attain its 

 maximum value when the armature is in the same relative 

 position as regards the fields, i.e., the difference in phase 

 between armature current and back electromotive force 

 (the phase of which depends only on the position of the 

 armature) must be constant. Consequently, if the phase 

 of the armature current changes, the turning effort pro- 

 duced by a given armature current and field will change 

 also. 



This may be put in another way by saying that it 

 is only that component of the armature current which is 

 in phase with the back electromotive force of the motor 

 which can be looked upon as producing the torque, and 

 the component which is out of phase with this is 

 inoperative. 



Thus, if <j> be the angle of phase-difference 

 between the armature current and back electromotive 

 force of the armature, and if C be the total armature 

 current, then the component which may be looked upon 

 as producing the turning moment on the shaft is C cos 0. 



Thus, with a constant load on the motor shaft, the 

 product (field x C cos <P) must have a definite 

 value and remain constant for this load, and if the 

 field is varied the factor C cos <t> will change inversely in 

 order that the load may continue to be overcome. With 

 a constant value of </> the load will be proportional to the 

 product (field x current) exactly as in the case of the 

 direct-current motor with fixed brushes. 



Further, in the case of the alternating-current motor 

 the armature current 



~ _ resultant of applied and back voltage 



armature impedance 



in which the resultant voltage is the resultant, obtained 

 by the parallelogram law, of the applied and back volts. 

 The armature current will be this voltage divided by the 

 impedance of the armature. 



If the excitation be changed, a synchronous motor 

 cannot change its speed like the direct-current motor, 



