lo-A] 



SYNCHRONOUS MOTOR. 



305 



2. Principle of Operation. In a direct-current motor the cur- 

 rent in a particular armature conductor is reversed by the com- 

 mutator and flows in one direction under a north pole and in the 

 opposite direction under a south pole, so that the torque is always 

 in the same direction. In a synchronous motor this reversal is 

 caused not by a commutator but by the alternations of the supply 

 and for the motor to run there must be one alternation for each 

 pole passed, i. e., the motor must run at synchronous speed ; at any 

 other speed the torque is alternately positive and negative and the 

 mean torque is zero. 



3. In a shunt (direct-current) motor,* the counter-electro- 

 motive force E' varies with the speed and, as the load changes, the 

 counter-electromotive force and speed so adjust themselves as to 

 allow an armature current to flow which produces just the right 

 torque for the particular load. The resultant of the impressed 

 electromotive force E and the counter-electromotive force E' is 

 their algebraic sum, and the armature current that flows is equal 

 to this resultant divided by the resistance of the armature ; or 



4. In a synchronous motor the speed is constant and the 

 counter-electromotive force E' is constant in value for a particular 

 field excitation. The phase of ', however, depends upon the 

 running position (or mechanical phase position) of the armature 

 and shifts with the load ; as the load changes, the armature drops 

 back or advances a few degrees and shifts the phase of E' so that 

 an armature current flows with a power component that produces 

 just the right torque (8) for the particular load. In other 

 words, the action of a synchronous motor depends upon the shift- 

 ing of the phase of the counter-electromotive force rather than 

 upon its change in value as in a shunt motor. 



The resultant of the impressed electromotive force E and the 

 counter-electromotive force E' is their geometric sum, Ez, and the 



* Compare 1-5, Exp. 2-A. 



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