CH. V SERIES-WOUND MOTORS 119 



of the car in miles per hour on 33-inch wheels. The 

 intercept on the axis of tension is the term '71 , giving 



the least tension that will move the car at the corresponding 

 load. The tractive efforts specified in the figure are 

 obtained from the curve of torque available for useful 

 effort given in Fig. 21. The diagram enables us to find 

 at once the speed of the motor in miles an hour for 

 any given load and terminal tension. 



Example 29. A car is equipped with two of these 

 motors, the tension of the line being 500 volts. If the 

 load is 800 pounds of useful horizontal pull per motor, 

 the speed, with the motors in parallel, will be 9*4 miles an 

 hour. If the motors are now thrown in series the speed 

 will be reduced to 4*2 miles per hour. If the car com- 

 mences to ascend an incline of 3 '5 per cent., the increased 

 load per motor will be 1,200 pounds, and the speed up the 

 incline will be 3'5 miles per hour. Suppose now that in 

 consequence of the drop in the loads the tension of the 

 line falls to 440 volts, the speed will fall to 2-9 miles per 

 hour, the current from the line being 55 amperes, since 

 the motors are in series. Should this speed be too low, 

 the motors must be put in parallel. The terminal tension 

 will then be 440 volts, the speed will rise to 7 '2 miles per 

 hour, while the current will increase to 110 amperes. 



When two motors, A and B, are connected in 

 series electrically, but not coupled, the load on the 

 one will be mechanically independent of that on the 

 other. The current is the same in each, hence for uni- 

 form motion in both motors the loads must be in the ratio 

 of the induction factors. For if the load on one motor, 

 say on A, is fixed, the current in A is fixed, for uniform 



