CHAPTKi; X 



THE SYNCHRONOUS MOTOR 



123. The Synchronous Motor. It will be remembered that 

 the direct-current generator operates satisfactorily as a motor. 

 Moreover, there is practically no difference in the construction 

 of the direct-current generator and the direct-current motor, 

 ami there is no substantial difference in the rating of a machine 

 whether it is operated as motor or as generator. 



Similarly, an alternator will operate as a motor without any 

 changes being made in its construction. When so operated, the 

 machine is called a xiinchronous motor. 



The design of a synchronous motor and of an alternator, each 

 of the same rating and speed, may differ somewhat in details 

 owing to the desirability of securing the best operating character- 

 istirs for each. Moreover, synchronous motors are almost 

 always salient-pole machines, whereas alternators may be either 

 of the salient-pole or of the non-salient -pole type. 



124. Principles of Operation. Figure 2S1 shows a conductor a 

 under a north pole and carrying a current flowing towards the 

 observer. By the well-known law of motor action, a torque 

 dev lops tending to drive the conductor from left to right. If 

 the current be alternatii 



it will reverse its direc- 

 tion for the next half-cycle 

 and the torque then acts 

 from right to left . Theie- 

 ihe net toi'(|ue OVCr 

 any given number of 





Fio. 284. Tor.|ur cii-vi-lopnl !>y synchronous 



in' 



;o and no continuous motion can n-iilt. 'This 

 is the condition existing in a synchronous motor when at stand- 

 still. The armature conductors carry alternating current and 



Bed polarity. l>eiim excited \\ith direct current. 



Then-fore, the synohronoufl motor, as such, develops no starting 

 torque. 



20 ;).- 



