CH. IX CONTROL 185 



II. The motors may be started and run in series. 



III. The motors may be started in series and run in 

 parallel. 



To compare the relative advantages of these methods 

 we shall take a practical case and work out the results for 

 each method. 



Suppose, then, that we have given the following data : 

 A motor-car weighing 40 tons is driven by two gearless 

 motors ; the wheel diameter is 33 inches ; the maximum 

 speed is to be 30 miles per hour ; the retarding torque is 

 200 inch-pounds per ton of load ; the track is straight and 

 level; the tension of the line is 500 volts; the minimum 

 resistance per motor is 0'4 ohm; the maximum current 

 per motor is 200 amperes ; the maximum current from the 

 line is 200 amperes ; the induction factor is to be constant. 

 We shall divide the process into steps, each being dis- 

 tinguished by a different law of acceleration. 



CASE I. Motors started and run in parallel. 

 Since the specification requires that the motors shall drive 

 the car at a maximum speed of 30 miles per hour, with a 

 frictional torque of 200 inch-pounds per ton of load, the in- 

 duction factor must be determined by these data. The car 

 weighs 40 tons, therefore the load per motor is 4,000 inch- 

 pounds of torque. As the wheels are 33 inches diameter, 

 and the velocity ratio unity, a tractive effort of 242 pounds 

 per motor is required. First suppose that the resistance 

 of the motor is as small as possible namely, 0'4 ohm. 

 Using Equation 21, we find that Jf=95'8. 



STEP I. Starting rheostat in circuit ; constant 

 acceleration. The motors are in parallel, the maximum 

 permissible current is 200 amperes, and the current per 

 motor is 100 amperes. As the resistance of the motor is 



