88 



COMMERCIAL ELECTRICAL TESTING 



With normal voltage held constant across the arma- 

 tures, the motors are run light for ten minutes in each 

 direction of rotation, readings of speed, and armature 

 and field currents being recorded. 



With rated voltage across the motors, the fields 

 should be weakened until about twice normal speed is 

 attained. Under these conditions the machine should 

 be run in each direction for five minutes, the same 

 readings as listed above being taken. 



Resistance measurements and high potential tests 

 must be made before and after this test. 



600 60 



Amperes 

 Fig. 28. Input- Output Curves for a 75 H.P., 600 Volt Railway Motor 



Care must be taken that the resistance and speed 

 at 25 deg. C. come within the prescribed limits 

 already mentioned. 



On all series motors, with the exception of railway 

 motors, standard efficiency tests are made by the method 

 of losses and the calculation of the efficiency is identical 

 with that for any other motor. In this case, of course, 

 amperes armature equal amperes line. 



In making an input-output test the motors are geared 

 and connected as for the load heat run and are usually 

 run under full load for one hour up to ordinary working 

 temperatures to get the bearings in good running con- 

 dition. Before the load is put on, careful measurements 

 of the armature and field resistance of the motor and 

 of the armature of the generator are taken by the 



