Dynamic braking
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Dynamic braking is the use of the electric traction motors of a railroad vehicle as generators to slow the vehicle. Radiators in the form of large fan cooled resistor banks (“brake grids”) dissipate the generated electric current as heat.
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[edit] Principle of operation
During braking, the traction motor connections are reversed to turn them into electrical generators. The motor fields are connected across the main traction generator (MG) and the motor armatures are connected across the brake grids. The rolling locomotive wheels turn the motor armatures, and if the motor fields are now excited by the MG, the motors will act as generators. For a given direction of travel, current flow through the motor armatures during braking will be opposite to that during motoring. Therefore, the motor exerts torque in a direction that is opposite from the rolling direction. Braking effort is proportional to the product of the magnetic strength of the field windings, times that of the armature windings.
[edit] Blended braking
Dynamic braking alone is insufficient to stop a locomotive, as its braking effect rapidly diminishes below about 10 - 12 mph (16 - 19 km/h) Therefore it is always used in conjunction with the regular Air Brake. Early designs had a separate dynamic brake handle, but nowadays the two braking systems are usually controlled by a single brake handle. This combined system is called blended braking.
Although blended braking combines both dynamic and air braking, the resulting braking force is designed to be the same as what the air brakes on their own provide. This is achieved by maximizing the dynamic brake portion and automatically regulating the air brake portion as the main purpose of dynamic braking is to reduce the amount of air braking required. This conserves air and minimizes the risks of over-heated wheels. One locomotive manufacturer, EMD, estimates that dynamic braking provides between 50% to 70% of the braking force during blended braking.
[edit] Regenerative braking
In electrified systems the similar process of regenerative braking is employed whereby the current produced during braking is fed back into the power supply system for use by other traction units, instead of being wasted as heat. It is normal practice to incorporate both regenerative and dynamic braking in electrified systems. If the power supply system is not "receptive", i.e. incapable of absorbing the current, the system will default to Dynamic mode in order to provide the braking effect.
[edit] Self-load test
It is possible to use the dynamic brake grids as a form of dynamometer to perform a "self load" test of locomotive engine HP. With the locomotive stationary, the MG output is connected to the grids instead of the traction motors. The grids are normally large enough to absorb the full engine output power, which is calculated from MG voltage and current output.
