

SEVERAL ROTARY CONVERTERS IN PARALLEL 223 



Moreover, the armature-reactions produced by the reactive current 

 produce the same effect on the resultant total field-excitation. The 

 alternating current produced by the converter, in such a case, generally 

 lags behind the (A.C.) E.M.F., hence it weakens the magnetic field, 

 and this, instead of causing a falling off in voltage, causes an increase 

 in speed. If the A.C. circuit happens to contain a considerable amount 

 of inductance, the reactance will increase with the frequency (due to 

 the increase of speed); and the lag will increase with the reactance. 

 The demagnetizing effect will therefore be increased. This will cause 

 a further increase of speed, a further increase of frequency, and again 

 a further increase of reactance; and the consequence of this cumulative 

 process may be that the converter will " run away " or attain an excess- 

 ive speed. It is therefore necessary, in such cases, to take special 

 precautions, even including the use of safety-devices such as automatic 

 safety stops, set so as to operate to open the (D.C.) supply- circuit 

 whenever the converter speed exceeds a certain limit. 



This want of stability can be avoided, and the converter can be 

 made to operate under synchronous conditions, as before, by simply 

 connecting to it (in parallel with its A.C. circuits), an alternator which 

 is driven at constant speed by an engine or motor, and which has 

 sufficient power to withstand any tendency to be itself driven as a motor 

 by the current produced by the converter. In that case, the alterna- 

 tor regulates the frequency, and it " sets the step " for the converter, 

 as it were, also furnishing to the converter the reactive current neces- 

 sary to bring back the magnetic field to the normal value, whatever 

 may be the excitation due to the field-windings. 



Other Special Applications of Converters. As already stated, 

 rotary converters may be used as motors for developing mechanical 

 power equal to a poriton of the electric power supplied to them either 

 on the A.C. or the D.C. side. In such a case, however, the active 

 currents entering and leaving the converter will no longer balance each 

 other, the difference between the energy-values which they represent 

 being converted into mechanical energy. The "transverse" reactions 

 no longer offset each other, and the heating effects are increased. 



This case can also be studied by the graphical methods previously 

 presented, because the diagram representing the conditions will be 

 simply intermediate between the two diagrams shown in Figs. 27 and 

 28, corresponding, respectively, to a synchronous motor and to a 

 converter. All that is needed is to change the lengths of the 

 segment OB, making it equal to a>L'(I w I w '} instead of coL'I w , in 



