84 COMMERCIAL ELECTRICAL TESTING 



are tested for commutation by suddenly reversing the 

 direction of rotation under various loads. 



Development tests consist of general tests and special 

 tests, and are made when an entirely new type of 

 machine is being developed. 



SPECIAL TESTS consist of speed curves, core loss, 

 and saturation tests. 



In taking a speed curve two similar motors are placed 

 on a testing stand and the pinion of each is meshed 

 in the same gear mounted on a shaft. One motor 

 drives the other as a separately excited generator and 

 is run loaded until the motor is heated to about 50 deg. 

 C. rise. The speed curve is then taken on the motor 

 rotating in first one direction and then the other, the 

 voltage being held constant. The resistances of arma- 

 ture and field should be measured both before and after 

 taking the curve. 



Core loss should be taken by the belted method, as 

 on any other machine, except that the test should be 

 made at about five speeds. (Fig. 24.) The lowest 

 speed should correspond to about 175 per cent full 

 load amperes (taken from speed curves) and the highest 

 at about 200 per cent full load speed. During this 

 test the machine is separately excited. 



A saturation curve may be taken on a series motor 

 just as on any other machine by separately exciting 

 the field. Saturation curves at different speeds may 

 be obtained from data taken during the core loss test. 



The speed curves, core losses and saturation are 

 calculated as previously explained. The speed curves 

 and core losses should be plotted on the same sheet 

 against amperes line as abscissae and revolutions per 

 minute and watts as ordinates. From these two sets 

 of curves another curve can be developed, which will 

 give the core loss of the motor at any speed or current. 



The thermal characteristic should be obtained by 

 making a series of heat runs at varying current values 

 for a sufficient time to get a temperature rise of 75 deg. 

 C. All runs should be made at the same constant voltage, 

 the current value for each run varying from 50 to 150 

 per cent normal. If a sufficient number of heat runs 



