SYNCHRONOUS MOTORS. 289 



If the machines are connected in series, i.e., terminals 

 of unlike sign together, the conditions will be unstable, since 

 a lagging machine would receive currents tending to retard 

 instead of to accelerate it. The machines would thus not con- 

 tinue to work in series (unless mechanically coupled together), 

 but would rapidly alter their phase relations until they run 

 in parallel. If two machines connected as in Fig. 122 are 

 connected in series they will, of course, produce a short 

 circuit in the connections between the machines. They 

 could only work in series with each other when also in series 

 with the load circuit. 



Case 2. Synchronous Motor Driven by Alternator. When 

 the motor is excited so as to give a voltage equal to that 

 of the alternator, the case is exactly similar to that 

 just described. The motor will always lag slightly 

 behind exact opposition in phase to the alternator ; but 

 since it runs synchronously, its back electromotive force 

 will be constant and equal to that of the alternator. The 

 phase relations and value of the current flowing between 

 the machines will be the same as those discussed in 

 Case 1. The phase and amount of the current will vary 

 with the load on the motor, which will lag behind the 

 generator by exactly the amount necessary to give the 

 current its value necessary to overcome the load. 



It will frequently occur, however, that owing to 

 armature reactions and other causes the back voltage of 

 the motor becomes less than that of the alternator. In 

 such a case the electromotive forces of the circuit become 

 those represented in Fig. 137, where the motor voltage 

 represented by E 2 is smaller than OE^ The resultant 

 voltage is found as before, and from this the phase of the 

 current is determined as already described. Again, the 

 power given out by the alternator is the product (current 

 x E^ x cos POEJ, while the watts absorbed by the 

 motor = current x E^ x cos P E^. 



Fig. 137 is drawn with the voltages of the two machines 

 in the same phase relations as Fig. 136, the only difference 

 between the diagrams being the decreased voltage of OE^. 

 The effect of this change is seen to be the rotation back- 

 wards of the current vector OC, so that the current in Fig. 

 137 is almost in phase with the voltage OE t . The power 

 given out by the alternator is seen to have increased, since 

 the component current. OCt 1 is greater. The power given 



