To slow down large prime movers like locomotives, aprocess
Chapter 8, Problem 8.91(choose chapter or problem)
To slow down large prime movers like locomotives, aprocess termed dynamic electric braking is used toswitch the traction motor to a generator mode inwhich mechanical power from the drive wheels isabsorbed and used to generate electrical current. Asshown in the schematic, the electric power is passedthrough a resistor grid (a), which consists of an arrayof metallic blades electrically connected in series (b).The blade material is a high-temperature, high electri-cal resistivity alloy, and the electrical power is dissi-pated as heat by internal volumetric generation. Tocool the blades, a motor- fan moves high-velocity airthrough the grid.(a) Treating the space between the blades as a rectan- gular channel of 220-mm?4-mm cross sectionand 70-mm length, estimate the heat removal rateper blade if the airstream has an inlet temperatureand velocity of 25C and 50 m/s, respectively,while the blade has an operating temperature of600C.(b) On a locomotive pulling a 10-car train, there maybe 2000 of these blades. Based on your result frompart (a), how long will it take to slow a train whosetotal mass is 106kg from a speed of 120 km/h to50 km/h using dynamic electric braking?
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