36 SYNCHRONOUS MOTORS 



tion of the generator is also constant. Each position of the point A\ 

 defines the vector of the E.M.F. Ei=OAi and the vector of the cur- 

 rent ZI=A 2 Ai. Knowledge of any two whatever of the elements 

 EI, 6, I, (f> leads to that of the two others. 



The diagram can be employed when E 2 varies, but the point O then 

 varies at the same time as A i, so that the locus of A\ no longer suffices 

 to define the conditions of operation. There is then some advantage, 

 if EI is constant, in having recourse to the second kind of diagram. 



Line of Equal Power Occurring with Constant Excitation. The 

 electric power of the motor, being equal to the product of the E.M.F. 

 E 2 by the watted current I w , will, when the excitation is constant, be 

 proportional to the segment A 2 D. When the motor-excitation remains 

 constant, all outputs corresponding to equal watted currents are repre- 

 sented by points, A\, situated on a single right line A\D, perpendicular 

 to the axis A 2 Y. This right line A-J) is, therefore, a line of constant 

 power for the motor, whatever may be the E.M.F. of the generator, 

 which may be variable. The same thing is true for all parallel right 

 lines. It is possible to represent, on the diagram, a series of such right 

 lines, each bearing the indication of the corresponding power E 2 I W 

 under constant excitation; and then, for each position of the point 

 A i, the corresponding power will be known. 



The mechanical power is equal to the electric power less the 

 losses due to eddy currents and hysteresis. (The heat-losses are 

 not included in the electric power E 2 I W ). The effect of these losses 

 is itself equivalent to a certain appreciable watted current, jo, and 

 to a wattless current which can be practically neglected. There- 

 fore it will only be necessary to draw on A 2 Y a vector, A 2 d = Zjo, 

 which represents the " lost " current; and the power that is really 

 useful will be proportional to the segment dD. Although the loss 

 under consideration varies slightly with the output, it can, without 

 material error in constructing the diagram, be considered as constant 

 for a given excitation, and as equal to the power necessary to operate 

 the motor without load. The electric power supplied by the generator 

 is deduced immediately from the electric power of the motor by adding 

 the resistance-losses (I 2 R) in the circuit. 



Lines of Equal Phase. Since the phase </> of the current is measured 

 by the angle A\A 2 D, all the points representing outputs AI, located 

 on the same right line issuing from A 2 , correspond to the same angle 

 of lag </>. Any line issuing from A 2 therefore constitutes a line of equal 

 phase. 



