64 SYNCHRONOUS MOTORS 



to the impedance of the armature. The segment ZI then extends as 



j^-. 



far as A 1 2 , for example, making an angle F, which is such that tan F= 



K 



and the E.M.F., OA'^, necessary to obtain this output may be superior 

 to the preceding value. Eq. (25) gives the value thereof. It is nec- 

 essary to foresee this condition, in studying an electric transmission 

 project, whenever the impedance of the line is of the same order as 

 that of the alternators. 



IV. INFLUENCE OF MOTORS ON THE GENERAL OPERATION OF 

 AN A.C. ELECTRICAL DISTRIBUTION SYSTEM 



Effect of Current of Synchronous Motors on Distribution-Systems. 

 There is a tendency, at the present time, and with good reason, to use 

 synchronous motors under conditions which permit them, as if they were 

 condensers, to compensate for the inductance of neighboring motors. 

 This method, which consists in overexciting the synchronous motors, 

 whether they be utilized for the production of mechanical power, or 

 whether they be running without load, appears to have been first 

 advocated in 1891 by Swinburn; and it has been used since by many 

 engineers in Europe and in America. Following the advice of Dolivo- 

 Dobrowolsky it was used for the Bulach-Oerlikon power- transmission 

 in 1893 (see letter of Dolivo-Dobrowolsky to the Elektrotechnische 

 Zeitschrift, 4. Oct., 1894, p. 555); and Lahmeyer also used it at Bocken- 

 heim (see Deutscher Verband der Elektrotechnischen Ingenieure, Con- 

 gress of June, 1894), where he was thus enabled, it would seem, to 

 double the output of the generators by compensating all inductance- 

 effects in the distributing system by the reaction of synchronous motors. 

 Some time after, Professor S. P. Thompson called attention to this 

 method (" Some Advantages of Alternating Currents ", by Professor S. P. 

 Thompson, British Association, 1894). 



To study this question, we must ascertain how the reactive current 

 varies with respect to the voltage at the terminals of the generator. 



A polar diagram referred to the line-voltage as a fixed axis gives 

 the necessary values most accurately. 1 Let us take (Fig. 27, p. 44) 



i We use here this form of diagram to take the ohmic loss of the motor into 

 account. In practice, the latter can be neglected without great inconvenience, 

 and the diagram referred to the motor E.M.F. E?, which is more simple, will answer 

 the purpose. We can then see more readily that the conditions corresponding to 



