2-A1 SPEED CHARACTERISTICS. 37 



If there are many series turns and no shunt is used, the cur- 

 rent taken by a differential motor may become excessive as the 

 load increases, thus weakening the field so that the motor races, 

 or even reversing the field so that the motor suddenly reverses. 



15. Queries. For increasing the speed, is the field current increased 

 or decreased? Why? What is the use of the starting box? In starting, 

 why do you not close the field and armature circuits simultaneously? Why 

 is the starting box connected in series with the armature and not in series 

 with the line? Why is a strong field needed for starting? Does this 

 become of more or of less importance when starting under load? Would 

 an added series winding be an advantage or a disadvantage in starting? 

 Why would it be dangerous to break the field circuit? What is the effect 

 of shifting the brushes? What is the proper position for the brushes? 

 What is effect of interchanging positive and negative supply lines? What 

 changes in connections are necessary to reverse the direction of rotation 

 of the armature? (Be careful not to run more than a moment in the 

 reverse direction if the brushes would thus be damaged.) 



PART III. SPEED CHARACTERISTICS. 



1 6. Shunt, Compound and Differential Motor. It is the pur- 

 pose of the experiment to determine the variation of speed with 

 load for the same motor connected in three ways, shunt, com- 

 pound and differential ; the line voltage is constant throughout 

 the three runs. The brushes should be in one position during 

 all the runs (8), or the amount of any change noted. 



With the motor connected as a shunt machine, Fig. I, adjust 

 the field current by means of the field rheostat so that the motor 

 runs, on no load, at the speed for which it is designed, and keep 

 the field current constant at this value during the run. For the 

 other two runs, compound and differential, adjust the field cur- 

 rent for this same no-load speed* and keep the field current 

 constant during each run. 



* ( i6a). Starting with the same no-load speed, and making runs from 

 no load to full load, gives the three speed characteristics of Fig. 2 coincid- 

 ing at no load ; this is the best procedure for instruction purposes. 



In commercial testing, the field should be adjusted so that the motor 

 runs at rated speed at full load. The curve is then taken from full load 

 to no load ; the maximum per cent, variation in speed from its full load 

 value is the per cent, speed regulation. (Standardization Rules, 195.) 



