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ELECTRICAL MACHINERY 



necessary to change a from zero to about 20 and for these 

 small values of a it is approximately true that the load on 

 the alternator is directly proportional to the angle a. Ex- 

 perimental results, illustrating this point, are given in 

 Fig. 167. It is shown there that for a given load increase 

 a somewhat smaller change in a is required when the gen- 

 erator is over-excited, than when it is under-excited. 



The electrical operator in a large power plant generally 

 has control of the steam supply to the various prime movers 

 so that he can by variation of the supply, shift the load 

 from one alternator to another as he desires. 



-4" 0" 10" 20 30 



Phase Position in Electrical Degrees 



FIG. 167. Curves Showing Phase Shift with Load Variation. 



It sometimes happens that two alternators will not operate 

 satisfactorily in parallel. The division of load is not con- 

 stant; the load shifts quickly from one machine to the 

 other and back again. The rush of current between the 

 two machines as the load shifts from one to the other may 

 be sufficient to open the circuit breakers and so throw 

 the two machines out of step. Such machines are said 

 to have a low synchronizing power whereas machines which 

 operate smoothly, maintaining a fairly uniform division 

 of load are said to have a high synchronizing power. 



