136 



SYNCHRONOUS MOTORS 



excitation-current, if the armature-core is slotted, but they also depend 

 on the phase-angle of the induced current, especially in the case of 

 single-phase motors. It can be readily understood that, in the case 

 of single -phase motors, the variation in the reaction-flux in front of the 

 pole is all the greater the more rapidly the armature-current varies in 

 the position of maximum mutual induction. For the same current in 

 the armature, the losses will, therefore, be greater the smaller the phase- 

 angle (lag) of the current. 



By the aid of these assumptions, the losses can always be approx- 

 imately separated, when the conditions of operation are precise, such 

 as when the current-supply has a constant voltage and the excitation- 

 current is constant. In such cases, the losses by friction are measured 



100% 



4O SO 6O 7O SO 90 



FIG. 68. 



by running the machine without excitation, by means of a small D.C. 

 motor whose efficiency curve has been obtained. On exciting the 

 machine, the loss becomes increased by an amount equal to the hys- 

 teresis and eddy currents, in the armature and the fields, other than 

 those due to armature-reaction. They may be distinguished, as usual, 

 by running at various speeds. The motor is then run with a load 

 applied at the brake. The difference between the losses noted and 

 those previously measured consists of two portions: the loss due to 

 resistance-heating in the armature, which can be easily calculated 

 from the resistance and the current, which are known ; and the increase 

 of loss due to armature-reaction, which is obtained, more or less 

 approximately, by taking the difference. 



