52 DIRECT CURRENT MOTORS. [Exp. 



course no run can be made. To obtain this, plot a curve showing 

 Oa iy 0a 2 , 0a 3 , etc., for various field currents and continue the curve 

 back to zero field current so as to get a value for Oa by extra- 

 polation. 



(b) It will be found by experience that the value of Oa found by 

 a run at a very low field excitation will differ but little from the 

 desired value Oa for zero excitation; that is, the iron losses at very 

 low excitation are negligible. Instead of a series of no-load runs 

 and extrapolation, one no-load run is taken at as low an excitation 

 as possible ; the value Oa obtained from this run is taken as the value 

 of Oa which would be obtained at zero excitation. 



Referring to Fig. 2, Oa obtained by either procedure just described 

 is the torque to overcome friction and windage, for at zero excitation 

 there is no hysteresis loss. To obtain watts loss in friction and 

 windage at any speed, multiply Oa by S; this is independent of 

 excitation. To obtain watts loss in hysteresis at any speed for some 

 particular excitation, multiply a a (for that excitation) by S. 



21. Determination and Separation of Losses; Machine Driven 

 Mechanically by an Auxiliary Driving Motor. This method, with the 

 machine driven mechanically, is not limited to testing direct current 

 machines; it can be used in testing alternators, synchronous motors, 

 etc. By this method separate values are found for the iron losses and 

 for the mechanical losses; that is, for hysteresis and eddy currents 

 combined and for friction and windage combined. 



The preceding method, with the machine driven electrically (7), 

 gave directly the eddy current loss and the combined hysteresis, fric- 

 tion and windage (17). Each method has its advantages; in the 

 one hysteresis is combined with eddy current, in the other with fric- 

 tion and windage. 



The procedure is as follows: (i) The machine to be tested is sepa- 

 rately excited and is driven as a generator* on no load at normal 

 speed and excitation by means of a shunt motor; compare 25. The 

 motor input is measured. (2) The generator field circuit is broken 

 and motor input again measured; the diminutionf in motor input 



*The armature winding is idle; this test therefore can be made for 

 finding iron loss and friction of a machine with armature unwound. 



t(2ia). This assumes that the motor losses remain constant. The 

 small change in armature Rl* loss will usually be negligible; if not 



