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Dynamic braking
 
 
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Dynamic braking is the use of the electric traction motors of a railroad vehicle as generators when slowing the vehicle. It is termed rheostatic if the generated electrical power is dissipated as heat in brake grid resistors and regenerative
Regenerative brake

A regenerative brake is a mechanism that reduces vehicle speed by converting some of its kinetic energy into another useful form of energy. This captured energy is then stored for future use or fed back into a power system for use by other vehicles....
 if the power is returned to the supply line. Dynamic braking lowers the wear of friction braking components and additionally regeneration can also lower energy consumption.

ng braking the motor fields
Stator

The stator is the stationary part of a rotordynamics system, such as in an electric generator or electric motorDepending on the configuration of a spinning electromotive device the stator may act as the field magnet, interacting with the armature to create motion, or it may act as the armature, receiving its influence from moving...
 are connected across either the main traction generator (Diesel-electric loco
Diesel locomotive

A Diesel locomotive is a type of railroad locomotive in which the prime mover is a Diesel engine. Several types of Diesel locomotive have been developed, the principal distinction being in the means by which the prime mover's mechanical power is conveyed to the driving wheels ....
) or the supply (Electric locomotive
Electric locomotive

An electric locomotive is a locomotive powered by electricity from an external source. Sources include overhead lines, third rail, or an on-board electricity storage device such as a battery or flywheel energy storage system....
) and the motor armatures
Armature (electrical engineering)

In electrical engineering, an armature generally refers to one of the two principal electrical components of an electromechanical machine - a Electric motor or Electrical generator, but may also mean the pole piece of a permanent magnet or electromagnet, or the moving iron part of a solenoid or relay....
 are connected across either the brake grids or supply line.






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Dynamic braking is the use of the electric traction motors of a railroad vehicle as generators when slowing the vehicle. It is termed rheostatic if the generated electrical power is dissipated as heat in brake grid resistors and regenerative
Regenerative brake

A regenerative brake is a mechanism that reduces vehicle speed by converting some of its kinetic energy into another useful form of energy. This captured energy is then stored for future use or fed back into a power system for use by other vehicles....
 if the power is returned to the supply line. Dynamic braking lowers the wear of friction braking components and additionally regeneration can also lower energy consumption.

Principle of operation

During braking the motor fields
Stator

The stator is the stationary part of a rotordynamics system, such as in an electric generator or electric motorDepending on the configuration of a spinning electromotive device the stator may act as the field magnet, interacting with the armature to create motion, or it may act as the armature, receiving its influence from moving...
 are connected across either the main traction generator (Diesel-electric loco
Diesel locomotive

A Diesel locomotive is a type of railroad locomotive in which the prime mover is a Diesel engine. Several types of Diesel locomotive have been developed, the principal distinction being in the means by which the prime mover's mechanical power is conveyed to the driving wheels ....
) or the supply (Electric locomotive
Electric locomotive

An electric locomotive is a locomotive powered by electricity from an external source. Sources include overhead lines, third rail, or an on-board electricity storage device such as a battery or flywheel energy storage system....
) and the motor armatures
Armature (electrical engineering)

In electrical engineering, an armature generally refers to one of the two principal electrical components of an electromechanical machine - a Electric motor or Electrical generator, but may also mean the pole piece of a permanent magnet or electromagnet, or the moving iron part of a solenoid or relay....
 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. 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
Torque

Torque is the tendency of a force to rotate an object about an axis . Just as a force is a push or a pull, a torque can be thought of as a twist....
 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.

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.

Regenerative braking

In electrified systems
Railway electrification system

A Railway electrification system supplies Electric potential energy to railway locomotives and multiple units so that they can operate without having an on-board Prime mover ....
 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
Default

Default, as in failing to meet an obligation, may refer to:* Default **Default judgment* Default , failure to satisfy the terms of a loan obligation or to pay back a loan...
 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 Canadian Pacific Railway
Canadian Pacific Railway

The Canadian Pacific Railway , known as CP Rail between 1968 and 1996, is a Canada Class I railroad operated by Canadian Pacific Railway Limited....
, BNSF Railway
BNSF Railway

The BNSF Railway , often referred to as the Burlington Northern and Santa Fe Railway, headquartered in Fort Worth, Texas, Texas, is one of the four remaining transcontinental railroads and one of the largest railroad networks in North America....
, Kansas City Southern Railway
Kansas City Southern Railway

The Kansas City Southern Railway , owned by Kansas City Southern Industries, is the smallest and second-oldest Class I railroad company still in operation....
 and Union Pacific Railroad
Union Pacific Railroad

The Union Pacific Railroad , headquartered in Omaha, Nebraska, is the largest railroad network in the United States. James R. Young is president, CEO and Chairman....
.

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
Air brake (rail)

An air brake is a conveyance brake applied by means of Gas compressor. Modern trains rely upon a fail-safe air brake system that is based upon a design patented by George Westinghouse on March 5, 1872....
. 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, 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
Dynamometer

A dynamometer or "dyno" for short, is a machine used to measure torque and rotational speed from which power produced by an Heat engine, motor or other rotating Wiktionary:prime mover can be calculated....
 or load bank
Load bank

A load bank is a device which develops an External electric load, applies the load to an electrical power source and converts or dissipates the resultant power output of the source....
 to perform a "self load" test of locomotive engine HP
Horsepower

Horsepower is the name of several non-International System of Units units of power . It was originally defined to allow the output of steam engines to be measured and compared with the power output of draft horses....
. 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.

External links

  • , Railway Gazette International
    Railway Gazette International

    Railway Gazette International is a monthly business journal covering the railway, rapid transit, light rail and tram industries worldwide. Available by annual subscription, the magazine is read in over 140 countries by transport professionals and decision makers, railway managers, consultants and suppliers to the rail industry....
     July 2007