[edit]The motor as a generator
Regenerative braking is used on hybrid gas/electric automobiles to recoup some of the energy lost during stopping. This energy is saved in a storage battery and used later to power the motor whenever the car is in electric mode.
[1]Early examples of this system were the
front-wheel drive conversions of horse-drawn
cabs by Louis Antoine Krieger (1868–1951). The Krieger electric
landaulet had a drive motor in each front wheel with a second set of parallel windings (
bifilar coil) for regenerative braking.
[2] In England, the Raworth system of "regenerative control" was introduced by tramway operators in the early 1900s, since it offered them economic and operational benefits as explained by A. Raworth of Leeds in some detail.
[3][4][5] These included tramway systems at Devonport (1903), Rawtenstall, Birmingham, Crystal Palace-Croydon (1906) and many others. Slowing down the speed of the cars or keeping it in hand on descending gradients, the motors worked as generators and braked the vehicles. The tram cars also had wheel brakes and track slipper brakes which could stop the tram should the electric braking systems fail. In several cases the tram car motors were shunt wound instead of series wound, and the systems on the Crystal Palace line utilized series-parallel controllers.
[6] Following a serious accident at Rawtenstall, an embargo was placed on this form of traction in 1911. Twenty years later, the regenerative braking system was reintroduced.
[5]Regenerative braking has been in extensive use on railways for many decades. The Baku-Tbilisi-Batumi railway (Transcaucasian railway or Georgian railway) started utilizing regenerative braking in the early 1930s. This was especially effective on the steep and dangerous Surami Pass.
[7] In Scandinavia the Kiruna to Narvik railway carries iron ore from the mines in Kiruna in the north of Sweden down to the port of Narvik in Norway to this day. The rail cars are full of thousands of tons of iron ore on the way down to Narvik, and these trains generate large amounts of electricity by their regenerative braking. From Riksgränsen on the national border to the Port of Narvik, the trains use only a fifth of the power they regenerate. The regenerated energy is sufficient to power the empty trains back up to the national border.
[8] Any excess energy from the railway is pumped into the power grid to supply homes and businesses in the region, and the railway is a net generator of electricity.
[edit]Limitations
Traditional
friction-based braking is used in conjunction with mechanical regenerative braking for the following reasons:
- The regenerative braking effect drops off at lower speeds; therefore the friction brake is still required in order to bring the vehicle to a complete halt. Physical locking of the rotor is also required to prevent vehicles from rolling down hills.
- 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 two-wheel drive car) and regenerative braking power only applies to such wheels because they are the only wheels linked to the drive motor, 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 and if the battery or capacitors are already charged. For this reason, it is normal to also incorporate dynamic braking to absorb the excess energy.
- Under emergency braking it is desirable that the braking force exerted be the maximum allowed by the friction between the wheels and the surface without slipping, over the entire speed range from the vehicle's maximum speed down to zero. The maximum force available for acceleration is typically much less than this except in the case of extreme high-performance vehicles. Therefore, the power required to be dissipated by the braking system under emergency braking conditions may be many times the maximum power which is delivered under acceleration. Traction motors sized to handle the drive power may not be able to cope with the extra load and the battery may not be able to accept charge at a sufficiently high rate. Friction braking is required to dissipate the surplus energy in order to allow an acceptable emergency braking performance.
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
EV-1 was the first commercial car to do this. Engineers Abraham Farag and Loren Majersik were issued two patents for this
brake-by-wire technology.
[11][12][edit]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).
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.
Savings of 17% are claimed for
Virgin Trains Pendolinos.
[13] There is also less wear on friction braking components. The
Delhi Metro saved around 90,000 tons of
carbon dioxide (
CO2) from being released into the atmosphere by regenerating 112,500 megawatt hours of electricity through the use of regenerative braking systems between 2004 and 2007. It is expected that the Delhi Metro will save over 100,000 tons of
CO2 from being emitted per year once its phase II is complete through the use of regenerative braking.
[14]Another form of simple, yet effective regenerative braking is used on the
London Underground which is achieved by having small slopes leading up and down from stations. The train is slowed by the climb, and then leaves down a slope, so kinetic energy is converted to
gravitational potential energy in the station.
Electricity generated by regenerative braking may be fed back into the traction power supply; either offset against other electrical demand on the network at that instant, or stored in
lineside storage systems for later use.
[15][edit]Comparison of dynamic and regenerative brakes
Main article:
Dynamic brakeDynamic brakes ("rheostatic brakes" in the UK), unlike regenerative brakes, dissipate the electric energy as heat by passing the current through large banks of variable
resistors. Vehicles that use dynamic brakes include
forklifts,
Diesel-electric locomotives, and
streetcars. This heat can be used to warm the vehicle interior, or dissipated externally by large
radiator-like cowls 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 and increased maintenance cost of the lines. 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
rectified for DC motors).
A small number of
mountain railways 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.
[edit]Kinetic Energy Recovery Systems
Kinetic Energy Recovery Systems (KERS) were used for the motor sport
Formula One's
2009 season, and are under development for road vehicles. KERS was abandoned for the 2010 Formula One season, but re-introduced for the 2011 season. As of the 2011 season, 9 teams are using KERS, with 3 teams having not used it so far in a race.
[16] One of the main reasons that not all cars use KERS is because it adds an extra 25 kilograms of weight, while not adding to the total car weight, it does incur a penalty particularly seen in the qualifying rounds, as it raises the car's center of gravity, and reduces the amount of
ballast that is available to balance the car so that it is more predictable when turning.
[17] FIA rules also limit the exploitation of the system. The concept of transferring the vehicle’s kinetic energy using
flywheel energy storage was postulated by physicist
Richard Feynman in the 1950s
[citation needed] and is exemplified in complex high end systems such as the
Zytek, Flybrid,
[18] Torotrak
[19][20] and Xtrac used in F1 and simple, easily manufactured and integrated
differential based systems such as the Cambridge Passenger/Commercial Vehicle Kinetic Energy Recovery System (CPC-KERS).
[21]Xtrac and Flybrid are both licensees of Torotrak's technologies, which employ a small and sophisticated ancillary gearbox incorporating a
continuously variable transmission (CVT). The CPC-KERS is similar as it also forms part of the driveline assembly. However, the whole mechanism including the flywheel sits entirely in the vehicle’s hub (looking like a drum brake). In the CPC-KERS, a differential replaces the CVT and transfers torque between the
flywheel, drive wheel and road wheel.
[edit]Use in motor sport
[edit]History
A Flybrid Systems Kinetic Energy Recovery System
The first of these systems to be revealed was the Flybrid
[citation needed] This 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, 80.4 HP) for 6.67 seconds is available. The 240 mm diameter flywheel weighs 5.0 kg and revolves at up to 64,500 rpm. Maximum torque is 18 Nm (13.3 ftlbs). The system occupies a volume of 13 litres
[citation needed]Two minor incidents have been reported during testing of KERS systems in
2008. The first occurred when the
Red Bull Racing team tested their KERS battery for the first time in July: it malfunctioned and caused a fire scare that led to the team's factory being evacuated.
[22] The second was less than a week later when a
BMW Sauber mechanic was given an electric shock when he touched
Christian Klien's KERS-equipped car during a test at the
Jerez circuit.
[23]Formula One have stated that they support responsible solutions to the world's environmental challenges,
[24] and the
FIA allowed the use of 81 hp (60 kW; 82 PS) KERS in the regulations for the
2009 Formula One season.
[25] Teams began testing systems in 2008: energy can either be stored as mechanical energy (as in a
flywheel) or as electrical energy (as in a battery or
supercapacitor).
[26] With the introduction of KERS in the 2009 season, only four teams used it at some point in the season:
Ferrari,
Renault,
BMW, and
McLaren. Eventually, during the season, Renault and BMW stopped using the system.
Vodafone McLaren Mercedes became the first team to win a F1 GP using a KERS equipped car when
Lewis Hamilton won the Hungarian Grand Prix on July 26, 2009. Their second KERS equipped car finished fifth. At the following race, Lewis Hamilton became the first driver to take pole position with a KERS car, his team mate,
Heikki Kovalainen qualifying second. This was also the first instance of an all KERS front row. On August 30, 2009,
Kimi Räikkönen won the Belgian Grand Prix with his KERS equipped Ferrari. It was the first time that KERS contributed directly to a race victory, with second placed
Giancarlo Fisichella claiming "Actually, I was quicker than Kimi. He only took me because of KERS at the beginning".
[27]Although KERS was still legal in F1 in the 2010 season, all the teams had agreed not to use it.
[28] New rules for the 2011 F1 season which raised the minimum weight limit of the car and driver by 20 kg to 640 kg,
[29] along with the FOTA teams agreeing to the use of KERS devices once more, meant that KERS returned for the 2011 season.
[30] This is still optional as it was in the 2009 season; as of the 2011 season 3 teams have elected not to use it.
[16]As of 2014, the power storage of the KERS units will increase from 60 kW to 120 kW. This will be to balance the sport's move from 2.4 litre V8 engines to 1.6 litre V6 engines.
[31][edit]Autopart makers
Bosch Motorsport Service is developing a KERS for use in motor racing. These electricity storage systems for hybrid and engine functions include a
lithium-ion battery with scalable capacity or a
flywheel, a four to eight kilogram
electric motor (with a maximum power level of 60 kW/80 hp), as well as the KERS controller for power and battery management. Bosch also offers a range of electric hybrid systems for commercial and light-duty applications.
[32] [edit]Carmakers
Vodafone McLaren Mercedes began testing of their KERS in September 2008 at the Jerez test track in preparation for the 2009 F1 season, although at that time it was not yet known if they would be operating an electrical or mechanical system.
[35] In November 2008 it was announced that
Freescale Semiconductor would collaborate with
McLaren Electronic Systems to further develop its KERS for
McLaren's Formula One car from 2010 onwards. Both parties believed this collaboration would improve McLaren's KERS system and help the system filter down to road car technology.
[36] [edit]Motorcycles
KTM racing boss
Harald Bartol has revealed that the factory raced with a secret Kinetic Energy Recovery System (KERS) fitted to
Tommy Koyama's motorcycle during the 2008 season-ending 125cc
Valencian Grand Prix. This was illegal and against the rules, so they were later banned from doing it afterwards.
[38] 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
LMP1 that will be equipped with a kinetic energy recovery system. "
[39] Peugeot was the first manufacturer to unveil a fully functioning LMP1 car in the form of the 908 HY at the 2008 Autosport 1000 km race at Silverstone.
[40] [edit]Use in compressed air cars
