Regenerative Braking

Regenerative Braking

What is regenerative braking?



Conventional braking systems convert kinetic energy into heat, usually via friction. And this does waste a great deal of energy!  Regenerative braking system reclaims and enables the storing of the kinetic energy, by converting it into another form of energy[electrical energy generally] in a reusable manner.  Many modern electric drive vehicles including electric locomotives and HEV’s[Hybrid Electric Vehicles] have regenerative braking systems.

Working : When a Vehicle has forward momentum :  The wheels are coupled to the rotors of electric traction motors in an EV. Instead of a current being applied to the motor to turn the rotors, the rotors are turned by the wheels of the EV

 The rotors experience opposing torque as current is induced in the motor coils.

 This opposing torque slows the vehicle, and the generated energy is then stored for further use.

Options for storing the Energy:  Recharging vehicle battery packs. State of charge influences how much of the generated current can be safely stored  Charging an array of capacitors/super-capacitors  Pro: Very high charge/discharge rate (high power density)  Con: Much lower energy density than batteries  Allows for regenerative braking if batteries are at a high state of charge

 Implemented

on

some

 Non-electrical methods  Fluid compression  Flywheels

Regenerative Braking: Slowing the Vehicle and Generating Electricity :

buses

Friction is the enemy of motion. The armature of the M/G is slowed by the force of inducing current in the windings as it passes over the opposing poles in the magnets in the stator (it's constantly battling the push/pull of the opposing polarities), and all this happens just when the brake is applied. It is this magnetic friction that slowly saps the vehicle's kinetic energy and helps scrub off speed.

Limitations :

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 2-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. Regenerative braking can only occur if no other electrical component on the same supply system is drawing power and only if the battery or capacitors are not fully 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.

Used in today’s world ? Kinetic energy recovery systems Kinetic energy recovery systems (KERS) were used for the motor sport Formula 1 and the FIA allowed the use of 81 hp (60 kW; 82 PS) KERS in the regulations since the 2009 season. Think of it as A hatchback say Hyundai i20 pushing the F1 car from behind with its full might ; The full power of a hatchback retrieved by jus recycling the wasted power of braking.

BoschMotorsport 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.

Toyota has used a super capacitorfor regeneration on Supra HV-R hybrid race car that won the 24 Hours of Tokachi race in July 2007, along with them is Toyota Prius,Honda Insight, Ford Escape Hybrid, Tesla Roadster, Chevy Volt, and many such cars today use the concept of regenerative braking.

Trains : When the driver of a train wants to stop or slow down a train, he applies regenerative braking. The motors of the locomotives which are normally pulling the train, change their mode of operation from motor to a generator. The mechanical energy of the train is converted to electrical energy by the generator, which then goes to the power

supply grid. This feature is called regenerative braking. Thus, even braking losses are gainfully utilized in three phase locomotives.

Compiled and edited By, Kartavya Bhagat Semester IV.