CTAILED STUDY OF OPERATION WITH NORMAL LOAD 63 



Let r equal the armature-resistance, x the armature-reactance, 

 armature-impedance, U the voltage at the terminals, 7 the max- 

 im allowable current. The internal E.M.F. of the corresponding 

 aerator, necessary to produce this current, will be obtained (Fig. 

 35) by combining the vector U with a vector A.\A = zI drawn below 



U at an angle = 7-, such that tan 7-= . In order that the same machine, 



when operating as a motor, at the same voltage, U, may furnish the 

 maximum power, 77, with the same current, 7, the latter must have 

 the same difference of phase, 7-, with respect to U, but in the opposite 

 direction. The internal E.M.F., E 2 =OA 2 , will therefore be obtained 

 by compounding OA with a vector z7 equal, but opposed, to the pre- 

 ceding one. 



Fig. 35 thus shows that the alternator corresponds to the same 



B, rl 



FIG. 35. 



electric power at its terminals when operating as a motor and as a 

 generator, and this with substantially the same excitation. Assuming 

 the losses to be the same in both cases, it is seen that by increasing 

 slightly the E.M.F. E 2 , the same normal power may be counted upon 

 for a motor as for a generator. (The normal power here designated 

 is the power which the alternator can furnish on a dead resistance. 

 This is the figure given in the catalogues of manufacturers of syn- 

 chronous motors.) 



(2) If now we consider the case where power is to be transmitted 

 to a distance, the impedance Z which counts -in the operation of 

 the motor is that of the entire circuit and it can be notably superior 



