lo-A] 



SYNCHRONOUS MOTOR. 



Swrtch 



Switch 



FIGS. 4 and s.t Lamps for 

 indicating " fast " or " slow." 



not be synchronized with the lamps bright without the use of 

 transformers. 



19. Synchroscopes. There are various commercial synchro- 

 scopes* which not only indicate when the motor is running syn- 

 chronously and is in the proper phase but also show whether the 

 motor is running too fast or too slow 

 when it is not in synchronism. Knowl- 

 edge in regard to the latter saves time 

 and annoyance in bringing the motor 

 to the proper speed. 



20. Lamps for Indicating When 

 Motor is Fast or Slow. When a poly- 

 phase motor is near synchronism, 

 lamps can be used to show whether it 

 is running too fast or too slow. (When 

 the speed is much too high or too low, however, the flicker of the 

 lamps will be too rapid to follow.) 



Figs. 4 and 5 show two methods of connecting lamps for a 

 3-phase motor. When the motor is in the proper phase for clos' 

 ing the switch, a particular lamp (the lower one) will be dark 

 and the other two will be of equal brightness. If the speed is 

 not synchronous, one lamp after the other will be dark, the 

 sequence depending on whether the motor is too fast or too slow. 

 With the lamps arranged in a circle, or triangle, this gives the 

 appearance of rotation in one direction or the other. Other 

 arrangements of lamps can be made to give similar effects with 

 polyphase motors of any number of phases. 



*(iQa). Automatic synchronizers are also used. For description of 

 synchroscopes and synchronizers, see hand and text books; also Electric 

 Journal, Vol. V., p. 538, where other references are given. 



fFig. 5 shows the speed-indicating lamps only; although not shown, a 

 lamp is required across each switch blade making five lamps in all. 



