4 DIRECT CURRENT MOTORS. [Exp. 



19. Torque. Since torque varies as flux X current, the torque 

 would vary as / 2 , if flux were proportional to current. For small 

 currents below saturation this is more or less true. For large 

 currents after saturation the flux is practically constant and the 

 torque increases directly as /. The torque curve, Fig. 3, is there- 

 fore at first more or less parabolic and then becomes a straight line. 



20. Speed. From equation (5) it is seen that speed varies in- 

 versely with flux. For small currents, if we consider RI negligible 

 and flux proportional to current, speed varies as 1/7; the speed curve 

 (Fig. 3) would then be an hyperbola. For larger currents satura- 

 tion is reached, the flux becomes practically constant and the speed 

 more nearly constant. On account of RI drop, speed continues to 

 gradually decrease as current increases, even after saturation is 

 reached. Series motors are sometimes overwound, that is, wound 

 so that saturation (and hence more constant speed) is soon reached. 



21. Test. The load is varied between an overload (determined 

 by maximum safe current) and an underload (determined by maxi- 

 mum safe speed). The line voltage is constant; a series resistance 

 is used for starting and may be used for adjusting voltage. Any 

 method for loading can be used. If a shunt generator is used as a 

 load, its output may be absorbed in resistance or pumped back into 

 the line. (See 26, Exp. 2-B.) The pumping back method has 

 been modified by A. S. McAllister, so as to form a convenient method 

 for determining the torque of any kind of motor, direct or alternating 

 (Standard Handbook, 3-239 and 8^151; McAllister's Alternating 

 Current Motors, p. 185). 



22. Power. Power is equal to El and, when E is constant, power 

 is directly proportional to /. In Fig. 3, power would be represented 

 by a straight line passing through the origin. It will be seen, there- 

 fore, that the power required does not increase as rapidly as does the 

 torque. 



