80 



quently reducing the plant-efficiency too 

 much. The investigation also showed 

 that, under the most favorable conditions, 

 the plant- efficiency of such a motor could 

 not be more than 70 per cent. This 

 figure would be obtained if the electro- 

 motive force of self-induction of the whole 

 machine were by some means reduced, so 

 much as to be only equal to the counter 

 electromotive force developed in the 

 armature, a condition which is extremely 

 difficult, if not impossible, to fulfill in 

 practice. For this reason, it would seem 

 that a motor of this type must always be 

 very much larger, more costly, and heavier 

 than a direct-current motor of the same 

 power. It has the advantage of being 

 self- starting, and not requiring any ac- 

 cessory apparatus for its excitation or 

 regulation. On the other hand, there is 

 the liability of the armature burning up, 

 if the motor should fail to overcome its 

 load almost at once. When the armature 

 is at rest, and an alternating current 

 passes through the machine, those arma- 

 ture coils which at the time happen to be 



