. Electric traction and transmission engineering . o 3to 10 The choice of gear ratio for the trial equipment shouldbe such that^ the peripheral velocity of the motor armaturewhen the car is running at its highest speed will not beexcessive. The ratio of the maximum speed to the schedulespeed varies between and , this ratio increasing asthe runs become shorter and the duration of stops becomeslonger. This enables the selection of the proper gear ratio. 23. Plotting Speed Curves. — To understand the methodcommonly used in plotting speed curves consider the dif- SPEED CURVES. 57 ferent por
. Electric traction and transmission engineering . o 3to 10 The choice of gear ratio for the trial equipment shouldbe such that^ the peripheral velocity of the motor armaturewhen the car is running at its highest speed will not beexcessive. The ratio of the maximum speed to the schedulespeed varies between and , this ratio increasing asthe runs become shorter and the duration of stops becomeslonger. This enables the selection of the proper gear ratio. 23. Plotting Speed Curves. — To understand the methodcommonly used in plotting speed curves consider the dif- SPEED CURVES. 57 ferent portions of the curve in Fig. 28 and the followingformula for the car acceleration (see § 11): Tm ~ Tt A = (I) 100 W where Tm is the tractive effort exerted by a motor, Tt is thetrain resistance per motor, and W is the weight of the caror train per motor. Then Tm = Tt+ 100 WA. (2) The starting part of a speed curve is taken as a straightline, and it passes through O, the origin of time, at anangle Oa with the horizontal such that Sa = tanM, where. Fig. 28. A is the assumed constant rate of acceleration at terminates at the point A having a speed ordinate takenfrom the motor characteristic curves for full voltage cor-responding to the tractive effort T^ calculated from equa-tion (2), in which Tt is based on half schedule speed. The motor part of the speed curve is considered as madeup of a series of elements which are. themselves speed ordinate of the upper end of any element is 58 TRACTION AND TRANSMISSION. assumed, while that of its lower end is the same as for theupper end of the preceding element. This element makeswith the horizontal an angle 6^ = tan~^^„, where Anis the average of the accelerations corresponding to thespeeds at the terminals of the element and each calculatedby means of formula (i). The calculation of these ele-ments is greatly facilitated by two auxiliary curves, oneshowing the relation between motor tractive effort and speedand
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