. A text-book of electrical engineering;. 450 * 14-6 20 490 21-3 25 505 Motor 27-3 la £ = 500 - 2-75 . 7„ = 5 486-2 970 10 472-5 590 15 4587 460 20 445 410 25 431-2 390 220 Electrical Engineenng By means of these figures we the speed at which a tramcarof given weight will ascend a given incline, as well as the current taken indoing so. Let p(7 ijg ^j^g weight of the car in metric tons (i metric ton = looo kg*),/the tractive coefficient, the force in kg required to move oneton at a steady speed on the level,and s the slope or gradien
. A text-book of electrical engineering;. 450 * 14-6 20 490 21-3 25 505 Motor 27-3 la £ = 500 - 2-75 . 7„ = 5 486-2 970 10 472-5 590 15 4587 460 20 445 410 25 431-2 390 220 Electrical Engineenng By means of these figures we the speed at which a tramcarof given weight will ascend a given incline, as well as the current taken indoing so. Let p(7 ijg ^j^g weight of the car in metric tons (i metric ton = looo kg*),/the tractive coefficient, the force in kg required to move oneton at a steady speed on the level,and s the slope or gradient in parts per thousand; then the pull required on the level will be W .f. On the slope, the weightAB (Fig. 203) acting vertically downwards can be resolved into two com-ponents, one normal to the surface and the other AO, which has to be directlyovercome by the tractive pull. We have from the figure AO = ^. If we substitute for sin ^ its value 1000 and for AB the weight of the car in kilogrammes, we have AO = xoooW.^ = The pull on the car must therefore be given by the formula F = + l^{/+s)kg*.If the radius of the car-wheels be r metre, the torque on the axles must^^ {f+ s) .y If the gear is such that the motor runs k times as fast as the car axles, thetorque of the motor, by the conservation of energy, must be a ^th part ofthat on the axles. If we assume, further, that the gearing has an efficiency7], we obtain the following formula for the total torque of all the motorson the car: p ^ (f+s) h. 2M,= In our present example the radius of the wheels was 0-39 metre and theratio of the gearing 4-9. If we assume that the weight of the car is 8 tonsand the tractive coefficient 12 kg per ton, we have YM - ^ • °^^ • ^^^ ^ ^^ ^ °^^^ • ^^^ ^ ^^ ij * See footnote on page 4^. 68. Example of Traction Motor Calculation 221 In our case, as is almost universal, each car was provided with twomotors, so that each motor had to produce hal
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