66 THK D1KECT-CURHKNT .MoTOK C!l. 1 V 



In many cases the reduction of M due to armature 

 reaction may of itself compensate for the falling off of 

 speed due to the heat drop, without the use of series 

 winding. Hence a motor in which M decreases, as 

 the armature current increases, will give better speed 

 regulation than one in which M remains constant at all 

 loads. 



Example 25. If the high-tension dynamo of Fio. M 

 has an armature resistance of 8 ohms, it will run at 

 772 r.p.m, at no load on a 900 volt circuit, and at 78 I 

 r.p.m., with a load of 9 amperes. 



Equation 2o gives us the change in the speed of ;i 

 motor when the tension of the line is varied, the 

 induction factor and the resistance remaining constant. 

 If the load is constant and incapable of reversing the 



73J 



motion, the speed will be nothing until E = -~1 . As 



E increases, n will increase, the relation between the two 

 being represented by a straight line such as ah in Fig. 16, 



Tit 



where 0= '71 - Since the load is constant, the cur- 

 M 



rent is also constant. 



Variations in the speed consequent upon 

 changes in the value of jfmay be investigated experi- 

 mentally by connecting the motor through a rheostat to a 

 line of constant tension. The resistance should be adjusted 

 so that the motor runs at a certain measured speed; the 

 current and the tension at the terminals of the motor 

 should be noted. The resistance should then be altered, 

 the load being adjusted so that the current remains 

 constant. Since the tension at the terminals of the 

 rheostat is equal to the product of the current and the 



