2-B] EFFICIENCY. 43 



current. No account will be taken of a possible difference be- 

 tween the contact resistance with machine running and that 

 measured with armature stationary. 



3. Load Run (Shunt Motor). This run is made* to ascer- 

 tain the working conditions for which the losses are to be deter- 

 mined, that is, to ascertain the load current and hot resistances 

 for calculating copper losses and to ascertain the normal speed 

 and excitation for which the iron and friction Losses are to be 

 determined in the no-load run. (The load run is a repetition 

 of the run made in Exp. 2-A for obtaining speed characteristics.) 



4. Connect the motor to the supply lines, the voltage of 

 which should remain practically constant during the run. (See 

 Fig. I of Exp. 2-A.) Adjust the field current by means of the 

 field rheostat so that the motor runs at its rated full-loadf speed 

 (or the speed for which its efficiency is desired) and keep the 

 field current constant at this value during the run. Care in 

 keeping the field current constant will increase the accuracy of 

 the results; it is not sufficient to leave the rheostat in one posi- 

 tion and assume the field current constant because it is very 

 nearly so. 



* (3a). Omission of Load Run. It will be seen that the load run is 

 not essential and that the method may be employed even when the load 

 run is impossible. Whenever it is possible, however, the load run should 

 be taken, since it serves to get the machine " down to its bearings," that is, 

 down to its working condition of friction as well as of temperature. 



When the load run is omitted, cold resistances are measured and hot 

 resistances determined by suitable temperature corrections or assumptions. 

 Values of field current and speed are determined for no load; values are 

 assumed for full load which it is believed will most nearly represent the 

 operating conditions for which the efficiency is to be obtained. In a 

 motor, for example, we may assume a constant excitation and a constant 

 'speed, or a speed which is say 5 per cent, lower at full load, etc. In a 

 generator we may assume a constant speed and a constant excitation, or 

 an excitation which is a certain amount lower (shunt generator) or higher 

 (compound generator) at full load. 



t(4a). For commercial testing the speed should be adjusted to its 

 rated value at full load; in laboratory practice the adjustment, when de- 

 sired, may be made at no load. 



