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Dynamic braking is the use of an electric traction motor as a generator when slowing a vehicle such as an electric or diesel-electric locomotive. It is termed "rheostatic" if the generated electrical power is dissipated as heat in brake grid resistors, and "regenerative" if the power is returned to the supply line. Dynamic braking reduces wear on friction-based braking components, and regeneration lowers net energy consumption. Dynamic braking may also be used on railcars with multiple units, light rail vehicles, electric trams, and electric and hybrid electric automobiles.
Principle of operation
When braking, the motor fields are connected across either the main traction generator (diesel-electric locomotive) or the supply (electric locomotive) and the motor armatures are connected across either the brake grids or supply line. The rolling locomotive wheels turn the motor armatures, and if the motor fields are now excited, the motors will act as generators.
During dynamic braking, the traction motors, which are now acting as generators, are connected to the braking grids (large resistors), which put a large load on the electrical circuit. When a generator circuit is loaded down with resistance, it causes the generators to slow their rotation. By varying the amount of excitation in the traction motor fields and the amount of resistance imposed on the circuit by the resistor grids, the traction motors can be slowed down to a virtual stop (approximately 3-5 MPH).
For permanent magnet motors, dynamic braking is easily achieved by shorting the motor terminals, thus bringing the motor to a fast abrupt stop. This method, however, dissipates all the energy as heat in the motor itself, and so cannot be used in anything other than low-power intermittent applications due to cooling limitations. It is not suitable for traction applications.
The electrical energy produced by the motors is dissipated as heat by a bank of onboard resistors. Large cooling fans are necessary to protect the resistors from damage. Modern systems have thermal monitoring, so, if the temperature of the bank becomes excessive, it will be switched off, and the braking will revert to friction only.
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 rheostatic braking in electrified systems. If the power supply system is not "receptive", i.e. incapable of absorbing the current, the system will default to rheostatic mode in order to provide the braking effect.
Yard locomotives with onboard energy storage systems which allow the recovery of some of this energy which would otherwise be wasted as heat are now available. The Green Goat model, for example, is being used by Canadian Pacific Railway, BNSF Railway, Kansas City Southern Railway and Union Pacific Railroad.
On modern passenger locomotives equipped with AC inverters pulling trains with sufficient Head End Power loads braking energy can be used to power the train's on board systems as a form of regenerative braking if the electrification system is not receptive or even if the track is not electrified to begin with. The HEP load on modern passenger trains is so great that some new electric locomotives such as the ALP-46 were designed without the traditional resistance grids.
Dynamic braking alone is insufficient to stop a locomotive, as its braking effect rapidly diminishes below about 10 to 12 miles per hour (16 to 19 km/h). Therefore, it is always used in conjunction with the regular air brake. This combined system is called blended braking. Li-ion batteries have also been used to store energy for use in bringing trains to a complete halt.
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, Electro-Motive Diesel (EMD), estimates that dynamic braking provides between 50% to 70% of the braking force during blended braking.
It is possible to use the brake grids as a form of dynamometer or load bank to perform a "self load" test of locomotive engine horsepower. With the locomotive stationary, the main generator (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.
Diesel engined locomotives with hydraulic transmission may be equipped for hydrodynamic braking. In this case, the torque converter or fluid coupling acts as a retarder in the same way as a water brake. Braking energy heats the hydraulic fluid, and the heat is dissipated (via a heat exchanger) by the engine cooling radiator. The engine will be idling (and producing little heat) during braking, so the radiator is not overloaded.
- Professor Satoru Sone, Kogakuin University (2007-07-02). "Wayside and on-board storage can capture more regenerated energy". Railway Gazette International.