4 2 DIRECT CURRENT MOTORS. [Exp. 



A further advantage of this method is that a load run is not 

 an essential, as will be seen later, and hence may be omitted. 

 Conditions often arise, as in testing large machines, when a load 

 test is impossible and this advantage then becomes important. 

 It is always best, however, to make the load run when this can 

 be done. 



The method of losses is general and can be applied for deter- 

 mining the losses, and hence the efficiency, of a shunt, compound, 

 differential or series wound motor or generator. In the follow- 

 ing paragraphs the directions are full and explicit for testing 

 a shunt-wound motor. Modifications are outlined in the Ap- 

 pendices for applying the method to other types of motors and 

 generators. 



2. For testing any machine two runs are made : a load run 

 to ascertain working conditions, and a no-load run (or runs) to 

 determine losses under these same conditions. 



In making the no-load run for losses the machine can be driven 

 electrically as a motor or mechanically as a generator. The for- 

 mer method is used in this experiment ( 7) ; the latter method 

 is described in 21 of Appendix I. 



The resistance* of the armature is to be found by the fall of 

 potential method both before and after the load run, in order 

 that it may be determined both cold and hot (see 17, Exp. 

 I A). Since this includes the resistance of the brushes and of 

 brush contact, which varies with current, to be exact it would 

 be necessary to measure the armature resistance for each load 



their reading. Assume the true output to be 95 when the true input is 

 100. The output, as measured, might vary from 94.05 to 95.95 and the 

 input, from 99 to 101 ; hence the efficiency, determined from output, 

 might vary from 93.1 to 96.9 per cent. On the other hand with the same 

 percentage error in their determination, the measured losses might vary 

 from 4.95 to 5.05 and the measured input from 99 to 101 ; hence the 

 efficiency, determined from losses, could only vary from 94.9 per cent, to 

 95.1 per cent. 



* In measuring armature resistance the voltmeter is to be connected to 

 the same points as in the load run. 



