8 



SYNCHRONOUS MOTORS 



The effect of this wattless current is, therefore, to produce, in the 

 motor, a supplemental positive or negative E.M.F., which adds itself 

 to its own induced E.M.F., in such a way as to produce, at the terminals, 

 a difference of potential equal to that of the generator. We can con- 

 clude from this, without further argument, that when the motor gen- 

 erates an E.M.F. which is too low, the current of the generator tends 

 to over-excite it and that, in the contrary case, it tends to under-excite it. 



The action of the current on the generator itself produces inverse 

 effects. 



The effects are more complicated still when resistances or induct- 

 ances are added in the circuit between the machines, with the general 



Excitation Current Amperes 

 FIG. 6. 



effect of lowering the voltage. Synchronous operation remains possible, 

 nevertheless, even when the resistance attains high values. 1 



When the circuit includes reactance it is observed that, by over- 

 exciting the motor, the voltage will be raised at its terminals, and even 

 at the terminals of the generator, so as to attain values which are higher 

 than the E.M.F. of the generator, measured with open circuit. On the 

 contrary, by under-exciting the motor, it is possible to produce increas- 

 ing and rapidly exaggerated voltage-drop along the whole line. 



1 When the resistance is high, say 75 per cent, it may be necessary to 

 add s<>nu- external reactance, so as to cause the current to lag more behind 

 the cross E.M.F. (E. J. B.) 



