What are dynamic brakes & how do they work ?

Question

What are dynamic brakes & how do they work ?

Answer 1

Excellent answers!  I'll just add a little something.

In addition to the above, dynamic brakes are further refined into two types, "taper" and "flat".  Flat dynamic brake is controlled the same as taper, with the difference being the amount of retarding effort is a function of throttle position, while taper dynamic is dependent not only on controller position but the speed at which the train is traveling as well.  Taper dynamic is strongest at 40 MPH and drops off as the speed decreases.

Flat dynamic "tapers off" as well, but not until a much lower speed does it really take a nose dive.  The electrical designing engineers compensated for this problem with the principal of "extended range".  This a method where, at around 12 MPH, retarding effort not only is maintained, but increased as the train slows to a stop.  There is a feature called the IPS, or "independent pressure switch", where as the stop is completed and the engineer applies the independent (locomotive) air brake, the extend range portion of the dynamic brake is nullified to help protect against sliding wheels.

The dynamic braking on the newer AC locomotives is even more improved in that there is a great deal of retarding effort right down to a stop.  The older extended range drops off around 4 MPH.

One use of dynamic not mentioned is in its operation in conjunction with the train's automatic brake during a "surprise stop", which is the way a train is stopped the quickest without using the emergency brake feature, which we all know is dangerous.  It is most likely to be used when some jerk has his car stuck on the tracks or being somewhere else he shouldn't be.

But, even under these circumstances, he's going to get hit.  Though the dynamic brake is extremely efficient, it takes a number of seconds before the engineer can switch from power to dynamic brake, to allow for electrical decay in the traction motors from the power position, a minimum of ten seconds before the engineer can begin to set up the dynamic, then time to load, etc.  So, figure at least 1/3 mile or more at 60 MPH before the train begins to slow.

So, once again, it is up to you to keep yourself out of the way.  As pointed out in another answer recently by wittster, every 18 seconds someone gets smacked by a train.  How much time do you have?

Answer 2

Dynamic brakes are banks of large resistors on locomotives.  During heavy braking, such as on a long downhill grade with a heavy train behind the locomotive, the traction motors on each locomotive axle can be used as generators, creating electric current.  This current can be routed into the dynamic brake resistor grids, turned into heat and dissipated into the air by fans.

Dynamic brakes are very effective because as long as the resistor grids does not burn out, they can provide continous braking down a long hill.  In contrast, friction brakes on the train car wheels can overheat and lose braking effectiveness if applied continuously for a long time.

Answer 3

Dynamic brakes (rheostatic brakes in the UK) convert the electric energy back into heat by passing the current through large banks of variable resistors. Vehicles that use dynamic electrical brakes include forklifts, diesel-electric locomotives, and streetcars. If designed appropriately, this heat can be used to warm the vehicle interior. When the energy is meant to be dissipated externally, large radiator-like cowls can be employed to house the resistor banks.

Regenerative brakes in electric railway vehicles feed the generated electricity back into the grid. In battery electric and hybrid electric vehicles, the energy is stored in a battery or bank of capacitors for later use.

The main disadvantage of regenerative brakes when compared with dynamic brakes is the need to closely match the electricity generated with the supply. With DC supplies this requires the voltage to be closely controlled and it is only with the development of power electronics that it has been possible with AC supplies where the supply frequency must also be matched (this mainly applies to locomotives where an AC supply is rectified for DC motors).

It is usual for vehicles to include a 'back-up' system such that friction braking is applied automatically if the connection to the power supply is lost. Also in a DC system or in an AC system that is not directly grid connected via simple transformers special provision also has to be made for situations where more power is being generated by braking than is being consumed by other vehicles on the system.

A small number of mountain railways have used 3-phase power supplies and 3-phase induction motors and have thus a near constant speed for all trains as the motors rotate with the supply frequency both when giving power or braking.

Answer 4

On North American diesel-electric locomotives, every motorized axle has a traction motor on it.

The diesel prime mover (engine) generates electricity, which is applied to the traction motors to get them to move forward or reverse.  Traction motors can be used to either consume electricity (pulling/pushing), or generate it (dynamic braking).

When the engineer "sets up" the dynamics, the traction motors begin generating electricity, which is fed through panels of electrical resistor grids.  These generate resistance, basically making it harder to turn an axle, and slowing the locomotive(s) down.

There are various stages of dynamic braking that can be used.  Throttle, which runs from "Idle" to settings 1 through 8, cannot be applied while in dynamic braking.  The dynamics run from "Set up" to settings 1 through 5, usually.  1 is minimal braking, 5 is maximum.

The resistance created is dissipated as heat, so when a locomotive has the dynamics running, they can get quite loud.  It's kind of a low pitched humming, almost sounding like a box fan x 10,000.