60 DIRECT CURRENT MOTORS. [Exp. 



33. Series Motor. A series motor may be tested for losses in 

 substantially the same manner as a shunt motor. So far as losses 

 are concerned, a series motor is like a shunt motor in that the losses 

 are the copper losses, which can be computed, and the rotation losses 

 W which depend only upon speed and excitation. In a series motor, 

 however, speed and excitation vary greatly with load. 



Load Run. A load run as a series motor is taken to obtain speed 

 and current for different loads; see Appendix I., Exp. 2-A. (If the 

 no-load run is to be taken as in 36, the load run is not a necessity.) 



34. No-load Runs for Obtaining Rotation Losses; General Pro- 

 cedure. No-load runs may then be made at different constant exci- 

 tations and W found for different speeds by varying the armature 

 voltage and measuring armature input in the usual manner. Read- 

 ings are taken of field current, armature current and armature volt- 

 age. (The procedure is sometimes to take runs with constant arma- 

 ture voltage and varying excitation.) Any convenient means may 

 be employed for obtaining the proper constant excitation and the 

 desired armature voltage; the armature and field can best be sup- 

 plied separately and not in series (see also 37). Curves are plotted 

 for each excitation, as in Fig. 5, showing IV for different speeds. 

 Instead of speeds, armature voltage is commonly plotted as abscissae. 



35. No-load Run for Obtaining Rotation Losses; Special Pro- 

 cedure. No-load runs, taken as in 34, gives curves (Fig. 5) which 

 tell the complete story, giving rotation losses for different speeds and 

 field currents. As a matter of fact such complete information is 

 often unnecessary; for, with constant potential supply, a series motor 

 has a definite counter-electromotive force and a definite speed for 

 any particular current (see Fig. 3 of Exp. 2-A). It is necessary, 

 therefore, to get the rotation losses with each field current for the 

 one corresponding speed only, this speed being obtained by supplying 

 the armature with the proper voltage. 



36. This proper voltage to supply the armature could be found 

 by trial (being adjusted until the speed in the no-load run for a par- 

 ticular field current is the same as in the load run for the same 

 current). It is easier, however, to compute this voltage without 

 making a load run. 



We know that in any run (load or no load) speed is proportional 

 to counter-electromotive force for the same excitation. For a par- 



