SEVERAL ROTARY CONVERTERS IN PARALLEL 221 



\e line (diameter) of commutation to the other; and, as the brushes 

 cannot follow these oscillations, sparking occurs, just as it does in the 

 case of a dynamo whose brushes are not at the right position for spark- 

 less running. 



To overcome or diminish these difficulties it is desirable to avoid 

 as much as possible using alternators driven by engines whose speed 

 is unsteady or irregular, and to use prime movers giving a speed which 

 is as uniform and constant as possible. However, speed-variations 

 may be produced in the best-regulated steam engines and turbines, 

 by sudden and large changes of load. For this reason it is desirable 



I to apply the remedy to the converter itself, even though, in consequence, 

 a small fraction of the useful energy may be lost. 



This remedy, which is quite simple, is well known since the " damp- 

 ers " of Hutin & Leblanc have been introduced. The principle in- 

 volved is that the speed-oscillations themselves cause eddy currents 

 to be produced either in the pole-pieces or else in " damping circuits " 

 placed suitably around or about the pole-pieces. 



A noticeable damping effect will already be obtained by merely 

 making the pole-pieces of solid annealed wrought iron. The damping 

 effect produced is not always sufficient, however; and as eddy currents 

 are also produced in such pole-pieces in consequence of the irregularity 

 of the revolving field caused by the two-phase or three-phase currents 

 employed, the loss of energy due to these eddy currents is not always 

 utilized in improving the stability of running of the converter. 



In attempting to use laminated pole-pieces with the object of eliminat- 

 ing these eddy current losses and of improving the efficiency of the 

 converter, it was soon found that the stability of operation of the con- 

 verter was, thereby, made altogether insufficient. The proper stability 

 was restored to the converter by placing copper or brass rings around 

 the pole-pieces, or else simply by placing the field-excitation windings 

 on copper spools or forms constituting a closed electric circuit around 

 the field-cores and poles. 



These " damper-circuits " are adjusted in such a manner as to 

 reduce as much as possible the energy-loss produced by the currents 

 induced in them, while at the same time obtaining the damping effect 

 required. For example, Mr. Parshall has, in this way, adjusted the 

 damping of several converters of 900 KW. capacity by connecting them 

 in parallel on an artificial line. Even when supplied with currents 

 by an alternator whose speed was irregular they operated satisfactorily 

 with a loss of 3 per cent in the reactances interposed between them. 



