lo-A] SYNCHRONOUS MOTOR. 3O/ 



an over-excited motor takes a leading current; this weakens the 

 field and decreases the motor electromotive force until it is equal 

 to the electromotive force of the line. The operation of a syn- 

 chronous motor is thus explained by the magnetomotive force, or 

 ampere-turn, method. 



7. When the current in an armature conductor is in phase 

 with the electromotive force, the current comes to its maximum 

 value when midway under a pole piece, as does the electromotive 

 force (see Fig. 9, Exp. 3~B) and the current does not strengthen 

 or weaken the pole. When, however, the current is out of phase 

 with the electromotive force, it comes to its maximum before or 

 after reaching the middle of the pole and either strengthens or 

 weakens it (see Fig. 10, Exp. 3~B). It is the quadrature or 

 wattless component (7sin0) and not the in-phase or power 

 component (7cos0) that has the magnetizing and demagnetiz- 

 ing effect. 



The excitation of a synchronous machine depends not only 

 upon the field current but also upon the wattless component of 

 the armature current. Any change in the former makes a corre- 

 sponding change in the latter, the combined effect of the two 

 always being just sufficient to cause the counter-electromotive 

 force of the motor to equal the electromotive force of the line. 



8. Torque and Power. Torque is proportional to power 

 divided by speed (3b, Exp. 2-A). In a synchronous motor, 

 since speed is constant, torque is proportional to power, and cer- 

 tain conclusions can be reached either from a consideration of 

 torque or from a consideration of power. At constant voltage, 

 the torque (neglecting losses) is proportional to the power com- 

 ponent of current. 



When the current and the electromotive force are in phase, 

 their product that is, the instantaneous power is always posi- 

 tive ; also, since in this case the current in an armature conductor 

 remains in one direction while passing a pole, the instantaneous 



