DIRECT CURRENT MOTORS. 



[Exp. 



Vary* the load on the motor by steps between no load and 25 

 per cent, overload, reading line voltage, field current, armaturef 

 current (or else line current) and speed, for each step. Make 

 runs with the motor connected shunt, compound and differential. 



With current as abscissae (either line current or armature 

 current) and speed as ordinates, plot speed characteristics for 

 the three runs as in Fig. 2. 



1100 



1000 

 900 

 800 

 700 



sr 



400 

 300 



100 



8 



12 



36 40 



16 20 24 28 35 

 ARMATURE CURRENT 

 FIG. 2. Speed characteristics of a motor, shunt, compound and differential. 



17. It is instructive to take runs as a differential motor with 

 different resistances in shunt with the series coil; also, to take 

 the various runs (shunt, compound and differential) with the 

 field excitation above and below saturation. 



* ( i6b). This may be done by means of a brake, a blower, a belted 

 generator or other convenient load; if a generator is used, its output may 

 be absorbed in resistance or pumped back into the line (26, Exp. 2-B). 



f(i6c). If the armature current is measured, the field current is 

 added to give the line current; if the line current is measured, the field 

 current is subtracted to give the armature current. 



