4 8 



DIRECT CURRENT MOTORS. 



[Exp. 



values of W -r- speed, which will give the straight line ac in 

 Fig. 2. At very low speeds, there may be a deviation from a 

 straight line, due possibly to errors in assumptions as to friction, 

 etc., at these speeds. This, however, does not affect the accuracy 

 of the construction; the straight part of the curve is to be ex- 

 tended back to a. 



SPEED (R.P.M.) 



FIG. 2. Variation of rotation losses W (iron losses, friction and windage) 

 with speed, at constant field excitation. The torque dc to overcome rotation 

 losses is composed of db to overcome friction, windage and hysteresis and 

 be, to overcome eddy current loss. 



After plotting this line for W -r- speed, pick off values from 

 it and multiply by speed, thus getting as many* points as desired 

 for plotting the W curve. For fuller treatment, see Appendix I. 



14. Efficiency. For any load (corresponding to readings in 

 the load run, or assumed), the input is equal to the product of 

 line current and voltage. 



The losses are: the (variable) RI 2 loss in the armature for 

 the particular armature current /; the (constant) copper loss of 



check. It is always desirable to plot the results of any experiment, if 

 possible, as a straight line, arc of circle, or as some curve whose law is 

 known. The arc of a circle is much used in alternating current testing. 

 * Obtained in this way, more points may be used in plotting Wo than 

 the number of observations. 



