CORE Loss TESTS 23 



machine under test. The difference between these 

 two sets of readings will give the brush friction on the 

 machine being tested. Starting with zero field on 

 the machine under test, observations of the input to the 

 driving motor should be made at various values of the 

 field current, up to that which will give 125 per cent 

 normal load voltage. Correcting the motor input at 

 these various field strengths by subtracting the C 2 R 

 loss of the armature of the driving motor and the power 

 input to the driving motor for zero field, the core loss 

 corresponding to the various field strengths is found. 



In order to insure constancy of friction losses during 

 the entire test, the readings of the motor input for zero 

 field should be repeated at least three times during 

 the progress of the test; namely, at the beginning, 

 again near the middle point of the curve, and lastly 

 at the end of the test. Readings should also be taken 

 at the end of the test with normal voltage field current 

 and with brushes raised from the commutator, for 

 comparison with the reading in which the same field 

 was used with the brushes resting on the commutator, 

 and with the set of readings giving the brush friction. 



The tester should check these values, one against 

 the other, to see that they are consistent, before turning 

 in the results of the test. To check the results of the 

 core loss as the test proceeds, the power input to the 

 driving motor required by the core loss at a given 

 excitation should be plotted against volts armature 

 generated. This should give a curve similar to Fig. 6. 

 If a satisfactory curve is obtained, the driving motor 

 should be unbelted and a running free reading taken 

 upon it, holding the same amperes field as were used 

 during the test. The bearing friction and windage losses 

 of the machine under test can be separated, if desired. 



For a successful core loss test, all readings must be 

 made at absolutely constant speed, when the rotating 

 parts are neither accelerating nor decelerating. All 

 values of field current must be held constant while 

 taking readings. No pulsation or sudden variations 

 must occur in the armature current of the driving 

 motor, as these would vitiate the power readings. 



