THE SYNCHRONOUS MOTOR. l6l 



tion of the governors requires the speed of each engine to vary 

 with its load. 



Sharing of load between two engine-driven alternating-current generators 

 operating in parallel, (a) When one engine only is governed, the other engine 

 being set at a fixed cut-off. In this case the power output of the alternator which is 

 driven by the engine with the fixed cut-off is constant, and the variations of the power 

 output of the station are met by the governed engine. If the station output falls 

 below the constant output of the alternator which is driven by the fixed cut-off engine, 

 the other alternator takes in power as a synchronous motor, it may even take in 

 enough power to drive its engine ; and if the station output falls too low the fixed cut- 

 off engine may cause the entire system to race. This arrangement is seldom used in 

 practice. 



(b} When both engines are governed the distribution of load between them is 

 approximately as follows : Let a be the zero-load-speed of engine A, and let b 

 be the zero-load-speed of engine B. Let s be the common speed of both engines 

 when they are driving the two alternators. Let P / be the power delivered by engine 

 A to its alternator, and P ff the power delivered by engine B to its alternator. 

 Then, approximately: 



P* = m(a s} (i) 



and 



P = n(l,-s) (ii) 



These equations are based on the assumption that the speed of a governed engine 

 falls off in proportion to its output. The quantity /// is obtained by dividing the full- 

 load output of engine A by its full-load drop in speed. The quantity n is found by 

 dividing the full-load output of engine B by its full-load drop in speed. 



The total station output determines the combined output P f -j- P" of the two 

 engines, and equations (i) and (ii) determine / >/ , P /f and s. 



The engines share the load equally only when their zero-load speeds are equal and 

 when their full-load drop in speed is the same. 



78. The efficiency of transmission of power by means of an alternating-current 

 generator and a synchronous motor. The ratio P /f '/ P / of the motor intake to the 

 generator output is called the efficiency of transmission. This is not the net efficiency 

 of the system inasmuch as power consumed in field excitation of the two machines 

 and power lost by friction and by eddy currents and hysteresis are not considered. 

 The conditions necessary for maximum efficiency of transmission depend upon which 

 of the quantities A, B, X, R and P" are open to choice or capable of adjustment. 

 The quantities X and R are ordinarily fixed in value, while A and B may be 

 changed more or less by varying the field excitation of the respective machines, and 

 P" may be changed by varying the load on the motor. 



I. When the electromotive force A of the generating alternator is adjustable, 

 maximum efficiency is obtained when the current (and also RI Z ) is a minimum; 

 values of B, X, R and P ff being given. This minimum current is obtained when 

 A is adjusted until B and I are opposite to each other in phase. 



Proof. The motor intake is P" = BIcos (BI), according to equation (iv), Art. 

 69. Therefore, since P ff and B are given, the minimum value of / corresponds 

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