262 ELECTRICAL MACHINERY 



done because alternators operating in series are in unstable 

 equilibrium, unless the machines are mechanically coupled 

 together. If the two alternators (not mechanically coupled) 

 are connected in series to a load, the voltage on the load 

 will continually vary from zero to twice that of one alter- 

 nator. It is not thought worth while to further analyze 

 this point here as it has no commercial importance. 



In considering alternators operating in parallel we 

 have three points to investigate. What precautions are 

 necessary in connecting an alternator to bus bars already 

 alive? What will happen if the generated voltage of this 

 machine is made greater or less than that of the bus bars 

 to which it is connected? (We emphasize the word generated 

 because, of course, the terminal voltage of the alternator 

 cannot be made different from that of the bus bars to 

 which it is connected.) How may the load be divided 

 between the different alternators, as desired by the opera- 

 tor? These three points will be taken up in the order 

 named. 



65. Synchronizing. Suppose several alternators are 

 already operating in parallel on the same bus bars and 

 another alternator is to be connected to these same bus 

 bars; what conditions must be fulfilled before the switch, 

 connecting this incoming machine to the bus bars, may be 

 closed? They may be briefly stated thus: 



1st. A machine voltage equal to the voltage of the line 

 (bus bars). 



2d. A frequency for the machine which is the same as 

 that of the line. 



3d. The phase of the machine voltage opposite to that of 

 the line. 



4th. A machine e.m.f. wave form similar to that of the line. 



The first three conditions can be satisfied by various 

 manipulations which the operator can carry out but the 

 fourth cannot be accomplished by any variation of speed, 

 field excitation, etc. 



