SYNCHRONOUS MACHINERY 



281 



If the field winding is short circuited while coming up to syn- 

 chronism these large induced voltages will not exist but the starting 

 torque of the motor will in general be reduced. The short-cir- 

 cuited field winding tends to act in somewhat the same way as the 

 grids but its inductance is so high that the flux produced through 

 it by the armature m.m.f . is small and this reduction of the flux in 

 the field poles more than counterbalances the torque due to in- 

 duced currents. In the majority of cases, however, it is better 

 to start the motor with the field winding short circuited since the 

 safety of operation is thereby increased. When synchronous speed 

 is reached the full excitation should be applied and then the 

 impressed voltage raised to full value. 



170. Parallel Operation of Alternators. Before an alternator 

 is connected in parallel with another machine which is supplying 

 power, the incoming machine must be adjusted to give the same 

 voltage, must have the same frequency and must be in phase. 



Load 



Synchronizing 

 Lamps 



Incoming 

 Machine 



FIG. 259. Parallel operation of alternators. 



The condition of synchronism may be indicated by incandescent 

 lamps or by some form of synchroscope, Art. 176. 



In Fig. 259 when the incoming machine B is in synchronism with 

 A there is no voltage across the switch and the lamps are dark. 

 The two machines are assumed to have been adjusted to give 

 equal voltages. 



If the frequency of B is lower or higher than that of A, there will 

 be a slow pulsation of the light showing the difference between the 

 frequencies. When the pulsations are very slow and the periods of 

 darkness long the switch may be closed and the field of B adjusted 



