METHODS OF TESTING ALTERNATORS 



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a pole. The system of two machines is driven by a meas- 

 uring motor whose duty it is to furnish the power necessary 

 for satisfying the losses. Between the two alternators A^A 2 

 (Fig. 18) whose terminals are connected each to each by very 

 short couplings of negligible imped- 

 ance, a volt-meter V is connected across, 

 an ammeter A and a wattmeter W being 

 inserted in series. The figure is drawn 

 upon the supposition of two single-phase 

 machines, but applies equally well to the 

 case of two similar three-phase machines 

 coupled by their three phases, testing upon 



a single phase, taking care that the phases remain balanced, in spite 

 of the measuring instruments. 



Let U=OB (Fig. 19) be the difference of potential observed at 

 the common terminals, OAi and OA 2 the direction of the generator 

 e.m.f.'s dephased relatively to each other by the angle a. By sym- 

 metry, the vector Ob, which represents the current, will also be directed 

 along OB, and the line A^A 2 drawn from B perpendicularly to U 

 and 7, will represent the double transverse reaction 2coL l I. There 

 will be a flow of current between the alternators 

 without the production of any external power. 

 The current will be in phase with the E.M.F. 

 at the terminals U, as if the alternators sup- 

 plied a conducting system devoid of induct- 

 ance; the flux density obtained in the armature 

 will be the same in both alternators, since it 

 gives rise to the same E.M.F. at the terminals 

 U. The power furnished by each will be 

 measured by the wattmeter W, and the total 



loss p will be furnished by the method of double-weighing, by means 

 of the measuring motor which drives both alternators. The efficiency 

 will then be the ratio 



UI 



To determine the excitations necessary for the two alternators 

 to produce the condition above described, it is sufficient to apply the 

 graphic method of two reactions, as follows (Fig. 19): from the point 



