54 DIRECT CURRENT MOTORS. [Exp. 



APPENDIX II. 



MISCELLANEOUS NOTES. 



22. Efficiency of a Generator. To find the efficiency of a machine 

 as a generator, a load run is made as a generator to ascertain the 

 working conditions of speed, excitation and voltage. A no-load run 

 as a motor is then made under these same conditions. The load run 

 should be made whenever possible, but it can be omitted ( 33). In 

 the load run, the field rheostat may be kept in one position ( 12, 

 Exp. i-B) or changed so as to maintain the desired terminal voltage 

 (26, Exp. i-B), according to what may be taken as the working 

 conditions of the machine. Commercially the latter is more usual. 

 For a compound generator, see 31. 



23. Efficiency of a Motor Generator.* A load run is to be made 

 when possible and measurements made of the various currents and 

 voltages for both motor and generator. (See 33.) A no-load run 

 is to be made if possible with the generator uncoupled; this deter- 

 mines the motor losses. Next make a run with the generator 

 coupled but not excited, the increase in losses over the no-load run 

 showing the friction and windage of the generator. Follow this with 

 a run in which the generator has its proper excitation, the increase 

 in losses over the preceding run showing the iron losses of the gen- 

 erator after copper losses have been taken into account. This last 

 run gives the combined rotation losses for both machines. The cop- 

 per losses are computed and added to these to get the total losses; 

 knowing these, the efficiencies are readily computed for the two 

 machines, combined and separately. As in the case of a generator 

 or motor, due care is to be taken in all the no-load runs to have the 

 proper speed and flux density in both machines. If the flux density 

 in either machine was not constant in all the runs (as would be the 

 case in a compound or differential machine), take note of Appendix 

 III. The test may be made by reversing the set, that is, running the 

 generator as a motor ; this makes it possible to determine the friction 

 and windage of the motor separate from iron losses. 



24. Calibrated Generator for Measuring Motor Output. The out- 



* The details of this test can be modified according to circumstance; 

 see 21. 



