92 THE DIRECT-CURRENT MOTOR CH. IV 



these points lie on a curve passing through a and g. In 

 the figure four such curves are taken for loads of 8, 10, 15, 

 and 20, the load on A represented by lid being taken 

 as 15. 



For any given load on the intermediate shaft, the loads 

 on A and B will be equal, and half of that of C, so that 

 the numbers on the curves may be taken to represent the 

 loads on the intermediate shaft, assuming all three wheel 

 diameters to be equal. 



Suppose that the motors are rotating in opposite direc- 

 tions. We can at once see from the diagram what the 

 values of the two induction factors must be to get any 

 required speed of the intermediate shaft for any given 

 load. Let the induction factor of A be kept constant 

 at jlf=6, with a constant load on each motor represented 

 by 15. If the induction factor of B is greater than 6, B 

 will run slower than A by an amount given by the 

 diagram. If the induction factor of B is less than 6, B 

 will run faster than A, the speed of the intermediate shaft 

 can thus be increased or diminished, and its direction 

 reversed, simply by altering the induction factor of B. 



The speed of -B is a maximum when 3/=3'4; if .V 

 is reduced below this value the speed diminishes, and 

 therefore also the speed of C ; when 3/=2'3 the two 

 speeds are again equal, and a further decrease in the 

 value of M will cause the rotation of C to be reversed. 

 When B has reached its maximum speed we may increase 

 the speed of by increasing the induction factor of A. 



The student should compare the curve of speed with 

 constant load and variable induction factor plotted on a 

 base of current as in Fig. 19, with the curve of speed 

 plotted on a base of induction factor as in Fig. 15. 



