CHAPTER IX 

 THE SYNCHRONOUS MOTOR 



79. Feasibility of Running an Alternator as a Motor. 

 If two alternating-current generators are operating in par- 

 allel on the same bus-bars and the driving power is taken 

 away from one of them, it will (in general) continue to run, 

 at exactly the same speed it had before the driving power was 

 taken off. 



Suppose two engine-driven generators, operating in 

 parallel, are running at 720 r.p.m. An accurate speed 

 indicating device is put on one of them so that its speed can 

 be read, then the steam is shut off from the engine to which 

 this generator is connected. We would naturally expect 

 the generator to slow down and stop if its driving source 

 is removed but, by watching the speed indicator while the 

 steam is being shut off from the engine, we may see that 

 the alternator not only does not stop but its speed never 

 changes while the steam is being shut off. The speed does 

 not even drop to 719.9 r.p.m. but remains at 720 r.p.m.* 



Reversal of Operation. Now a machine cannot rotate 

 unless it is being supplied with power, and as the steam 

 engine is delivering no mechanical power it is evident 

 that the machine must be receiving electrical power. Sup- 

 pose two alternators operating in parallel as shown in 

 Fig. 200 and that the power output of No. 2 is indicated by 

 the wattmeter W- It is supposed that W is so connected 

 that when alternator No. 2 is helping No. 1 to carry the load 



* This statement holds good only if the speed of the other alter- 

 nator is held constant during the operation. 



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