312 



SYNCHRONOUS MACHINES. 



[Exp. 



Supply Line 



so high that single lamps cannot be used directly across the switch 

 blades. In this case step-down transformers are often used, these 

 being connected as in Fig. 3 for synchronizing* with the lamp 

 dark or with one transformer coil reversed for synchronizing 

 with the lamp bright. A voltmeter or other indicator can be used 

 in place of the lamp. 



17. Synchronizing a Polyphase Motor. 

 If lamps are used, a lamp may be placed 

 across eachf switch blade as in the case of 

 a single-phase motor, the proper time for 

 closing the switch being indicated by the 

 lamps being dark. If all the lamps do not 

 become dark simultaneously, the motor is 

 running counter, to the direction it would 

 have if driven by the line. It is usually 

 not convenient or desirable to reverse the 

 direction of rotation of generator or motor, 

 but the leads may be interchanged in such a manner as to make 

 the cyclic changes of the electromotive forces of line and motor 

 similar. In the case of a 2-phase motor, this can be brought 

 about by reversing the leads to one phase of the motor; in the 

 case of a 3-phase motor, it is sufficient to interchange any two 

 leads to the motor. 



18. In order to have the lamps bright instead of dark at 

 the proper moment for closing the switch, in the case of a 4-wire 

 2-phase motor two terminals of the lamps on each phase can be 

 interchanged, as will be seen by considering the 2-phase motor 

 as consisting of two single-phase motors. A 3-phase motor can- 



* In synchronizing with the lamp dark (or bright) the connections should 

 be made so that the lamp will be dark (or bright) when the brushes on 

 the motor are lifted and the switch is closed. 



f Lamps across only two switch blades will be sufficient when it is known 

 (as in permanent installations) that the connections and direction of rota- 

 tion are correct. 



Motor 



FIG. 3. Transform- 

 ers for synchronizing. 



