Regenerative brake
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2008-07-14T18:33:50Z
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/* See also */ regenerative shock
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.
For example, electrical regenerative brakes in electric [[railway]] vehicles feed the generated electricity back into the [[Railway electrification system|supply system]]. In [[Battery electric vehicle|battery electric]] and [[hybrid vehicle|hybrid electric]] vehicles, the energy is stored in a [[battery (electricity)|battery]] or bank of [[capacitors]] for later use. Other forms of energy storage which may be used include compressed air and [[Flywheel energy storage|flywheels]].
Regenerative braking should not be confused with ''[[dynamic braking]]'', which dissipates the electrical energy as heat.
==Limitations==
Traditional [[friction]]-based braking is still used with electrical regenerative braking for the following reasons:
* The regenerative braking effect rapidly reduces at lower speeds, therefore the friction brake is still required in order to bring the vehicle to a complete halt.
* The friction brake is a necessary back-up in the event of failure of the regenerative brake.
* Most road vehicles with regenerative braking only have power on some wheels (as in a 2WD car) and regenerative braking power only applies to such wheels, so in order to provide controlled braking under difficult conditions (such as in wet roads) friction based braking is necessary on the other wheels.
* The amount of electrical energy capable of dissipation is limited by either the capacity of the supply system to absorb this energy or on the state of charge of the battery or capacitors. No regenerative braking effect can occur if another electrical component on the same supply system is not currently drawing power or if the battery or capacitors are already charged. For this reason, it is normal to also incorporate dynamic braking to absorb the excess energy.
* For these reasons there is typically the need to control the regenerative braking and match the friction and regenerative braking to produce the desired total braking output. The GM [[General Motors EV1|EV-1]] was the first commercial car to do this. Engineers Abraham Farag and Loren Majersik were issued 2 patents for this 'Brake by Wire' technology.<ref>GM patent [http://www.google.com/patents?id=VzshAAAAEBAJ&dq=5775467 5775467] – ''Floating electromagnetic brake system''.</ref><ref>GM patent [http://www.google.com/patents?id=_A0kAAAAEBAJ&dq=5603217 5603217] – ''Compliant master cylinder''.</ref>
==The motor as a brake==
Regenerative braking utilizes the fact that an electric motor can also act as a [[Electrical generator|generator]]. The vehicle's electric [[traction motor]] is reconnected as a generator during braking and its output is connected to an electrical load. It is this load on the motor that provides the braking effect.
An early example of this system was the [[Energy Regeneration Brake]], developed in 1967 for the [[Amitron]]. This was a completely [[Battery electric vehicle|battery]] powered urban [[concept car]] whose batteries were recharged by regenerative braking, thus increasing the range of the automobile.<ref> Time Magazine, Business Section, ''Next: the Voltswagon?'', [[December 22]], [[1967]]. </ref>
==Electric railway vehicle operation==
During braking, the [[traction motor]] connections are altered 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 load. The MG now excites the motor fields. The rolling locomotive or multiple unit wheels turn the motor armatures, and the motors act as generators, either sending the generated current through onboard resistors ([[dynamic braking]]) or back into the supply (regenerative braking)<br /> 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.
When rail operator [[c2c]]'s began using regenerative braking with a fleet of [[Bombardier]] [[British Rail Class 357 |Class 357]] EMUs, monitoring over the first two weeks showed an immediate energy saving of 15%. Savings of 17% are claimed for [[Virgin Trains]] [[British Rail Class 390 |Pendolinos]].<ref>{{cite news | title=Regenerative braking boosts green credentials | url =http://www.railwaygazette.com/features_view/article/2007/07/7577/regenerative_braking_boosts_green_credentials.html | work =[[Railway Gazette International]] | date = [[July 2]], [[2007]] |accessdate=2008-03-21 | author=[[Roger Ford]]}}</ref> There is also less wear on friction braking components.
==Comparison of dynamic and regenerative brakes==
{{main|Dynamic brake}}
Dynamic brakes ("rheostatic brakes" in the UK), unlike regenerative brakes, dissipate the electric energy as heat by passing the current through large banks of variable [[resistor]]s. Vehicles that use dynamic brakes include [[forklift]]s, [[Diesel-electric]] [[locomotive]]s and [[streetcar]]s. If designed appropriately, this heat can be used to warm the vehicle interior. If dissipated externally, large [[radiator]]-like cowls are employed to house the resistor banks.
The main disadvantage of regenerative brakes when compared with dynamic brakes is the need to closely match the generated current with the supply characteristics. With DC supplies, this requires that the voltage be closely controlled. Only with the development of [[power electronics]] has this been possible with AC supplies, where the supply frequency must also be matched (this mainly applies to locomotives where an AC supply is [[rectifier|rectified]] for DC motors).
A small number of [[mountain railway]]s have used [[3-phase]] power supplies and 3-phase [[induction motors]]. This results in a near constant speed for all trains as the motors rotate with the supply frequency both when motoring and braking.
==Use in motor sport==
[[Max Mosley]] of the [[FIA]] has announced that all cars will become hybrid by 2013, along with other changes to the vehicles. The governing body of international [[motor sport]], the [[FIA]], has allowed the use of 60 kW "Kinetic Energy Recovery Systems" (KERS), in the regulations for the [[2009 Formula One season]].<ref>{{cite web | url =http://www.fia.com/resources/documents/1151088479__2009_F1_TECHNICAL_REGULATIONS.pdf |format=PDF | title =2009 Formula One Technical Regulations | publisher =FIA | date =[[December 22]], [[2006]] | accessdate =2006-12-22 | language = }}</ref><ref>{{cite web |url=http://www.evworld.com/syndicated/evworld_article_1160.cfm |publisher=EVWorld |author=Chris Ellis |title=Formula One: 'Braking' New Ground |date=[[December 26]], [[2006]] |accessdate=2008-03-21}}</ref>
[[Automobile Club de l'Ouest]], the organizer behind the annual [[24 Hours of Le Mans]] event and the [[Le Mans Series]], is currently "studying specific rules for [[Le Mans prototype|LMP1]] which will be equipped with a kinetic energy recovery system."<ref>{{cite web | url =http://www.lemans.org/sport/sport/reglements/ressources/auto_2008/cdc_reglement_lmp_fr_gb_2008.pdf |format=PDF |title =ACO Technical Regulations 2008 for Prototype "LM"P1 and "LM"P2 classes, page 3| publisher =Automobile Club de l'Ouest (ACO) | date =2007-12-20 | accessdate =2008-01-20 }}</ref>
The hybrid system that will be implemented in Formula 1 is known as KERS, which stands for Kinetic Energy Recovery System. The maximum power that can be released from such a system is restricted to 60kW by the FIA. Energy can either be stored as mechanical energy (as in a flywheel) or can be stored as electrical energy (as in a battery or supercapacitor). <ref>{{cite web |url=http://www.fia.com/resources/documents/1151088479__2009_F1_TECHNICAL_REGULATIONS.pdf|title=2009 FORMULA ONE TECHNICAL REGULATIONS|publisher=FIA| author= FIA management |date=[[December 22]], [[2006]] |accessdate=2008-07-8}}</ref>
The first of these systems to be revealed was the Flybrid<ref>{{cite web |url=http://www.racecar-engineering.com/articles/f1/182014/f1-kers-flybrid.html |title=F1 KERS: Flybrid |publisher=Racecar Engineering |author=Charles Armstrong-Wilson |date=[[February 21]], [[2008]] |accessdate=2008-03-21}}</ref> which appeared in an article in Racecar Engineering magazine.
The Flybrid F1 KERS System weighs 24 kg and has an energy capacity of 400 kJ after allowing for internal losses. A maximum power boost of 60 kW (81.6 PS) for 6.67 sec is available. The 20-cm diameter flywheel weighs 5.0 kg and revolves at up to 64,500 rpm. Maximum torque is 18 Nm. The system occupies a volume of 13 liters.
Toyota has used a supercapacitor for regeneration on a race car that won the Tokachi race in July 2007. http://www.greencarcongress.com/2007/07/toyota-hybrid-r.html
==Use in compressed air cars==
Regenerative brakes are being used in [[compressed air car]]s to refuel the tank during braking.
== See also ==
* [[Brake (railway)]]
* [[Electromagnetic brake]]
* [[Dynamic braking]]
* [[Regenerative shock absorber]]
* [[Supercapacitor]]
==References==
<references />
[[Category:Vehicle braking technologies]]
[[Category:Brakes]]
[[Category:Locomotive parts]]
[[Category:Electric vehicles]]
[[cs:Rekuperace]]
[[de:Nutzbremse]]
[[es:Freno regenerativo]]
[[fr:Frein régénérateur]]
[[ko:회생 제동]]
[[it:Freno rigenerativo]]
[[hu:Energia-visszatáplálás]]
[[ja:回生ブレーキ]]
[[no:Regenerativ bremsing]]
[[pl:Hamowanie rekuperacyjne]]
[[pt:Frenagem regenerativa]]
[[ru:Рекуперативное торможение]]
[[sk:Rekuperácia (dopravný prostriedok)]]
[[zh:再生制動]]