Selection of Motor Equipment by Principle of Similar Speed-Time Curves

Data for making preliminary estimates of speed of cars required and capacity of motor to do a given service have been rather limited. The object of this paper is to show a method whereby it is possible to provide curves which are simple to calculate and easy to apply. No mathematical formula has ever been derived to express the speed-time or current-time curves for a railway motor. Similarity between speed-time curves has been resorted to and this paper shows how this method can be used to give very accurate results by proper consideration of the various factors which influence the shape of the speed-time curve. Previous methods for estimating motor capacities are based on the horsepower of motor required, but with the advent of the self-ventilated and blown motors it is necessary to calculate the r. m. s. current for a given cycle and select a motor having this continuous rating. A method for doing this is proposed, based on similar current-time curves. The limiting values of average running speeds for given rates of acceleration and braking, and the data for obtaining motor capacity necessary at these limits are indicated on each curve. This helps one to visualize why it is not practicable to perform certain schedules on a fixed rate of acceleration and braking basis.