OPERATION OF SYNCHRONOUS MOTORS. HUNTING 125 



In the particular case where the motor is supplied from a circuit of 



Pi 2 



reat output-capacity the term ~ can be considered as very small in com- 



AI 



2 



Darison with -, and, consequently, the expression for the period of 

 A 2 



le oscillations, as a function of the constants of the machine and of 

 ic inertia of the armature and of the parts mounted on the shaft, is 

 laterially increased. 



This formula shows that the oscillations are all the more rapid 

 ind of lower amplitude the lower the self-induction and the mechanical 

 icrtia. 



Consequently, from this point of view, it would be more profitable 

 select motors with a low armature-reactance, a conclusion which we 

 lad already reached by another process. The oscillations not only 

 lave the effect of causing the motor to fall out of step, and of thus 

 limiting its maximum output, but also of producing oscillations of 

 and current, including change of phase (lag). These effects 

 can manifest themselves in a very objectionable manner, on the circuit 

 and on the generator, by producing fluctuations of voltage which are 

 often important, especially when the reactance of the circuit is large 

 and the circuit is thereby made very sensitive to variations in wattless 

 current. 



A synchronous motor which is osculating can therefore cause trouble 

 to its neighbors on the circuit, and it may even render lighting from 

 that circuit impossible. For this reason, great care should be taken 

 to regulate perfectly the speed and phase of the motor by means of a 

 phase-indicator, before connecting it to the supply-circuit. 



Every sudden change in load gives rise to a strong oscillation in 

 the phase-angle, causing 3 to go beyond the position of equilibrium 

 corresponding to the new value of the power. If this oscillation comes 

 from a reduction in load, it can cause no inconvenience; if, on the 

 contrary, it is caused by an excess of load, the phase-variation may, 

 in the first impulse, exceed the angle of stability, and the machine will 

 fall out of synchronism. 



Calculation shows (A. Blondel, loc. cit., Lumiere Electrique, 

 Vol. XLV, p. 24) that the relative increase of load J which can be 

 made suddenly, starting from an initial power ( P^ 0o does not depend 

 on inertia, but only on the two ratios 



E 2 , 



y and C os0=*; 



