162 TIIK I>IKK<T-<:ri!UKNT .MOTOR (II. VII 



the axles without gearing. The motors are connected 

 permanently in series, and are also series wound. Their 

 resistances are : magnets, O087 ohm ; armature, 0'3 

 ohm ; total resistance of each motor, 0'387 ohm. We 

 shall take the tension of the line as being 400 volts 

 throughout. The wheels are 27 inches diameter. The 

 weight of the train is 35 tons. 



A starting rheostat is used, and when this is all out, 

 the motors are left to speed up in series. The maximum 

 current per motor, and hence from the line, will be taken 

 as 150 amperes. 



The induction curve for these motors is plotted in 

 Fig. 39, the horizontal ordinates representing current, 

 and the vertical ordinates values of the induction factor. 

 We must first find the values of the total torque in inch- 

 pounds, and plot them on a speed base. The torque and 

 speed can be obtained from the usual equations, remember- 

 ing that since the motors are in series, the value of E 

 is 200 volts. The speed can be reduced from revolutions 

 per second of the motor to feet per second of the car by 

 multiplying by the factor 262'10~ 3 d, where d is 27 

 inches. 



The results are plotted as a curve, A, in Fig. 40, the 

 origin being at o, horizontal ordinates representing inch- 

 pounds of torque, and vertical ordinates feet per second. 

 When the speed is nothing, the current is 150 amperes 

 per motor, the total torque is 13,280 inch-pounds, and 

 remains constant until the rheostat is all out. At this 

 moment the speed of the car is found to be 16 feet per 

 second, using the ordinary equation for the speed in 

 r.p.s., putting E = 0*38 7, and C=lbO. The curve is thus 

 straight as far as the point where the speed is 16 feet per 



