220 GENERAL DIAGRAMS FOR SYNCHRONOUS MOTORS 



reduce the voltage-drop in the feeders to a small proportion, i.e., 2 to 

 3 per cent of the normal working voltage. 



INHERENT OSCILLATIONS OR PUMPING OF CONVERTERS 

 CONNECTED IN PARALLEL 



Speed-oscillations can be produced in rotary converters, same 

 as in synchronous motors, by periodic speed-variations in the 

 alternators supplying them with current, when these alternators are 

 driven by steam engines whose angular velocity varies at different 

 portions of each piston-stroke ; and they may also be produced by sud- 

 den changes or fluctuations of load. These speed-oscillations in 

 rotary converters may even have a much greater amplitude than in 

 other machines, because they are not opposed by a contrary variation 

 of the torque acting upon the armature-shaft. As already stated, 

 when the speed of a converter increases, its current is thrown out of 

 phase and becomes leading with respect to the induced E.M.F. This 

 causes the magnetic field to become stronger and the induced E.M.F., 

 to rise; consequently, the secondary (direct cxirrent) output increases. 

 The primary (input) power increases, however, in substantially the same 

 proportion; hence, there is no other variation in the torque acting 

 upon the armature shaft than that which can result from the varia- 

 tions of the losses with the load; and this effect is always quite small 

 in comparison with the inertia of the armature. 



The amplitude of the oscillations may, therefore, be very large; 

 sometimes it is sufficient to make the converter " fall out of step." 

 More often, its effects are made evident by fluctuations in the (D.C.) 

 E.M.F. of the converter which have amounted to as much as 30 per 

 cent and also by sparking at the commutator. 



The E.M.F. fluctations are easily explained by the variations of 

 speed and reactive current which occur simultaneously with them. 

 The sparking is due to a much more complicated action. Owing to 

 the inertia of the armature, every increase in speed, causing additional 

 kinetic energy to be stored in the armature-mass, makes the primary 

 (input) power greater than the secondary (output) power; and every 

 decrease in speed, causing the armature to lose some of the kinetic 

 energy stored in its mass, produces the contrary effect of making the 

 secondary power greater than the primary power. 



In both cases the transverse armature-reactions produced by the 

 primary and secondary active currents no longer balance each other, 

 and the axis of the resultant magnetic field oscillates from one side of 



