)ETAILED STUDY OF OPERATION WITH NORMAL LOAD 53 



(i) Suppose a given motor-load, and suppose the E.M.F.'s. EI 

 and E 2 to be nearly equal. The stability will first increase with the 

 reactance until we have 



i.e., until Z approaches zR; and it will then decrease if is greater. 



R 



Z 



Inasmuch as is always greater than 2 in ordinary alternators, any 



increase of inductance is objectionable in a motor which is connected 

 directly to the source of current. It could only be useful if the motor 

 was connected to the generator by a very long line of exceptionally 

 high resistance. 



(2) Suppose a given motor-load and suppose the external E.M.F. 

 EI to be constant. The stability of operation will always vary inversely 



with R; but the result will depend upon whether cos f be greater or 



/ X\ 



smaller than |, i.e., whether the reactance-factor ( tan r=-^ } will be 



_ \ K / 



higher or lower than \/3. When cos y >i an increase of the internal 

 E.M.F. E 2 until it exceeds the external voltage E\ will cause an increase of 

 stability, whereas, when cos y < ^ the increase will cause a reduction of 

 stability. This observation explains someapparent contradictions of ex- 

 perience. When a motor is connected on a constant potential line or on a 

 line of low resistance, it is more difficult to make it fall out of step when it 

 is over-excited. If, on the other hand, the same motor is used for power- 

 transmission over a line of high resistance, it will be observed that increas- 

 ing the excitation will also increase its tendency to fall out of step. 



In practice, the first case is the more frequent, and it is advisable, 

 in order to improve the stability, to increase the excitation with the 

 load either by hand or automatically. 



At the same time, it is seen that, in the most frequent practical cases, 

 it is necessary, in order to have good synchronous motors, to diminish 

 both the resistance and the reactance of the armature. This means 

 that the field must be powerful and the armature -reaction must be low. 



If we were to make Z constant by hypothesis, we would be led 

 to reduce as much as possible the resistance and to increase the 

 reactance; but this case does not occur in practice. 



Numerical Example. Let us take, for example, two Mordey alter- 

 nators of 37.5 kilowatts electrical output 1 coupled for electrical trans- 



1 The power available at the shaft must be reduced in proportion to the 

 efficiency (exclusive of the excitation-energy). 



