SYNCHRONOUS MOTORS. 287 



be assumed to be equal unless a statement to the con- 

 trary is made. 



Case I. Two Alternators in Parallel without Load. When 

 running in parallel connected in the manner shown in 

 Fig. 122, each alternator will tend to send current in 

 opposition to the other machine round the circuit which 

 connects the two machines together. There will thus be two 

 opposite electromotive forces acting on the circuit, and so 

 long as the machines remain truly in parallel (i.e., in exact 

 opposition of phase as regards the local circuit joining 

 them) and give equal voltages, there will be zero resultant 

 electromotive force, and no current. 



When represented graphically, the voltages of the 

 alternators would form two lines OE V and OE exactly 

 equal, and opposite to each other. The resultant of these 

 lines would be zero. 



If one alternator begins to lag behind the other, the 

 two electromotive forces no longer remain opposite in 

 phase, and the conditions in the circuit joining the 

 machines then become similar to that shown in Fig. 136, 

 where the alternator voltage E, 2 has assumed the posi- 

 tion shown instead of its original position indicated by 

 the dotted line. 



The resultant voltage acting on the circuit is now seen 

 to be OE T) which is the diagonal of the parallelogram 

 drawn with E l and J 2 as sides. 



In this case there will be a current in the circuit, the 



resultant voltage 



value of which is . 3 ? . ., Since the resis- 



impedance of circuit. 



tance of the conductors between the alternators will be very 

 small, the value of the current will be practically 



half of resultant electromotive force 

 impedance of armature of one alternator. 

 The phase of the current can be determined by resolving 

 this resultant voltage into its components. In order to 

 do this, we must know either the energy or idle component 

 of the current. 



Usually the energy component is not easy of deter- 

 mination, since, in addition to the component spent in 

 overcoming the resistance of the circuit and armatures, there 

 may be an unknown energy component spent in iron 



