228 II IK DIRECT-CURRENT MOTOR (II. XI 



The greatest possible value of is when Ji is 



tv 



nothing ; hence the least possible accelerating current that 

 would satisfy the conditions is given by the equation 



If we determine the value of c,, by this equation, and 

 insert it in Equation 103, we shall find that the running 

 conditions require that R should be nothing. We must 

 then make c,, a little larger than this. 



Example 54. A car weighing 40 tons has to be 

 equipped with two motors controlled on the parallel method, 

 operating on a line having a tension of 500 volts. The 

 frictional and other resistances retarding the motion 

 amount to 4,950 inch-pounds of torque on the car axles. 

 The motors have to be designed to start up from rest and 

 cover 400 yards in 80 seconds. 



The least possible accelerating current is given by 

 Equation 105, where D is expressed in feet, and W in 

 tons. Inserting the given values we get c,, = 35'8 

 amperes. As the motors must be of some finite resistance, 

 put c a equal to 40 amperes, and use this value in Equa- 



!// 

 tion 103 ; we find that the best value of is 5*2. 



(3L 



If the motors are to be gearless the induction factor is 

 thus found to be as 5-2 times the wheel diameter. To 

 make the motors as light as possible, take d as small as 

 possible, say 30 inches, then M becomes 156. If gearing 

 is introduced the motors can be made lighter ; thus if 

 <= 4, and the wheel diameter is GO inches, the induction 

 factor is 78. We shall assume v to be unity. 



