Dynamic braking is the use of an electric

traction motor A traction motor is an electric motor used for propulsion of a vehicle, such as locomotives, electric vehicle, electric or hydrogen vehicles, or electric multiple unit trains. Traction (engineering), Traction motors are used in electrically powe ...

as a generator when slowing a vehicle such as an

electric Electricity is the set of physical phenomena associated with the presence and motion of matter possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwel ...

or diesel-electric

locomotive A locomotive is a rail transport, rail vehicle that provides the motive power for a train. Traditionally, locomotives pulled trains from the front. However, Push–pull train, push–pull operation has become common, and in the pursuit for ...

. It is termed " rheostatic" if the generated electrical power is dissipated as heat in brake grid

resistor A resistor is a passive two-terminal electronic component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active e ...

s, and " regenerative" if the power is returned to the supply line. Dynamic braking reduces wear on

friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. Types of friction include dry, fluid, lubricated, skin, and internal -- an incomplete list. The study of t ...

-based braking components, and regeneration lowers net energy consumption. Dynamic braking may also be used on

railcar A railcar (not to be confused with the generic term railroad car or railway car) is a self-propelled railway vehicle designed to transport passengers. The term "railcar" is usually used in reference to a train consisting of a single coa ...

s with

multiple unit A multiple-unit train (or multiple unit (MU)) is a self-propelled train composed of one or more Coach (rail), carriages joined, and where one or more of the carriages have the means of propulsion built in. By contrast, a locomotive-hauled ...

light rail vehicle Light rail (or light rail transit, abbreviated to LRT) is a form of passenger urban rail transit that uses rolling stock derived from tram technology National Conference of the Transportation Research Board while also having some features from ...

electric tram A tram (also known as a streetcar or trolley in Canada and the United States) is an urban rail transit in which vehicles, whether individual railcars or multiple-unit trains, run on tramway tracks on urban public streets; some include segment ...

trolleybus A trolleybus (also known as trolley bus, trolley coach, trackless trolley, trackless tramin the 1910s and 1920sJoyce, J.; King, J. S.; and Newman, A. G. (1986). ''British Trolleybus Systems'', pp. 9, 12. London: Ian Allan Publishing. .or troll ...

es, and

and hybrid electric automobiles.

Principle of operation

Converting

electrical energy Electrical energy is the energy transferred as electric charges move between points with different electric potential, that is, as they move across a voltage, potential difference. As electric potential is lost or gained, work is done changing the ...

to the

mechanical energy In physical sciences, mechanical energy is the sum of macroscopic potential and kinetic energies. The principle of conservation of mechanical energy states that if an isolated system is subject only to conservative forces, then the mechanical ...

of a rotating shaft (electric motor) is the inverse of converting the mechanical energy of a rotating shaft to electrical energy (electric generator). Both are accomplished through the interactions of armature windings with a (relatively) moving external magnetic field, with the armature connected to an electrical circuit with either a power supply (motor) or power receptor (generator). Since the role of the electrical/mechanical energy converting device is determined by which interface (mechanical or electrical) provides or receives energy, the same device can fulfill the role of either a motor or a generator. In dynamic braking, the traction motor is switched into the role of a generator by switching from a supply circuit to a receptor circuit while applying electric current to the field coils that generate the magnetic field ( excitation). The amount of resistance applied to the rotating shaft (braking power) equals the rate of electrical power generation plus some efficiency loss. That is in turn proportional to the strength of the magnetic field, controlled by the current in the field coils, and the rate at which the armature and magnetic field rotate against each other, determined by the rotation of the wheels and the ratio of power shaft to wheel rotation. The amount of braking power is controlled by varying the strength of the magnetic field through the amount of current in the field coils. As the rate of electrical power generation, and conversely braking power, are proportional to the rate at which the power shaft is spinning, a stronger magnetic field is required to maintain braking power as speed decreases and there is a lower limit at which dynamic braking can be effective depending on the current available for application to the field coils. The two main methods of managing the electricity generated during dynamic braking are rheostatic braking and regenerative braking, as described below. 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, such as in cordless power tools. It is not suitable for traction applications.

Rheostatic braking

The electrical energy produced by the motors is dissipated as heat by a bank of onboard

s, referred to as the ''braking grid''. Large cooling fans are necessary to protect the resistors from damage. Modern systems have thermal monitoring, so that if the temperature of the bank becomes excessive it will be switched off, and the braking will revert to being by

only.

Regenerative braking

In electrified systems the process of

regenerative braking Regenerative braking is an energy recovery mechanism that slows down a moving vehicle or object by converting its kinetic energy or potential energy into a form that can be either used immediately or stored until needed. Typically, regenerativ ...

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 the energy which would otherwise be wasted as heat are now available. The Green Goat model, for example, is being used by

Canadian Pacific Railway The Canadian Pacific Railway () , also known simply as CPR or Canadian Pacific and formerly as CP Rail (1968–1996), is a Canadian Class I railway incorporated in 1881. The railway is owned by Canadian Pacific Kansas City, Canadian Pacific Ka ...

BNSF Railway BNSF Railway is the largest freight railroad in the United States. One of six North American Class I railroads, BNSF has 36,000 employees, of track in 28 states, and over 8,000 locomotives. It has three Transcontinental railroad, transcontine ...

Kansas City Southern Railway The Kansas City Southern Railway Company is an American Class I railroad. Founded in 1887, it operated in 10 Midwestern United States, Midwestern and Southeastern United States, Southeastern U.S. states: Illinois, Missouri, Kansas, Oklahoma, Ark ...

and

Union Pacific Railroad The Union Pacific Railroad is a Railroad classes, Class I freight-hauling railroad that operates 8,300 locomotives over routes in 23 U.S. states west of Chicago and New Orleans. Union Pacific is the second largest railroad in the United Stat ...

. On modern passenger locomotives equipped with AC

inverter A power inverter, inverter, or invertor is a power electronic device or circuitry that changes direct current (DC) to alternating current (AC). The resulting AC frequency obtained depends on the particular device employed. Inverters do the op ...

s pulling trains with sufficient

head-end power In rail transport, head-end power (HEP), also known as electric train supply (ETS), is the electrical power distribution system on a passenger train. The power source, usually a locomotive (or a generator car) at the front or 'head' of a train, ...

(HEP) loads, braking energy can be used to power the train's on board systems via 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.

Blended braking

Dynamic braking alone is not enough to stop a locomotive, because its braking effect rapidly diminishes below about . Therefore, it is always used in conjunction with another form of braking, such as an air brake. The use of both braking systems at the same time 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 the air brakes on their own provide. This is achieved by maximizing the dynamic brake portion, and automatically regulating the air brake portion, because the main purpose of dynamic braking is to reduce the amount of air braking required. That conserves air and minimizes the risks of over-heated wheels. One locomotive manufacturer,

Electro-Motive Diesel Electro-Motive Diesel (abbreviated EMD) is a brand of diesel-electric locomotives, locomotive products and diesel engines for the rail industry. Formerly a division of General Motors, EMD has been owned by Progress Rail since 2010. Electro-Motive ...

(EMD), estimates that dynamic braking provides between 50% and 70% of the braking force during blended braking.

Plug braking

A third method of electric braking is plug braking or 'plugging', under which a reverse torque is applied for a short time. It is the most rapid form of electric braking, but comes at the disadvantage of applying significant transient stresses to motors and mechanical components. It is typically abrupt and 'jerky', the braking equivalent of a 'jog' in forward motion, and plug braking is never applied in electric traction applications. Nonetheless, it has been applied widely to applications such as long-travel and cross-travel drives of direct current and alternating-current powered overhead traveling cranes; hoist drives on such cranes typically use rheostatic braking. Reversing drives with (intentional) plug braking typically use rheostatic control for acceleration, and always have resistance in the motor circuit, when plug braking is applied, to limit the reverse (braking) torque. Plugging is usually achieved by moving the controller, briefly, to the first step of the opposite direction, and then back to the off position. After zero speed is reached, plugging must cease to avoid the drive running in reverse, and this function may be provided automatically, by a 'plugging relay'. Plugging does not fit well with inverter-controlled drives; it is becoming less common, and it is actively discouraged in modern crane operation.

Self-load test

It is possible to use the brake grids as a form of

dynamometer A dynamometer or "dyno" is a device for simultaneously measuring the torque and rotational speed ( RPM) of an engine, motor or other rotating prime mover so that its instantaneous power may be calculated, and usually displayed by the dyna ...

or load bank to perform a self-load test of the power output of a locomotive. 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 power output, which is calculated from MG voltage and current output.

Hydrodynamic braking

Diesel locomotive A diesel locomotive is a type of railway locomotive in which the prime mover (locomotive), power source is a diesel engine. Several types of diesel locomotives have been developed, differing mainly in the means by which mechanical power is con ...

s with hydraulic transmission may be equipped for hydrodynamic braking. In this case, the

torque converter A torque converter is a device, usually implemented as a type of fluid coupling, that transfers rotating power from a prime mover, like an internal combustion engine, to a rotating driven load. In a vehicle with an automatic transmission, the ...

fluid coupling A fluid coupling or hydraulic coupling is a hydrodynamics, hydrodynamic or 'hydrokinetic' device used to transmit rotating mechanical power.

acts as a retarder in the same way as a water brake. Braking energy heats the

hydraulic fluid A hydraulic fluid or hydraulic liquid is the medium by which power is transferred in hydraulic machinery. Common hydraulic fluids are based on mineral oil or water. Examples of equipment that might use hydraulic fluids are excavators and backho ...

, 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 A radiator is a heat exchanger used to transfer thermal energy from one medium to another for the purpose of cooling and heating. The majority of radiators are constructed to function in cars, buildings, and electronics. A radiator is always a ...

is not overloaded.

References

External links

Blended braking

, ''

Railway Gazette International ''Railway Gazette International'' is a British monthly business magazine and news website covering the railway, metro, light rail and tram industries worldwide. Available by annual subscription, the magazine is read in over 140 countries by tran ...

'' July 2007 {{DEFAULTSORT:Dynamic Braking Railway brakes