07-07-2012, 02:48 PM
REGENERATIVE BRAKING SYSTEMS
REGENERATIVE BRAKING SYSTEMS.DOC (Size: 350 KB / Downloads: 151)
ABSTRACT:
A regenerative energy recovery mechanism is that reduces vehicle energy losses by converting some of its kinetic energy into a useful form of energy instead of dissipating it as heat, as in a conventional brake. The converted energy is stored for future use or fed back into a power system.
Electrical regenerative brakes in electric railways feed the generated electricity back into the supply system. In battery electric and hybrid electric vehicles, the energy is stored in a battery or bank of capacitors for later use. Energy may also be stored by compressing air or by a rotating fly wheel.
Regenerative braking is not the same as dynamic braking, which dissipates the electrical energy as heat and does not maintain energy in a usable form.
Simply, regenerative braking enables the vehicle to capture some of the energy that would otherwise be lost when the vehicle slows to a stop. In conventional brake designs, the forward momentum of the vehicle is lost when the brakes are applied. This energy is converted to heat via friction and is dissipated into the surrounding air.
Theoretical investigations of a regenerative braking system show about 25% saving in fuel consumption .The lower operating and environment costs of a vehicle with regenerative braking system should make it more attractive than a conventional one. The exhaust emission of vehicle using the regenerative braking concept would be much less than equivalent conventional vehicles as less fuel are used for consumption.
Energy recovery:
Includes any technique or method of minimizing the input of energy to an overall system by the exchange of energy from one sub-system of the overall system with another. The energy can be in any form in either subsystem, but most energy recovery systems exchange thermal energy in either sensible or latent form.
What Is Regenerative Braking?
If you've been paying attention to the development of hybrid electric vehicles, you've noted that a number of these cars are designed to use "regenerative braking." If you're not an engineer, you may not know what regenerative braking is or how it works.
About Regenerative Braking:
Simply, regenerative braking enables the vehicle to capture some of the energy that would otherwise be lost when the vehicle slows to a stop. In conventional brake designs, the forward momentum of the vehicle is lost when the brakes are applied. This energy is converted to heat via friction and is dissipated into the surrounding air.
In hybrid cars with regenerative braking, some of the forward momentum of the car is captured and stored in batteries
that provide power for accessories or locomotion.
Regenerative brakes will slow the car and capture some of the "lost" energy, but they will not reliably bring the car to a complete halt.
Regenerative braking systems use the energy that would ordinarily be dissipated as heat to power a motor that is also wired to function as a generator. When the vehicle is in motion, the wheels turn the motor, generating current that aids the forward momentum of the vehicle. When the brakes are applied, the current flow in the motor reverses and causes the motor to oppose the vehicle's forward motion.
The applied braking force is proportional to the current that opposes forward motion. The current produces a force in the motor called torque, which slows the vehicle and allows the motor to charge the battery pack. The regenerative braking process slows the vehicle but not enough to bring the car to a precise stop.
For safety reasons, a regenerative braking system is typically used in parallel with a traditional friction brake design. Under most normal driving conditions, regenerative braking would be desirable in a hybrid electric vehicle. Under emergency braking conditions, however, the vehicle requires immediate maximum braking force, which can be delivered only by conventional friction brakes.
In hybrid electric vehicles, coordinating the actions of two braking systems requires extremely precise control. Mechanical components don't react fast enough to provide precise braking controls. A network of electronic controllers, sensors and actuators distributed throughout the vehicle is increasingly used in automotive braking designs. Electronic braking control is a major departure from the largely mechanical process of braking. Inclusion of these "brake-by-wire" technologies also enables other braking and safety technologies like collision detection and corrective braking.
Within an electromechanical braking system, a bevy of sensors track the wheel speed, the positions of the brake pedal and the emergency brakes, the actions of the throttle and steering systems, and the position and movement of the vehicle. Electromechanical braking employs other sensors to control how forcefully the brake should be applied and to measure the build-up of heat within the brakes.
What are regenerative brakes?
Electric trains, cars, and other electric vehicles are powered by electric motors connected to batteries. When you're driving along, energy flows from the batteries to the motors, turning the wheels and providing you with the kinetic energy you need to move. When you stop and hit the brakes, the whole process goes into reverse: electronic circuits cut the power to the motors. Now, your kinetic energy and momentum makes the wheels turn the motors, so the motors work like generators and start producing electricity instead of consuming it. Power flows back from these motor-generators to the batteries, charging them up. So a good proportion of the energy you lose by braking is returned to the batteries and can be reused when you start off again. In practice, regenerative brakes take time to slow things down, so most vehicles that use them also have ordinary (friction) brakes working alongside (that's also a good idea in case the regenerative brakes fail). That's one reason why regenerative brakes don't save 100 percent of your braking energy.
Artwork: Regenerative braking in a nutshell: Left: When you drive, energy flows from the batteries to the wheels via the electric motor. Right: When you brake, energy flows from the wheels to the batteries via the motor, which works as an electric generator. Next time you switch on the power, you can reuse the energy you stored during braking.
The motor as a generator:
Vehicles driven by electric motors use the motor as a generator when using regenerative braking: it is operated as a generator during braking and its output is supplied to an electrical load; the transfer of energy to the load provides the braking effect.
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.
Early examples of this system were the front-wheel drive conversions of horse-drawn cabs by Louis Antoine Krieger (1868-1951). The Krieger electric landau let had a drive motor in each front wheel with a second set of parallel windings (bifilar coil) for regenerative braking.
An Energy Regeneration Brake was developed in 1967 for the AMC Amitron.
This was a completely battery powered urban car whose batteries were recharged by regenerative braking, thus increasing the range of the automobile.
Many modern hybrid and electric vehicles use this technique to extend the range of the battery pack. Examples include the hybrids Toyota Prius, Honda Insight, and the Vectrix electric maxi-scooter.
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, although malfunction of a dynamo can still provide resistance for a while. 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 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 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.