FIELD-EXCITATION OF ROTARY CONVERTERS 207 



that the best excitation under these conditions of operation with falling 

 voltage would be one which neutralizes the reactive current dP at 

 all loads; but this mode of operation is not of great practical interest. 

 It has been proposed to use converters with a falling voltage-char- 

 acteristic in connection with storage batteries, so as to make the batteries 

 discharge when the load is heavy, and make them charge when the load 

 is light. With the usual method of cur rent- supply this mode of opera- 

 tion would cause variations of TO, 20 per cent, or more, in the supply 

 voltage. Dr. F. B. Crocker l has devised an ingenious method of regulat- 

 ing the supply-voltage which enables a wide range of E.M.F. regulation 

 to be obtained in a rotary converter, either with a falling or a rising 



r 



FIG. 22. 



characteristic, without affecting the E.M.F. of the source of supply. 

 The same result is also accomplished by the so-called " split-pole " 

 rotary converter, in another way. 



If we add a series-winding which causes the right line PQ to incline 

 toward the left, the variations of s will evidently become smaller. We 

 proceed to find the condition which reduces these variations of r to a 

 minimum, i.e., which makes the converter give a substantially constant 

 voltage. 



It is evidently necessary that the lag-characteristic PQ should 

 coincide as far as possible, within the practical load limits, with a circular 

 arc described from the point O as a center situated at the end of the 

 line BO=E. Let F (23) represent the zero-load condition; let G 

 represent the average load, and M the maximum load. A right line 



1 See U. S. Patents to F. B. Crocker, on "Automatic Regulation of Rectifiers 

 and Rotary Converters," No. 891,797, June 23, 1908, and No. 1,012,524, 

 Dec. 19, 1911. 



