i6o 



ELEMENTS OF ELECTRICAL ENGINEERING. 



be fallowed in getting machine B into operation as a synchronous 

 motor. Machine A being in operation, machine B is started 

 up by starting its driving engine, its' field is excited, and its speed 

 is adjusted until the synchronizing lamp shows (i) that B is in 

 synchronism with A and (2) that the electromotive force of B 

 is opposite (opposite, that is, with reference to the circuit formed 

 by the two machines) to the electromotive force of A t and then 

 the double-pole switch ss f is closed. If machine B is to be 

 operated as a synchronous motor its voltage should be somewhat 



To receiving circuit 



To receiving circuit 



Fig. 134. 



less than the voltage of A, but if B is to be operated as a 

 generator in parallel with A its voltage should be nearly the 

 same as the voltage of A. 



A number of alternating-current generators operating in 

 parallel must run at absolutely the same speed, or rather, at 

 absolutely the same frequency, and the possibility of operating a 

 number of alternating-current generators in parallel depends 

 upon the fact that when one generator runs momentarily slower 

 than the others its consequent change of phase relative to the 

 other machines causes its share of the station load to be greatly 

 reduced thus counteracting its'tendency to run at a reduced speed. 



The necessity of absolute equality of speed of a number of 

 alternating-current generators operating in parallel makes it 

 impossible for the governors of the several engines to operate 

 independently of each other, inasmuch as the independent opera- 



