282 



ELECTRICAL ENGINEERING 



until it takes its proper share of the load. Lamps are not very 

 satisfactory since they do not show whether the incoming machine 

 is running too fast or too slow. 



171. Effect of Inequality of Terminal Voltage. If two alter- 

 nators have the same frequency and are in phase but have not 

 their fields adjusted to give the same terminal voltage, a watt- 

 less current will flow between the two machines leading and 

 magnetizing in the machine of lower field excitation and lagging 

 and demagnetizing in the machine of higher field excitation. 



If EZ the voltage of B is lower than EI the voltage of A then 

 E' the difference of the two will act in the local circuit through 

 the two armatures in series and will produce a current I f lagging 

 nearly 90 degrees behind E' and EI and leading EZ, Fig. 260. 



The circulating current is 



E' 

 I f = -A (269) 



where z\ r\ + j%i is the synchronous impedance of A and 

 z 2 = ?*2 + jx 2 is the synchronous impedance of B. 



This current lowers the terminal e.m.f. of A since it is lagging 

 in A and raises the terminal e.m.f of B since it is leading in B. 



The total current in A will lag behind the current in the load 

 circuit and the current in B will lead the current in the load cir- 

 cuit. In Fig. 260 



OEi = EI is the terminal e.m.f. of A before synchronizing. 



OEz = EZ is the terminal e.m.f. of B before synchronizing. 



OE f = E' is the e.m.f. which produces the circulating current. 



OI f = I' is the circulating current. 



01 ^ = 1 1^ is the load current of A. 



01 L, = !LZ is the load current of B. 



OIi = 1 1 is the total current of A. 



0/2 = /2 is the total current of B. 

 01 = I = I ^ + Ii* is the load current. 



