To slow down large prime movers like locomotives a process termed dynamic electric braking is used to switch the traction motor to a generator mode in which mechanical power from the drive wheels is absorbed and used to generate electrical current. As shown in the schematic, the electric power is passed through a resistor grid

(a) Which consists of an array of metallic blades electrically connected in series

(b) The blade material is a high-temperature, high electrical resistivity alloy, and the electrical power is dissipated as heat by internal volumetric generation. To cool the blades, a motor-fan moves high-velocity air through the grid.

(a) Treating the space between the blades as a rectangular channel of 220-mm x 4-mm cross section and 70-mm length, estimate the heat removal rate per blade if the air stream has an inlet temperature and velocity of 25°C and 50 m/s, respectively, while the blade has an operating temperature of 600°C.

(b) On a locomotive pulling a 10-car train there may be 2000 of these blades. Based on your result from part (a), how long will it take to slow a train whose total mass is 10 6 kg from a speed of 120 km/h to 50 km/h using dynamic electric braking?

Insulated support ring Motor-fan -Current flow in blades Resistor Insulated grid support ring Electrical power from drive generator L= 70 mm la) Fluid V, T= 25C Channel 220 mm x 4 mm (b)