CH. V SEKIES-WOUND MOTOKS 117 



It is possible that the term that has to be deducted 

 on account of the load may be so large that when the 

 motors are put in series they cannot run at all. Taking 

 Equation 47 as giving the speed of a motor for any ter- 

 minal tension E, we see that the speed will be nothing 



7~JL 



if E='7l-~. Hence unless the tension on the motor 

 M 



TJj. 



when in series with a second motor is greater than '71^, 



the motors will not run when put in series. The limiting 

 value of E may also be written 



where T is the horizontal tractive effort, d is the diameter 

 of the driving wheel in inches, and v the velocity ratio. 



By the use of Equation 47 we can obtain curves giving 

 the speed for different tensions with constant load. If we 

 have the curves of total and useful torque and the induc- 

 tion curve of a motor, we can draw a set of curves that 

 will give us a complete insight into the behaviour of the 

 motor under different conditions. 



Take the case of the G. E. 800 railway motor. The 

 curves of torque and induction are given in Fig. 21 ; from 

 these we can find the current required for any useful torque 

 or tractive effort on wheels of given diameter, and also the 

 corresponding value of M. Then by use of the equation, 

 remembering that t is the total torque for each current, we 

 can obtain two points on the speed line. 



The results for this motor are plotted in Fig. 29. 

 Vertical ordinates represent the tension at the terminals 

 of the motor. Horizontal ordinates represent the speed 



