GENERAL DIAGRAMS FOR SYNCHRONOUS MOTORS 183 



The reactive currents corresponding to these loads are therefore 

 entirely determined. 



It will be seen that there is an infinite number of possible solutions, 

 according to the value taken for the constant E.M.F. E. This E.M.F. 

 can be made high or low, as desired. It depends on the ratio of trans- 

 formation of the static transformers used for supplying current to 

 the converters. 



Moreover, the impedance Z can also be regulated, within certain 

 limits, at least, by the introduction of reactance in the current supply- 

 circuit. (It is obviously desirable to reduce the ohmic resistance to 

 the lowest possible value, to avoid lowering the efficiency.) It remains 

 to be seen what are the most suitable values for E and X. This point 

 will be discussed in the next two sections. 



Most Suitable Value of Current-Supply Voltage. This value is 

 evidently that which gives zero-lag (unity-power-factor) for the most 

 usual or frequent load, because we then have, for that load, a minimum 

 current-value, consequently better line-efficiency, and better " over-all " 

 efficiency. This condition of operation can be determined from the 

 known or expected conditions of service, for the particular apparatus. 



One might be tempted, on general principles, to take the maximum 

 load as the one for which the supply-voltage is to be specially adjusted, 

 so as to give the best results with that load, because it is precisely 

 then that it is most important to reduce the losses; but since, in 

 practice, it is necessary to consider the mean daily heating and effi- 

 ciency of the apparatus used, it is for the average or normal load, and 

 not for excessive loads of momentary duration only, that the supply- 

 voltage should be adjusted, with the object of obtaining unity power- 

 factor. 



The range of voltage-regulation desired in the rotary converter 

 can be obtained with more ease and precision when the point of 

 maximum power-factor (" no lag ") corresponds to the average load. 

 In fact, as will be seen later, the only converter-voltages (D.C.) which 

 can be obtained exactly, by practical methods of excitation, are those 

 corresponding to zero-lag and zero-load. 



Most Suitable Value of Reactance. It is important to note, at 

 the outset, that a certain minimum reactance is necessary in order 

 to obtain, with a constant (A.C.) E.M.F. E, a (D.C.) voltage which 

 will increase, or even remain constant, with the load. 



Let us suppose that there is no reactance in the circuit. This 

 makes f=o; the reference-line BY (Fig. 7), will then coincide with 



