90 SYNCHRONOUS MOTORS 



30 per cent. For this reason it is always desirable, in any extensive 

 installation, to consider the expediency of utilizing a sufficient number 

 of synchronous motors, to avoid this reduction of output. 



Comparison between Synchronous and Induction-Motors. The 

 preceding remarks show the great advantages which synchronous 

 motors present in comparison with induction -mo tors, in certain cases. 

 Their stability of operation is not inferior to that of induction-motors, 

 because a single-phase induction-motor, and even a polyphase induc- 

 tion-motor, also falls out of step when too much overloaded, unless 

 the secondary resistance has been made quite large, at the expense of 

 efficiency. The two points of inferiority of synchronous motors are, 

 first, the fact that they are not self-starting, and, second, their low 

 efficiency. In reality, their inferiority in these two respects is not 

 as great as supposed, because it is possible to make synchronous motors 

 which are self-starting without load, and as for their efficiency, it can be 

 made as high as that of induction-motors, if they are properly designed. 



On comparing a synchronous motor of Kapp type, with non- 

 laminated poles and expensive excitation, consuming, when running 

 without load, a current equal to one-third the full-load current, with 

 an induction -mo tor of the most perfect Oerlikon type (see article of 

 Kolben, Elektrotechnische Zeitschrift, Nov. i, 1894) it is not difficult 

 to establish the superiority of the latter. Bu L , if, in the said induction- 

 motor, the bars of the rotor were to be transformed into a D.C. exciting 

 circuit, it would be observed that the difference in efficiency becomes 

 very small. 



Synchronous motors have a great advantage in construction because 

 they admit of much higher magnetic densities than induction-motors. 



[Use of Synchronous Motors to Raise Power-Factor in America. 

 The compensating action of over-excited synchronous motors is 

 utilized extensively in America for improving the power-factor of 

 distribution-systems. 



As an interesting example of the practical application of synchro- 

 nous condensers some figures obtained from the Detroit Edison 

 Company are given in Appendix C. C. O. M.] 



