232 THE DIRECT-CURRENT MOTOR CJI. XI 



Let us take the case of a tramcar weighing 10 

 driven by two motors working on a line having a tension 

 of 500 volts. Suppose that we have to design an 

 arrangement by which the car will start up from rest and 

 travel 500 feet in 80 seconds. The motors are to be series 

 wound. 



First find the least possible accelerating current per 

 motor by Equation 107, remembering that TF=5 tons. 

 "We get c rt =29-5 amperes. Take 30 amperes to allow for 

 the resistance of the motor. From Equation 103 we find 



Mv 



that the best value of " is 5'15. For the present we 

 d 



may take f=4'78 and d='33 inches, giving 3f=35 - 5. 



The maximum speed is 25 feet per second or 17 miles 

 an hour. If the frictional and other resistances retarding 

 the motion amount to 3,580 inch-pounds of torque on the 

 car axle, the corresponding current will be 15 amperes, 

 and the resistance of each motor must therefore be 

 0'6 ohm. 



The initial acceleration will be 1-25 f.p.s. per second, 

 and the current of 45 amperes will be constant until the 

 starting rheostat is all out, at which point the speed of the 



.,, , 500-45x0-6 



motor will be given by n= --- -- =800 r.p.m. 



o5*5 



The speed of the car will therefore be 24'2 feet per second. 

 Thus we see that if the induction factor is constant, the 

 acceleration can be maintained constant up to a speed 

 of 97 per cent, of final speed ; after this point the motor 

 will speed up according to the law already given in 

 Chapter VII. ; the error involved in assuming that the 

 acceleration is constant up to full speed will be small, and 



