How do dynamic brakes work?
Dynamic brakes work by turning the traction motors into generators then putting a load on the generators(traction motors) that causes the generators to resist turning. The electrical load of the generator is a resistance grid, basicly a big heat element. This heat element requires a lot of air to keep cool so a portion of the energy is used to turn a big fan to blow the heat away from the grid. Basicaly the energy of motion is converted to electricity then to heat, then dissipated to the atmosphere.
Check out this site:
Real simple demonstration you can do in your driveway:
With vehicle running, note general sound of engine.
Turn on the headlights. Notice that engine is now working a little harder. It’s not real noticable, but you can hear the difference, especially at the point in time where you turn the lights on and off.
You’ve sort of duplicated the dynamic brakes side of the dynamic brakes + traction motor (as generator)/traction motor (as motor)+main generator(alternator) equation. The vehicle alternator is serving the same function as the traction motors when they are used as generators in dynamic braking, and the lights (actually the filaments) are serving as the “dynamic brake resistance grids.”
Don’t try using your headlights to slow down, though…[:P]
Notice that conventional brakes and dynamic brakes both convert the energy of motion to heat, the difference being in how it’s done.
Larry:
Observe the additional load on your engine at idle when you turn on your rear window defogger. The grid in the window is nichrome wiring (like your toaster) and creates quite a resistance similar to dynamic brake grids.
Good point. Didn’t think of it since my truck doesn’t have a rear window defroster… Either way it’s a good simple way to explain the principle.
Thanks very much!