44 DIRECT CURRENT MOTORS. [Exp. 



5. Beginning at about 25 per cent, overload, as estimated from 

 the input, vary the load by steps from overload to no load or 

 vice versa; at each step measure the line voltage, armature cur- 

 rent,* field current and speed. 



6. The motor may be loaded in any manner that is convenient. 

 A brake may be used for this, but it is frequently more con- 

 venient to load with a generator and to absorbf the output of 

 the generator by suitable resistances. 



7. No-load Run (Shunt Motor) ; Machine Driven Electric- 

 ally 4 For a shunt machine one|| no-load run is made; the 

 machine is operated as a motor at the same constant excitation 

 as in the load run. The object is to determine the losses for 

 different speeds at this constant excitation. Before taking read- 

 ings the motor should be run awhile so as to attain its normal 

 working condition of lubrication, temperature, etc. 



With the motor running unloaded, adjust the field current to 

 the same value as during the load run and hold constant at this 

 value during the no-load run. By varying the electromotive 

 force impressed on the armature terminals, vary the speed of the 

 motor -by steps so as to cover as wide a range of speed as pos- 

 sible; this will give more accurate results than if only the speed 

 range of the load run is covered. At each step measure the 



* See i6c, Exp. 2-A. 



flf a direct current generator of suitable voltage is used, the current 

 from the generator may be " pumped back " into the motor supply line 

 (26). 



$ (7a). This run can be made with the machine driven mechanically 

 (21) instead of electrically. 



|| (7b). Although a run at only one excitation is necessary for de-. 

 termining the efficiency of a shunt motor, runs at other excitations are 

 recommended. These additional runs may be taken by the two voltaic 

 method (?d). They are necessary if hysteresis loss is to be separated 

 (Appendix I.) or if flux density is variable (Appendix III.). If a run 

 is wanted at a very high saturation, a higher voltage may be supplied to 

 the field than the rated voltage supplied to the armature. 



