164 ALTERNATING CURRENTS 



Two alternators 1 and 2 are operating in parallel as shown in 

 Fig. 162 (a). If they are operating in parallel they must have 

 the same frequency and the same terminal voltage. Figure 162 

 (6) shows the speed-load curve of each of the prime movers 

 driving the alternators. (Instead of plotting speed in r.p.m., 

 the frequency or electrical speed is plotted. For example, a 

 six-pole alternator running at 1,200 r.p.m. would have the same 

 electrical speed as an eight-pole alternator running at 900 r.p.m.) 

 The speed-load curves of the prime movers are determined by 

 their respective governors, if they are steam-, water-, or gas- 

 driven units. If motor-driven, the speed-load characteristics 

 depend upon the motor speed-load characteristics. 



Let oc, Fig. 162 (6), be the frequency at which the system is 

 operating. By projecting horizontally to intersect the speed- 

 load curves, the load taken by each machine at this frequency is 

 obtained, oa is the load on machine 1 and ob is the load on 

 machine 2, as both machines are operating at system frequency. 

 Let the field of 1 be strengthened by means of its field rheostat. 

 At the same time weaken the field of 2 so that the line voltage 

 does not change. If these were direct-current generators, ma- 

 chine 1 would immediately take more load. But 1 cannot take 

 more load because its prime mover can deliver only the load oa 

 at this frequency. Machine 2 cannot drop any load because its 

 prime mover can deliver only the load ob at this frequency. 

 Therefore, the kilowatt load delivered by alternators in parallel can- 

 not be shifted appreciably by means of the generator fields. 



To change the kilowatt load of either machine, the speed-load 

 characteristic of its prime mover must be changed. In engine- 

 driven units this is done by changing the tension in the governor 

 spring or altering in some manner the governing device. Assume, 

 in Fig. 162 (6) that it is desired to make generator 1 take the same 

 load as 2. The governor spring of 1 is so adjusted that the charac- 

 teristic of 1 is raised, as shown in Fig. 163. Both machines now 

 deliver the same load oa' at a frequency oc'. Under the conditions 

 shown, Fig. 163, the frequency oc' is higher than the original 

 frequency oc, Fig. 162. If the original frequency is to be main- 

 tained, the speed-load characteristic of 2 must be lowered at the 

 same time that the characteristic of 1 is raised. Therefore, to 

 adjust the load between alternators in parallel, the speed-load 



