The diagram or function that defines the traction effort as a function of speed; dependent on the type of means of traction, also defines the usable power to the wheel rim (product of tensile stress x velocity), the conditions of the so-called hourly and continuous power, the tension variation as a function of the motors control system. Together with the resistance function, the characteristic of traction allows the calculation of the equation of motion, in the various phases of start-up and at operating speed, the maximum speed of the train and the construction of the driving tables
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