5 6 DIRECT CURRENT MOTORS. [Exp. 



in excitation, thus increasing the load on the driving motor as 

 desired. 



When the loading back method is thus used simply as a loading 

 method and not as a testing method (27), no measurements are 

 made on the generator; measurements are made on the motor the 

 same as though the generator were loaded with resistances. 



Since one machine takes power as a motor and the other returns 

 it as a generator, the net power taken from the supply line is only 

 that which is required to supply the losses in the two machines. 



27. Opposition Method for Testing Two Similar Machines. If 

 two similar machines are operated as in the preceding paragraph 

 and measurements are taken on both, they can be tested by Kapp's* 

 opposition method and their combined losses determined. 



There are various other opposition methods for accomplishing the 

 same object; in each of these two similar machines are run, one as 

 motor and the other as generator under load conditions. The two 

 machines are connected both electrically and mechanically, so that 

 power circulates between them and the only outside power taken is 

 that necessary to supply the combined losses. These losses may be 

 all supplied by the line (Kapp's method) or either partly or wholly 

 by an auxiliary motor or by an auxiliary booster, giving rise to the 

 various methodsf of Hopkinson, Potier, Hutchinson and Blondel. 



Although opposition methods are economical of power, they are 

 not economical of time or apparatus; they are accordingly limited to 

 testing pairs of large machines which could not be tested under load 

 conditions in any other way. Temperature runs, regulation and 

 efficiency tests are made in this way. Kapp's method is the simplest, 

 but (on account of the different field excitation of the two machines) 

 theoretically is not so accurate as some of the other methods. 



28. Point of Maximum Efficiency. Consider that a machine has 

 a certain constant loss (W^-\- field copper loss) and a variable loss 

 (armature RI 2 ) which varies as the square of the load current I and 



*This method and a modification by Prof. W. L. Puffer is fully de- 

 scribed in Foster's Electrical Engineering Pocketbook; see also 273. 



t(27a). For full description and complete references, see Swenson 

 and Franken field's Testing of Electromagnetic Machinery; see also R. E. 

 Workman, Electric Journal, Vol. L, 1004, pp. 244, 289, 363; Karapetoff's 

 Exp. Elect. Eng.; and various text and handbooks. 



