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Posted by tracksionmotor on Fri Apr 21 23:26:49 2006, in response to Re: SMEE, posted by VictorM on Fri Apr 21 18:55:18 2006.

Well, kind of. Primary braking always remains pneumatic. When the operator applies brakes in a DC traction motor set-up, the group box cycles the motor wiring as self excited generators (motors are series wound...field and rotor) to provide power in high rpm to the resistive grid. You can turn the shaft of a alternator/generator coupled to a load by hand without any resistance at a slow speed but once you reach a speed of electrical excitation, you encounter resistance. The resistance at higher speeds in a trainset is called 'dynamic braking.' In the group box there is a simple electrical mechanism that senses the generator current manufactured by the tracksionmotors. As the trainset slows down by dynamic braking, the amount of current generated gets smaller. When the current generated goes down to about 100 Amperes DC (10 MPH or so,)
pneumatic braking is engaged. IF a particular car has a dynamic braking failure, upon inspection the cars truck (s) will have very noticible brake shoe wear as compared to others in the trainset. In the event of dynamic braking failure, the specific car will have engaged pneumatic braking positively...very reason why primary braking is pneumatic. IF you were to Brake In Emergency at a high speed...engaging a propulsion failure...air is numbah ONE. Worst case scenario: loss of third rail/catenary/trainset total LVDC trainline battery and control systems...R142 master controller joystick won't play PackMan and bumping into emergency does nothing because POU magnet valves allow coast. Reach up and pull BIE chord...conductors valve evacuates 'straight air' to atmospheric pressure, magnet valves drop and reserve air engages emergency braking. Mucho FLATZ.

This has been Dynamic Braking 101. Tracksionmotor is RRCI Peter

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