CHAPTER VI 



HEATING OF ELECTRICAL MACHINES- 

 EQUIVALENT LOAD TESTS 



Very often it is found impossible to run actual load 

 tests, especially on large machines, on account of 

 limited facilities. Equivalent load tests have con- 

 sequently been devised in which the heating of the 

 machine at a certain load may be very closely ascertained 

 without actually loading it. One of five different 

 methods may be employed in making such a test; viz., 

 "open circuit," "short circuit" and "low voltage test," 

 "circulating open delta" or "phase control." 



Direct current machines can be satisfactorily tested 

 by short circuiting the armature upon itself, or through 

 the series field, so connected that it will not build up 

 as a series generator. The shunt field is separately 

 excited from an external source, until the required 

 current flows through the armature, or armature and 

 series field. This method is excellent for baking and 

 settling the commutator. Amperes armature and field, 

 and volts field should be read throughout the run. 



In the case of alternators, the machine is run open 

 circuited, with a field current that gives a predetermined 

 percentage over normal voltage. The run should be 

 continued until the rise in temperatures above the room 

 temperature is constant, after which the machine is 

 shut down and the final temperatures taken. The 

 armature is then short circuited, the machine started 

 again, and sufficient excitation applied to give a current 

 in the armature of a certain percentage over normal. 

 This run should also be continued until the rise in 

 temperatures above that of the room is constant, after 

 which the final temperatures are taken. The resistance 

 of the field should be carefully measured before and 



[45] 



