DIRECT CURRENT RAILWAY MOTORS 



89 



drop in potential method. Three different measure- 

 ments should be made of each, with as many different 

 values of current near normal load current. 



Holding normal voltage constant, 12 or 15 different 

 loads ranging from as low as possible to 150 per cent 

 load should be put on, the direction of rotation being 

 such that the motor tends to lift from its bearings. 

 Readings at each load should be taken of the amperes, 

 volts armature and speed of the motor, and amperes 

 and volts armature of the generator. The direction 



Fig. 29 

 Speed, Tractive Effort, Efficiency on a 75 H.P., 600 Volt Railway Motor 



of rotation should then be changed and several check 

 points taken in speed and amperes, after which the 

 machine should be shut down and hot resistance measure- 

 ments made. 



Table X and Fig. 28 show the method of working 

 and plotting the data obtained from the input-output 

 test. Unless otherwise specified, the tractive effort 

 and miles per hour are calculated for 33 in. wheels. 

 The formula used are: 



, ... R.p.m. X diam. of wheels in inches XTT 



Miles per hour = - vxincn 



Gear ratio XI 056 



Amps. X volts X efficiency X 252 



Tractive effort = 



Miles per hour X 500 



The gear ratio is that between the gear and pinion. 



