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Posted by tracksionmotor on Sat Apr 22 18:18:58 2006, in response to Re: SMEE, posted by Allan on Fri Apr 21 13:57:07 2006.

With application of brakes, relays engage contactors that rewire the motors into generators and 'push' electricity into the resistive grid banks (those cages undercar) and not the third rail. A electric circuit senses the amount of current generated and disconnects the grid, applying air brakes. when the generated current falls below an amount of 100 Amperes...a speed of about 10 MPH or so. This is why it is very important to check 'application and release' contacts for proper operation of the brake tower...trainset needs to see an electrical signal to control all brakes of all cars in SMEE. I teach this stuff to crew so they have a basic understanding of what the work is.

R142s I had worked on and the M7s I work on now eliminated all the electrical relays and power contactors I had become familiar with, The computer system senses wheel rotation (speed), braking (pressure sensors) and current generated to determine if its dynamic or pneumatic braking. The old trains had something called 'lock-out' where relays locked out the brakes when traction power was applied...kind of not being able to step on your cars accelator pedal and the brake at the same time. R142s and M7s remain SMEEs because trainline braking is basically electric signals.

You mentioned 'pushing electricity into the third rail.' This is called REGENERATIVE BRAKING. Idea is that power generated does not get wasted generating heat in the resistive grid but goes back into the third rail. Problem is the third rail is not a good conductor of electricity and a trainset engaging dynamic braking needs to find a closeby train to push power into....does not always work!

On hybrid automobiles, dynamic braking is standard, When you drive uphill, your engine basically provides the power to go up. When you start to go downhill and apply brakes to slow down, the computer rewires electric traction motor(s) as generators to charge batteries ONLY if you apply brakes and not coasting down the hill. Different systems work differently...some vehicles use an engine/generator/electric motor configuration (like Diesel locomotives) while others use a combination of two wheel drive and other two wheels are electric motor driven (like Ford Escape 4WD.) Concept remains the same.

Thanks for giving me the opportunity to 'jump.' My TARDIS is overdue for service. I love this stuff. RRCI Peter


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