THE SYNCHRONOUS MOTOR. 



155 



73. Starting of the synchronous motor. Let the machine A 

 which is to operate as a generator be running with its field fully 

 excited ; then to put the machine B into operation as a syn- 

 chronous motor it has to be driven by some independent source 

 of power, its field has to be properly excited, and its speed has 

 to be carefully adjusted until (a) it is exactly in synchronism with 

 machine A, and (b] its electromotive force is nearly opposite in 

 phase to the electromotive force of A and when these two condi- 

 tions are realized the circuit of the two machines A and B may 

 be closed, and the independent source of power which was used 

 to start B may be disconnected. 



In bringing about the two conditions (a) and (b) previous to 

 the closing of the circuit of the two machines, it is necessary to 

 use some sort of a device for indicating when the two machines 

 are in synchronism and for indicating their phase relation. Such 

 a device is called a synchronising device or a synchronizer. 



The simplest synchronizer is an ordinary incandescent lamp 

 connected in the circuit of the two machines as shown in Fig. 131. 



Fig. 131. 



This lamp pulsates in brightness as machine B is speeded up, 

 and the pulsations of brightness become slower and slower as 

 the frequency of B approaches the frequency of A. When the 

 lamp is at its maximum of brightness the phase difference $ of 

 A and B, see Fig. 125, is zero, and when the lamp is at its 

 minimum of brightness the phase difference of A and B is 

 1 80. When the pulsations of brightness become very slow the 



