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Dynamic Braking A train brake system where the traction motors are used to provide a braking force by reconnecting them in such a way that they become generators. Al Krug, referring to diesel-electric locomotive braking in a reply to a question in a newsgroup, put it this way. Dynamic brakes are fundamentally no different from locomotive air brakes. Both systems convert the energy of the rolling train into heat and then throw away that heat. If you apply the loco air brakes, the brake shoes are pushed against the wheel treads and the resulting friction produces heat. The energy required to produce this heat power makes the loco hard to keep moving. The heat power is thrown away into the air by radiating from the hot brake shoes and hot wheel treads into the surrounding atmosphere. A loco with air brakes applied is hard to keep moving but it will keep going, particularly if it has energy to move it in the form of a train pushing it from behind. The energy (kinetic energy, it's called) comes from the rolling train that is pushing it. The trouble with using engine brakes alone is that eventually (rather quickly actually) the shoes and wheels get very hot. Hot enough to destroy them. This is because heat is produced faster than it can be dissipated by radiating it into the air. So dynamic brakes are used to move the heat dissipation away from the brake shoes and wheel treads to the dynamic brake grids instead. Like an electric bathroom heater, the dynamic brake grids are designed to handle this amount of heat power (as long as the grid cooling blowers are operating). Train air brakes work in the same manner as loco air brakes. They convert the rolling energy of the train into heat and throw it away. But when using train brakes, the heat generated is dispersed through out the entire train. It is spread over (say) 800 wheels instead of just the few wheels of the loco. Because of this, the train's wheels do not normally get overheated. They will get warm or even hot but not normally so hot as to cause damage. On prolonged downgrades, however, the braking energy required is sufficient to overwhelm the heat dissipating ability of even all the train's wheels and overheating occurs. This is the main reason for using dynamic brakes, to move the heat dissipation away from the wheels to the dynamic brake grids. Remember that it takes a 3,000 HP diesel engine just to turn the generator on a 3,000 HP loco. Commercial generating power plants require 100s of thousands of HP to turn the generators that supply your household power. Generators are hard to turn when they are producing power. This is because you never get anything for free. If you take power out of a generator you must put at least equal power into it. (Actually more than equal since nothing is ever 100% efficient either). In locomotive dynamic brakes, the traction motors are acting as generators. That means the traction motors are hard to turn. The loco's wheels are what are turning the traction motors. They are geared to the traction motors. This means the loco's wheels are hard to turn. They resist turning because they are geared to the traction motors which are hard to turn when generating power, as they are doing when in dynamic braking. Because the loco's wheels are hard to turn when in dynamic braking the loco is hard to move or in other words it resists movement just as if the airbrakes were applied making the wheels hard to turn. The energy required to pu***his "hard to move" loco comes from the rolling train. This removes energy from the rolling train slowing it. Note that dynamic brakes are used by electric multiple unit trains as well. In these designs, careful blending with air braking is required to maintain a smooth braking profile. Electronic control is used to determine that the brake effort demanded by the brake controller is matched by the brake effort achieved by the train. Preference is given to the dynamic brake to save wear on brake blocks (shoes) or pads and air braking is added if necessary to achieve the braking rate required. Dynamic braking can be used on electric railways to convert the energy of the train back into usable power by diverting the braking current into the current rail or overhead line. This is known as regenerative braking. It is used in the same way as rheostatic braking but the energy can be used by other trains requiring power. The power developed by a braking train may not be accepted by the line if no other trains are drawing power so trains equipped with regenerative braking will usually have resistor grids as well to absorb the excess energy. The balance between regenerated current and rheostatic current is also controlled electronically
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