20 COMMERCIAL ELECTRICAL TESTING 



at full load. With this voltage impressed, the field 

 current is varied until normal speed is obtained, when 

 careful readings must be made of armature current, 

 armature voltage, field current, field voltage and speed. 



If the machine in test is a direct current motor, the 

 voltage applied to the armature should be equal to 

 the normal rated voltage of the motor, less the CR drop 

 in the armature under full load. The field current is 

 then adjusted to give normal speed, and electrical 

 and speed readings taken as outlined above for direct 

 current generators. 



The power supplied to machines running free will 

 equal that absorbed in bearing friction, brush friction, 

 windage, and core loss, when the armature C 2 R losses 

 have been subtracted. 



In making records of these tests, the testing record 

 should clearly show whether the running light current 

 consists of the armature current plus the shunt field 

 current, or whether it is the armature current alone. 

 To check this point, open the armature circuit with 

 the shunt field circuit closed, and note whether any 

 current is indicated on the ammeter, reading the power 

 supplied. If no current is indicated, the reading in- 

 dicates the armature current alone; otherwise, the 

 running light current is equal to the sum of the armature 

 and field currents. To obtain "running light" core 

 loss tests, only a single field winding must be used for 

 excitation; this must be a shunt field winding. 



In order to obtain running light core loss upon alter- 

 nating synchronous machines (in which class rotary 

 converters are not included, as the core loss test on 

 these machines is similar to that on direct current 

 machines), they should be operated as synchronous 

 motors at the proper frequency and rated voltage. 

 For the best results, both frequency and voltage must 

 have a steady value. 



With normal voltage on the armature, the direct 

 current field should be varied until minimum armature 

 current is obtained. Readings should then be taken of 

 amperes and voltage on all the phases. At minimum 

 input current unity power-factor obtains, and therefore 



