NATIONAL INSTITUTE OF TECHNOLOGY

Seminar ReportOn

“Regenerative Braking Systems”

Submitted By: Guided By :Kundan Kumar Dr. G.A. Harmain (Professor)Enroll: 196/06, Roll-24 Mechanical Engg. Deptt. Mechanical Engg. Deptt. NIT Srinagar.

NATIONAL INSTITUTE OF TECHNOLOGYHAZRATBAL, SRINAGAR- 190006 (J&K)

AKNOWLEDGEMENT

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I wish to express my profound gratitude to Dr . G.A. Harmain for his valuable

guidance ,inspiration and help without which it would have been difficult to achieve

anything worthwhile.

Lastly , I would like to thank everybody who

helped me directly or indirectly in preparation of my Seminar Report.

Kundan KumarEnroll: 196/06, Roll-24Mechanical Engg. Deptt. NIT Hazratbal Srinagar.

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CERTIFICATE

It is certified that the seminar report entitled “Regenerative Braking

Systems” is the work carried by Kundan Kumar under my guidance and

supervision. He has fulfilled all the requirements as per status of NIT for

the submission of this report.

Dr. G.A. Harmain (Professor) Mechanical Engg. Deptt. NIT Srinagar.

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CONTENTS

Serial No. Topics Page No.

1 ABSTRACT 6

2INTODUCTION TO

REGENERATIVE BRAKING SYSTEM

7-10

3 THE MOTOR AS AGENERATOR 11

4ELECTRIC RAILWAY VEHICLE

OPERATION12-13

5REGENERATIVE BRAKING

DIAGRAM13

6REGENERATIVE BRAKING

CIRCUITS14

7REGENERATIVE BRAKE

CONTROLLERS15

8REGENERATIVE BRAKING

EFFICIENCY16-17

9COMPARISON OF DYNAMIC

AND REGENERATIVE BRAKES18

10REGENERATIVE BRKING

LIMITATIONS AND DRAWBACKS

19-20

11 CONCLUSION 21

12 BIBLIOGRAPHY 22

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LIST OF FIGURES

Figures Page

FIGURE 1 REGENERATIVE BRAKES IN CAR 7

FIGURE 2ELECTRIC MOTOR CUM

GENERATOR11

FIGURE 3WORKING OF REGENERATIVE

BRAKES13

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1. ABSTRACT

Regenerative Braking Systems as the name describes is a system that regenerates

the braking energy. This system utilizes a storage battery that stores and supplies

power according to need. Now a day’s these systems are frequently used in

numerous field such as hybrid and electric vehicles, some Formula1 cars, heavy

trucks, high speed trains and some industrial appliances. Many renowned

automobile companies are making use of this system such as Toyota, BMW,

Bosch, Tesla and many others. Its efficiency is quite good and so helps in

increasing the fuel efficiency. It has certain limitations due to which it is not being

used in general automobiles running on petrol and diesel. It is based upon the

energy conservation and momentum conservation. Regenerative brakes uses

electrical storage battery, electrical motors which works as generator also and

brake controllers. Regenerative brakes more efficiently can be used in the heavy

vehicles having large momentums.

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2. Introduction to Regenerative Braking Systems

A regenerative brake is an apparatus, a device or system which allows a vehicle to

recapture and store part of the kinetic energy that would otherwise be 'lost' to heat

when braking.

Or,

A regenerative brake is a mechanism that reduces vehicle speed by converting

some of its kinetic energy into a storable form of energy instead of dissipating it as

heat as with a conventional brake. The captured energy is stored for future use or

fed back into a power system for use by other vehicles. Electrical regenerative

brakes in electric railway vehicles 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.

Figure 1 (Regenerative brakes in a car)

As the figure indicates the axle is coupled with the electric motor mechanically and

the motor is connected to the battery (The blue arrows in the figure denote the

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power supplied by the batteries and the green arrows denotes the power supplied to

the batteries for storage)

Every time you step on your car's brakes, you're wasting energy. Physics tells us

that energy cannot be destroyed. So when your car slows down, the kinetic energy

that was propelling it forward has to go somewhere. Most of it simply dissipates as

heat and becomes useless. That energy, which could have been used to do work, is

essentially wasted.

Is there anything that you, the driver, can do to stop wasting this energy? Not

really. In most cars it's the inevitable byproduct of braking and there's no way you

can drive a car without occasionally hitting the brakes. But automotive engineers

have given this problem a lot of thought and have come up with a kind of braking

system that can recapture much of the car's kinetic energy and convert it into

electricity, so that it can be used to recharge the car's batteries. This system is

called regenerative braking.

All throughout the country, gas prices are still climbing to numbers that seem to

unhinge wallets, yet jaws even wider. Car advertisements now highlight gas

mileage efficiency rather than the quality of steering and handling. Thus, the era of

the Hybrid has begun, and with it, revolutions in saving power and energy have

become the forefront innovations in recent cars. As hybrids rely primarily on the

battery, recharging this mechanism to exhume the maximum power is vital to

prolonging the existence of this car. One such way is via the braking system.

Everyone is quite familiar with the concept of friction, a force that resists the

direction of motion and reduces the force causing that motion. Now the braking

system of most hybrids transfers the torque from the wheels into the motor shaft

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through chains and gears. The electric motor inside, maintains the ability to

convert electric energy into mechanical (normal) as well as mechanical energy,

such as heat, a byproduct of friction, back into electric (regenerative). Located on

the shaft of the motor, also known as a rotor, are magnets that move past electric

coils on the stator, the stationary part of a motor. This creates electricity which is

delivered to the battery in the form of electrical energy. In laymen’s terms, this

process is defined as turning the electric motor backwards to convert the

mechanical energy into electric energy. The regenerative nature of the braking

system and the conversion of mechanical energy into electrical energy are very

efficient processes and contribute to saving the car’s power.

At present, these kinds of brakes are primarily found in hybrid vehicles like the

Toyota Prius, and in fully electric cars, like the Tesla Roadster. In vehicles like

these, keeping the battery charged is of considerable importance. However, the

technology was first used in trolley cars and has subsequently found its way into

such unlikely places as electric bicycles and even Formula One race cars.

In a traditional braking system, brake pads produce friction with the brake rotors to

slow or stop the vehicle. Additional friction is produced between the slowed

wheels and the surface of the road. This friction is what turns the car's kinetic

energy into heat. With regenerative brakes, on the other hand, the system that

drives the vehicle does the majority of the braking. When the driver steps on the

brake pedal of an electric or hybrid vehicle, these types of brakes put the vehicle's

electric motor into reverse mode, causing it to run backwards, thus slowing the

car's wheels. While running backwards, the motor also acts as an electric generator,

producing electricity that's then fed into the vehicle's batteries. These types of

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brakes work better at certain speeds than at others. In fact, they're most effective in

stop-and-go driving situations. However, hybrids and fully electric cars also have

friction brakes, as a kind of back-up system in situations where regenerative

braking simply won't supply enough stopping power.

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.

Understanding how regenerative braking works may require a brief look at the

system it replaces. Conventional braking systems use friction to counteract the

forward momentum of a moving car. As the brake pads rub against the wheels (or

a disc connected to the axle), excessive heat energy is also created. This heat

energy dissipates into the air, wasting up to 30% of the car's generated power. Over

time, this cycle of friction and wasted heat energy reduces the car's fuel efficiency.

More energy from the engine is required to replace the energy lost by braking.

Hybrid gas/electric automobiles now use a completely different method of braking

at slower speeds. While hybrid cars still use conventional brake pads at highway

speeds, electric motors help the car brake during stop-and-go driving. As the driver

applies the brakes through a conventional pedal, the electric motors reverse

direction. The torque created by this reversal counteracts the forward momentum

and eventually stops the car.

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3. The motor as a generator

Vehicles driven by electric motor 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. But regenerative braking does more than simply stop the car.

Electric motors and electric generators (such as a car's alternator) are essentially

two sides of the same technology. Both use magnetic fields and coiled wires, but in

different configurations. Regenerative braking systems take advantage of this

duality. Whenever the electric motor of a hybrid car begins to reverse direction, it

becomes an electric generator or dynamo. This generated electricity is fed into a

chemical storage battery and used later to power the car at city speeds.

Figure 2 (Electric motor cum Generator)

4. Electric railway vehicle operation

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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. 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.

Many stations on the London Underground are built so that the tracks entering the

platform are on a slight incline, and those leaving it on a decline. This saves energy

by letting gravity slow the train on its entry to the station without expending (as

much) energy and, helps it accelerate on departure. This could be seen as a form of

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regenerative braking where the energy is stored as potential energy (using gravity)

instead of electrical energy.

5. Regenerative Braking Diagram

This simple diagram shows how a regenerative braking system is able to recapture

some of the vehicle's kinetic energy and convert it into electricity. This electricity

is then used to recharge the vehicle's batteries. It also shows how the electric motor

on reversing the direction becomes a generator and the various positions of the

components used. It depicts the two cases when the vehicle is accelerated and

when the brakes are being applied.

Figure 3 (Working of a regenerative brake)

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6. Regenerative braking circuits

In a regenerative braking system, the trick to getting the motor to run backwards is

to use the vehicle's momentum as the mechanical energy that puts the motor into

reverse. Momentum is the property that keeps the vehicle moving forward once it's

been brought up to speed. Once the motor has been reversed, the electricity

generated by the motor is fed back into the batteries, where it can be used to

accelerate the car again after it stops. Sophisticated electronic circuitry is necessary

to decide when the motor should reverse, while specialized electric circuits route

the electricity generated by the motor into the vehicle's batteries. In some cases, the

energy produced by these types of brakes is stored in a series of capacitors for later

use. In addition, since vehicles using these kinds of brakes also have a standard

friction braking system, the vehicle's electronics must decide which braking system

is appropriate at which time. Because so much is controlled electronically in a

regenerative braking system, it's even possible for the driver to select certain

presets that determine how the vehicle reacts in different situations. For instance,

in some vehicles a driver can select whether regenerative braking should begin

immediately whenever the driver's foot comes off the accelerator pedal and

whether the braking system will take the car all the way to 0 mph (0 kilometers per

hour) or will let the car coast slightly.

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7. Regenerative brake controllers

Regenerative braking systems are particularly effective in stop-and-go driving

conditions.

Brake controllers are electronic devices that can control brakes remotely, deciding

when braking begins, ends, and how quickly the brakes need to be applied. In

towing situations, for instance, brake controllers can provide a means of

coordinating the brakes on a trailer with the brakes on the vehicle doing the

towing.

In vehicles that use these kinds of brakes, the brake controller not only monitors

the speed of the wheels, but it can calculate how much torque -- rotational force --

is available to generate electricity to be fed back into the batteries. During the

braking operation, the brake controller directs the electricity produced by the motor

into the batteries or capacitors. It makes sure that an optimal amount of power is

received by the batteries, but also ensures that the inflow of electricity isn't more

than the batteries can handle.

The most important function of the brake controller, however, may be deciding

whether the motor is currently capable of handling the force necessary for stopping

the car. If it isn't, the brake controller turns the job over to the friction brakes,

averting possible catastrophe. In vehicles that use these types of brakes, as much as

any other piece of electronics on board a hybrid or electric car, the brake controller

makes the entire regenerative braking process possible.

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8. Regenerative braking efficiency

The energy efficiency of a conventional car is only about 20 percent, with the

remaining 80 percent of its energy being converted to heat through friction. The

miraculous thing about regenerative braking is that it may be able to capture as

much as half of that wasted energy and put it back to work. This could reduce fuel

consumption by 10 to 25 percent. Hydraulic regenerative braking systems could

provide even more impressive gains, potentially reducing fuel use by 25 to 45

percent [source: HybridCars.com]. In a century that may see the end

of the vast fossil fuel reserves that have provided us with energy for automotive

and other technologies for many years, and in which fears about carbon emissions

are coming to a peak, this added efficiency is becoming increasingly important.

The beginning of the 21st century could very well mark the final period in which

internal combustion engines are commonly used in cars. Already automakers are

moving toward alternative energy carriers,

such as electric batteries, hydrogen fuel and even compressed air. Regenerative

braking is a small, yet very important, step toward our eventual independence from

fossil fuels. These kinds of brakes allow batteries to be used for longer periods of

time without the need to be plugged into an external charger. These types of brakes

also extend the driving range of fully electric vehicles. In fact, this technology has

already helped bring us cars like the Tesla Roadster, which runs entirely on battery

power. Sure, these cars may use fossil fuels at the recharging stage -- that is, if the

source of the electricity comes from a fossil fuel such as coal -- but when they're

out there on the road, they can operate with no use of fossil fuels at all, and that's a

big step forward.

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The added efficiency of regenerative braking also means less pain at the pump,

since hybrids with electric motors and regenerative brakes can travel considerably

farther on a gallon of gas, some achieving more than 50 miles per gallon at this

point. And that's something that most drivers can really appreciate.

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9. Comparison of dynamic and regenerative brakes

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 resistors. Vehicles that use dynamic brakes include forklifts, Diesel-

electric locomotives and streetcars. 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 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.

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10. Regenerative Braking limitations and drawbacks

Traditional friction-based braking is used 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.

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

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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 absorb the surplus energy in order to allow an

acceptable emergency braking performance.

The main disadvantage of regenerative brakes when compared with dynamic

brakes is the need to closely match the electricity generated with the supply.

With DC supplies this requires the voltage to be closely controlled and it is

only with the development of power electronics that it has 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).

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.

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11. CONCLUSION

From the above discussions the fact is clear that there is a larger scope of

regenerative braking in future where we will face the ending of the fossil fuel and

we have to depend upon these battery and electric operated hybrid vehicles. The

regenerative brake increases the efficiency of these vehicles by considerable

extents. But regenerative brake needs to be modified in order to being used

independently (i.e without the friction brakes). Now a day’s it is being used in

hybrid cars only but it can be used in heavy vehicles for greater recovery of

braking power. Electronic advancements can be done to increase the efficiency of

the brakes to a greater extent. It decreases the fuel consumption and brake wear

which saves the fuel as well as the material used in brakes. Electronic brake

controllers are used which decides the amount of charge being sent to the battery

up to its capacity. It also decides the switching of regenerative brakes and friction

brakes depending upon the intensity of power used to stop the vehicle, if the power

required is in the limit of regenerative brakes then it is used otherwise it switches

to friction brakes.

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12. BIBLIOGRAPHY

Online encyclopedia: www.wikipedia.com

Online search engine: www.google.co.in

Online engineering help site: www.howstuffworks.com

Website: www.Technizzel.com

Online site: www.pdfsearchengine.com

Pdf file by Brent Bolton

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