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 or hydrogen vehicles, elevators or electric multiple unit. Traction motors are used in electrically powered rail vehicles ( electric multip ...

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 that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by ...

or diesel-electric

locomotive A locomotive or engine is a rail transport vehicle that provides the motive power for a train. If a locomotive is capable of carrying a payload, it is usually rather referred to as a multiple unit, motor coach, railcar or power car; the ...

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

resistor A resistor is a passive two-terminal electrical 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. There are several types of friction: *Dry friction is a force that opposes the relative lateral motion of ...

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

railcar A railcar (not to be confused with a 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 coach (carriage, car), with a dri ...

s with

multiple unit A multiple-unit train or simply multiple unit (MU) is a self-propelled train composed of one or more carriages joined together, which when coupled to another multiple unit can be controlled by a single driver, with multiple-unit train cont ...

s, light rail vehicles, electric trams,

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 trol ...

es, and

and hybrid electric automobiles.

Principle of operation

Converting electrical energy to the mechanical energy 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 A field coil is an electromagnet used to generate a magnetic field in an electro-magnetic machine, typically a rotating electrical machine such as a motor or generator. It consists of a coil of wire through which a current flows. In a rotati ...

that generate the magnetic field (

excitation Excitation, excite, exciting, or excitement may refer to: * Excitation (magnetic), provided with an electrical generator or alternator * Excite Ballpark, located in San Jose, California * Excite (web portal), web portal owned by IAC * Electron ...

). 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. 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 into a form that can be either used immediately or stored until needed. In this mechanism, the electric traction mo ...

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 (french: Chemin de fer Canadien Pacifique) , 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 Canad ...

BNSF Railway BNSF Railway is one of the largest freight railroads in North America. One of seven North American Class I railroads, BNSF has 35,000 employees, of track in 28 states, and nearly 8,000 locomotives. It has three transcontinental routes that ...

Kansas City Southern Railway The Kansas City Southern Railway Company is an American Class I railroad. Founded in 1887, it operates in 10 midwestern and southeastern U.S. states: Illinois, Missouri, Kansas, Oklahoma, Arkansas, Tennessee, Alabama, Mississippi, Loui ...

and

Union Pacific Railroad The Union Pacific Railroad , legally Union Pacific Railroad Company and often called simply Union Pacific, is a freight-hauling railroad that operates 8,300 locomotives over routes in 23 U.S. states west of Chicago and New Orleans. Union Paci ...

. On modern passenger locomotives equipped with AC inverters pulling trains with sufficient head-end power (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 The Bombardier ALP-46 is an electric locomotive built in Germany by Bombardier between 2001 and 2002 for use in the United States. It is derived from the German Class 101. New Jersey Transit (NJT) is the only railroad to operate this locomotive ...

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 the regular air brake. This combined system is called blended braking.

Li-ion A lithium-ion or Li-ion battery is a type of rechargeable battery which uses the reversible reduction of lithium ions to store energy. It is the predominant battery type used in portable consumer electronics and electric vehicles. It also se ...

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 Progress Rail Locomotives, doing business as Electro-Motive Diesel (EMD), is an American manufacturer of diesel-electric locomotives, locomotive products and diesel engines for the rail industry. The company is owned by Caterpillar through its s ...

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

Self-load test

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

dynamometer A dynamometer or "dyno" for short, 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 ...

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 is a diesel engine. Several types of diesel locomotives have been developed, differing mainly in the means by which mechanical power is conveyed to the driving whee ...

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

torque converter A torque converter is 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 torque converter connects the p ...

or fluid coupling 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 backhoe ...

, 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 Radiators are heat exchangers 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 ...

is not overloaded.

References

External links

Blended braking

, ''

Railway Gazette International ''Railway Gazette International'' is a 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 transport ...

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