ABSTRACT:

An Electromagnetic Braking system uses Magnetic force to engage the brake, but the power

required for braking is transmitted manually. The disc is connected to a shaft and the

electromagnet is mounted on the frame .When electricity is applied to the coil a magnetic

field is developed across the armature because of the current flowing across the coil and

causes armature to get attracted towards the coil. As a result it develops a torque and

eventually the vehicle comes to rest. In this project the advantage of using the

electromagnetic braking system in automobile is studied. These brakes can be incorporated

in heavy vehicles as an auxiliary brake. The electromagnetic brakes can be used in

commercial vehicles by controlling the current supplied to produce the magnetic flux.

Making some improvements in the brakes it can be used in automobiles in future

One of the drawbacks of going anywhere fast is that you always have

to stop sooner or later. In an emergency, when you have to brake quickly, the

only thing that comes between safe stopping and disaster is the simple science

offriction: you slow to a halt when two surfaces rub together. Now

friction brakes have more than proved their worth: you'll find them in every

car, bicycle, airplane, and most factory machines. But they have a big drawback

too: every time you use them, they wear out a little bit, and that means they're

relatively expensive. What's the alternative? One option is to slow things down

with the force of electromagnetism instead of friction. It sounds like something

out of Flash Gordon or Buck Rogers, but it's the basic idea behind eddy-current

brakes, which can cost half as much to run over their lifetime as traditional,

friction brakes.

How ordinary (friction) brakes work

Motorcycle brakes: Like most vehicles, this bike brakes with friction. When

you pull on the brake handle, a hydraulic cable applies the brake pads to the

brake rotor disc, slowing the machine down by converting your kinetic

energy to heat. The tire doesn't normally play much part in braking unless

you brake really hard: then the wheel will lock completely and friction

between the tire and the road will bring you to a sudden halt, leaving a

rubber skid mark on the road. That's not a good way to brake: it'll wear out

your tires very quickly.

Moving things have kinetic energy and, if you want to stop them, you have to get

rid of that energy somehow. If you're on a bicycle going fairly slowly, you can

simply put your feet down so they drag on the ground. The soles of your feet act

as brakes. Friction (rubbing) between the rough ground and the grip on your soles

slows you down, converting your kinetic energy into heat energy (do it long

enough and your shoes will get hot). Brakes on vehicles work pretty much the

same way, with "shoes" that press rubber pads (brake blocks) against discs

mounted to the wheels. (Find out more about this in our main article on brakes.)

Even if you make brakes from super-strong, hard-wearing materials like Kevlar®,

they're still going to wear out sooner or later. But there are other problems with

friction brakes. The faster you go, the harder they have to work to get rid of your

kinetic energy, and the quicker they'll wear out. Use your brakes too often and

you may suffer a problem called brake fade, where heat builds up too much in the

brakes or the hydraulic system that operates them and the brakes can no longer

work as effectively. What if your brakes can't stop you in time?

What are eddy currents?

Before we can understand eddy current brakes, we need to understand eddy

currents! They're part of the science

of electromagnetism: electricity and magnetism aren't two separate things but

two sides of the same "coin"—two different aspects of the same underlying

phenomenon.

Electricity and magnetism go hand in hand

Wherever you get electricity, you get magnetism as well, and vice-versa. This is

the basic idea behind electricity generatorsand electric motors. Generators use

some kind of movement (maybe a wind turbine rotor spinning around) to make

an electric current, while motors do the opposite, converting an electric current

into movement that can drive a machine (or propel something like an electric

car or electric bike).

Both kinds of machine (they are virtually identical) work on the idea that you can

use electricity to make magnetism or magnetism to make electricity. To make

electricity, all you have to do is move an electrical conductor (like a copper wire)

through a magnetic field. That's it! It's called Faraday's law of induction after

English scientist Michael Faraday, who discovered the effect in the early 19th

century. If you connect the wire up to a meter, you'll see the needle flick every

time you move the wire (but only when you move it). If you were clever, you

could figure out some way of removing the electricity and storing it: you'd have

made yourself a miniature electric power plant.

How eddy currents are made

What if the conductor you're moving through the magnetic field isn't a wire that

allows the electricity to flow neatly away? You still get electric currents, but

instead of flowing off somewhere, they swirl about inside the material. These are

what we call eddy currents. They're electric currents generated inside a

conductor by a magnetic field that can't flow away so they swirl around instead,

dissipating their energy as heat.

One of the interesting things about eddy currents is that they're not completely

random: they flow in a particular way to try to stop whatever it is that causes

them. This is an example of another bit of electromagnetism called Lenz's law (it

follows on from another law called the conservation of energy, and it's built into

the four equationssummarizing electromagnetism that were set out by James

Clerk Maxwell).

Here's an example. Suppose you drop a coin-shaped magnet down the inside of

a plastic pipe. It might take a half second to get to the bottom. Now repeat the

same experiment with a copper pipe and you'll find your magnet takes much

longer (maybe three or four seconds) to make exactly the same journey. Eddy

currents are the reason. When the magnet falls through the pipe, you have a

magnetic field moving through a stationary conductor (which is exactly the same

as a conductor moving through a stationary magnetic field). That creates electric

currents in the conductor—eddy currents, in fact. Now we know from the laws of

electromagnetism that when a current flows in a conductor, it produces a

magnetic field. So the eddy currents generate their own magnetic field. Lenz's law

tells us that this magnetic field will try to oppose its cause, which is the falling

magnet. So the eddy currents and the second magnetic field produce an upward

force on the magnet that tries to stop it from falling. That's why it falls more

slowly. In other words, the eddy currents produce a braking effect on the falling

magnet.

It's because eddy currents always oppose whatever causes them that we can use

them as brakes in vehicles, engines, and other machines.

How does an eddy current brake stop something moving?

Suppose we have a railroad train that's actually a huge solid block of copper

mounted on wheels. Let's say it's hurtling along at high speed and we want to

stop it. We could apply friction brakes to the wheels—or we could stop it with

eddy currents. How? What if we put a giant magnet next to the track so the train

had to pass nearby. As the copper approached the magnet, eddy currents would

be generated (or "induced") inside the copper, which would produce their own

magnetic field. Eddy currents in different parts of the copper would try to work in

different ways. As the front part of the train approached the magnet, eddy

currents in that bit of the copper would try to generate a repulsive magnetic field

(to slow down the copper's approach to the magnet). As the front part passed by,

slowing down, the currents would start generating an attractive magnetic field

that tried to pull the train back again (again, slowing it down). The copper would

heat up as the eddy currents swirled inside it, gaining the kinetic energy lost by

the train as it slowed down. It might sound like a strange way to stop a train, but

it really does work. You'll find the proof of it in many rollercoaster cars, which use

magnetic brakes like this, mounted on the side of the track, to slow them down.

Artwork: Here's our simple copper block train moving from right to left, and

I've embedded a giant bar magnet in the track to stop it. As the train

approaches, eddy currents are induced in the front of it that produce a

repulsive magnetic field, which slows the train down. If the train is moving

really fast, this magnet might not stop it completely, so it'll keep moving

beyond the magnet. As it moves past the other end of the magnet, the

induced eddy currents will work to produce an attractive magnetic field that

tries to pull the train backward, but still tries to slow it down. The basic point

is simple: the eddy currents are always trying to oppose whatever causes

them. (Note that eddy currents are actually induced through the whole of

the copper block, but I've drawn only a few of them for clarity.)

Types of eddy current brakes

Real eddy current brakes are a bit more sophisticated than this, but work in

essentially the same way. They were first proposed in the 19th century by the

brilliant French physicist Jean-Bernard Léon Foucault (also the inventor of the

Foucault pendulum and one of the first people to measure the speed

of light accurately on Earth). Eddy current brakes come in two basic flavors—

linear and circular.

Linear brakes

Linear brakes feature on things like train tracks and rollercoasters, where the

track itself (or something mounted on it) works as part of the brake.

The simplest linear, eddy-current brakes have two components, one of which is

stationary while the other moves past it in a straight line. In a rollercoaster ride,

you might have a series of powerful, permanent magnets permanently mounted

at the end of the track, which produce eddy currents in pieces of metal mounted

on the side of the cars as they whistle past. The cars move freely along the track

until they reach the very end of the ride, where the magnets meet the metal and

the brakes kick in.

This kind of approach is no use for a conventional train, because the brakes might

need to be applied at any point on the track. That means the magnets have to be

built into the structure that carries the train's wheels (known as the bogies) and

they have to be the kind of magnets you can switch on and off (electromagnets, in

other words). Typically, the electromagnets move a little less than 1cm (less than

0.5 in) from the rail and, when activated, slow the train by creating eddy currents

(and generating heat) inside the rail itself. It's a basic law of electromagnetism

that you can only generate a current when you actually move a conductor

through a magnetic field (not when the conductor is stationary); it follows that

you can use an eddy current brake to stop a train, but not to hold it stationary

once it's stopped (on something like an incline). For that reason, vehicles with

eddy current brakes need conventional brakes as well.

Photo: The linear eddy-current brakes from a roller coaster. (The brakes are

the black things mounted on the side of the track.) Photo by Stefan Scheer

courtesy of Wikimedia Commons published under a Creative Commons

Licence.

Circular brakes

Like linear eddy current brakes, circular brakes also have one static part and one

moving part. They come in two main kinds, according to whether the

electromagnet moves or stays still. The simplest ones look like traditional brakes,

only with a static electromagnet that applies magnetism and creates eddy

currents in a rotating metal disc (instead of simple pressure and friction) that

moves through it. (The Shinkansen brakes work like this.) In the other design, the

electromagnets move instead: there's a series of electromagnet coils mounted on

an outer wheel that spins around (and applies magnetism to) a fixed, central

shaft. (Telma frictionless "retarder" brakes, used on many trucks, buses, and

coaches, work this way.)

How do these things work in practice? Suppose you have a high-speed factory

machine that you want to stop without friction. You could mount a metal wheel

on one end of the drive shaft and sit it between some electromagnets. Whenever

you wanted to stop the machine, you'd just switch on the electromagnets to

create eddy currents in the metal wheel that bring it quickly to a halt.

Alternatively, you could mount the electromagnet coils on the rotating shaft and

have them spin around or inside stationary pieces of metal.

With a linear brake, the heat generated by the eddy currents can be dissipated

relatively easily: it's easy to see how it would disappear fairly quickly in a brake

operating outdoors over a relatively long section of train track. Getting rid of heat

is more of an issue with circular brakes, where the eddy currents are constantly

circulating in the same piece of metal. For this reason, circular eddy current

brakes need some sort of cooling system. Air-cooled brakes have metal meshes,

open to the air, which use fan blades to pull cold air through them. Liquid-cooled

brakes use cooling fluids to remove heat instead.

Photo: Close-up of the circular eddy-current brake from the Shinkansen 700

train in our top photo. Although this resembles the motorcycle friction brake

up above, it works in a totally different way. You can see the brown brake

disc and the electromagnet that surrounds it at the top. When the brake is

applied, the electromagnet switches on and induces eddy currents in the

disc, which create opposing magnetic fields and stop it rotating. Unlike in the

motorcycle brake, there is no contact at all between the electromagnet and

the brake disc: there's an air gap of a few millimeters between them. Photo

courtesy of Wikimedia Commons published under a Creative Commons

Licence.

Where are eddy current brakes used?

Despite being invented over a century ago, eddy current brakes are still relatively

little used. Apart from rollercoasters, one area where they're now finding

applications is in high-speed electric trains. Some versions of the German Inter

City Express (ICE) train and Japanese Shinkansen ("bullet train") have

experimented with eddy-current brakes and future versions of the French TGV are

expected to use them as well. You'll also find eddy current brakes in all kinds of

machines, such as circular saws and other power equipment. And they're used in

things like rowing machines and gym machines to apply extra resistance to the

moving parts so your muscles have to work harder.

Advantages and disadvantages of eddy current brakes

On the plus side, eddy-current brakes are quiet, frictionless, and wear-free, and

require little or no maintenance. They produce no smell or pollution (unlike

friction brakes, which can release toxic chemicals into the environment). All this

makes them much more attractive than noisy friction brakes that need regular

inspection and routinely wear out. It's been estimated that switching an electric

train from friction brakes to eddy-current brakes could halve the cost of brake

operation and maintenance over its lifetime.

The drawbacks of eddy current brakes are more to do with how little experience

we have of using them in real-world settings. As Jennifer Schykowski noted in an

excellent review of the technology for Railway Gazette in 2008, the

electromagnetic parts of eddy current brakes have sometimes caused problems

by interfering with train signaling equipment. Although heat dissipation in rails

should not, theoretically, be an issue, if there's a busy section of track where

many trains brake in quick succession (something like the approach to a station),

the heating and expansion of rails could prove to be an issue, either reducing the

effectiveness of the brakes or leading to structural problems in the rails

themselves. Another important question is whether eddy-current brakes will ever

become widespread, given the growing interest inregenerative brakes that

capture and store the energy of moving vehicles for reuse (a much more energy-

efficient approach than turning energy into useless heat with eddy currents).

Some of the latest Shinkansen trains (series E5) use regenerative brakes where

earlier models used eddy-current technology.

INTRODUCTION

A brake is a device which inhibits motion. Its opposite component is a clutch.Most

commonly brakes use friction to convert kinetic energy into heat, though other methods

of energy conversion may be employed. For example regenerative braking converts

much of the energy to electrical energy, which may be stored for later use.

II. PRINCIPLE OF BRAKING SYSTEM

The principle of braking in road vehicles involves the conversion of kinetic energy into

thermal energy (heat). When stepping on the brakes, the driver commands a stopping

force several times as powerful as the force that puts the car in motion and dissipates

the associated kinetic energy as heat. Brakes must be able to arrest the speed of a

vehicle in short periods of time regardless how fast the speed is. As a result, the brakes

are required to have the ability to generating high torque and absorbing energy at

extremely high rates for short periods of time.

III.EXISTING CONDITION

A. Types of Braking System Brakes may be broadly described as using friction, pumping,

or electromagnetic. One brake may use several principles: for example, a pump may

pass fluid through an orifice to create friction.

B. Conventional Friction Brake The conventional friction brake system is composed of

the following basic components: the “master cylinder” which is located under the hood

is directly connected to the brake pedal, and converts the drivers’ foot pressure into

hydraulic pressure. Steel “brake hoses” connect the master cylinder to the “slave

cylinders” located at each wheel. Brake fluid, specially designed to work in extreme

temperature conditions, fills the system. “Shoes” or “pads” are pushed by the slave

cylinders to contact the “drums” or “rotors,” thus causing drag, which slows the car. Two

major kinds of friction brakes are disc brakes and drum brakes

C. Characteristics

Brakes are often described according to several characteristics including: Peak force is

the maximum decelerating effect that can be obtained. The peak force is often greater

than the traction limit of the tires, in which case the brake can cause a wheel skid.

Continuous power dissipation Brakes typically get hot in use, and fail when the

temperature gets

too high. The greatest amount of power (energy per unit time) that can be dissipated

through the brake without failure is the continuous power dissipation. Continuous

power dissipation often depends on e.g., the temperature and speed of ambient cooling

air. Fade As a brake heats, it may become less effective, called brake fade. Some designs

are inherently

prone to fade, while other designs are relatively immune. Further, use considerations,

such as cooling, often have a big effect on fade

. IV.EXISTING CONDITION

A. Brake fading effect: The conventional friction brake can absorb and convert enormous

energy values (25h.p. without self-destruction for a 5-axle truck, Reverdin1974), but only

if the temperature rise of the friction contact materials is controlled. This high energy

conversion therefore demands an appropriate rate of heat dissipation if a reasonable

temperature and performance stability are to be maintained.

B. Brake fluid leakage If your vehicle has worn brake pads or brake shoes, the fluid level

in your brake fluid reservoir will be low. But let's say you have relatively new brake pads

and you recently topped-off your brake reservoir only to notice a few days later that the

fluid level has dropped noticeably. If that's the case, it's a good bet you have a leak

somewhere in your brake system -- which means that you likely have bigger brake issues

than something as simple as worn brake pads.

C. Other major problems And other problems include the brake fluid vaporization and

brake fluid freezing though vaporization occurs only in rare cases. Freezing is quite

common in colder places like Scandinavian countries and Russia etc…… where the

temperature reaches as low as -50°C to−65°C, in such cases there is a need for some

anti-freezing agents and increases the complexity and cost of the system

B. Magnetic Effect of Current The term "Magnetic effect of current" means that "a

current flowing in a wire produces a magnetic field around it". The magnetic effect of

current was discovered by Oersted in 1820. Oersted found that a wire carrying a current

was able to deflect a magnetic needle Fig. 2 Magnetic Field Lines

C. Electromagnet An electric current can be used for making temporary magnets known

as electromagnets. An electromagnet works on the magnetic effect of current. It has

been found that if a soft iron rod called core is placed inside a solenoid, then the

strength of the magnetic field becomes very large because the iron ore is magnetized by

induction

D. Factors affecting strength of an Electromagnet The strength of an electromagnet is:

Directly proportional to the number of turns in the coil.

Directly proportional to the current flowing in the coil.

Inversely proportional to the length of air gap between the poles

. In general, an electromagnet is often considered better than a permanent magnet

because it can produce very strong magnetic fields and its strength can be controlled by

varying the number of turns in its coil or by changing the current flowing through the

coil.

. V. WORKING PRINCIPLE

A. Electromagnetism Electromagnetism is one of the four fundamental interactions in

nature. The other three are the strong interaction, the weak interaction and

gravitation. Electromagnetism is the force that causes the interaction between

electrically charged particles; the areas in which this happens are called

electromagnetic fields.

VI.ELECTROMAGNETIC BRAKES

Electromagnetic brakes operate electrically, but transmit torque mechanically. This is why they used

to be referred to as electro-mechanical brakes. Over the years, EM brakes became known as

electromagnetic, referring to their actuation method. The variety of applications and brake designs

has increased dramatically, but the basic operation remains the same. Single face electromagnetic

brakes make up approximately 80% of all of the power applied brake applications A. Characteristics

of Electromagnetic Brakes It was found that electromagnetic brakes can develop a negative power

which represents nearly twice the maximum power output of a typical engine, and at least three

times the braking power of an exhaust brake. These performances of electromagnetic brakes make

them much more competitive candidate for alternative retardation equipment’s compared with

other retarders. The brake linings would last considerably longer before requiring maintenance, and

the potentially “brake fade” problem could be avoided. In research conducted by a truck

manufacturer, it was proved that the electromagnetic brake assumed 80 percentage of the duty

which would otherwise have been demanded of the regular service brake. Furthermore, the

electromagnetic brake prevents the dangers that can arise from the prolonged use of brakes beyond

their capability to dissipate heat. This is most likely to occur while a vehicle descending a long

gradient at high speed.

VII. CONSTRUCTION

The parts of Electromagnetic Disc Brake are:

dc Motor Disc Frame Electromagnet Pulleys & Belt Shaft

dc motor

One of the first electromagnetic rotary motors was invented by Michael Faraday in 1821

and consisted of a free-hanging wire dipping into a pool of mercury. A permanent magnet

was placed in the middle of the pool of mercury. When a current was passed through the

wire, the wire rotated around the magnet, showing that the current gave rise to a circular

magnetic field around the wire. This motor is often demonstrated in school physics classes,

but brine(salt water) is sometimes used in place of the toxic mercury. This is the simplest

form of a class of electric motors called homopolar motors. A later refinement is the

Barlow's Wheel.

Another early electric motor design used a reciprocating plunger inside a switched solenoid;

conceptually it could be viewed as an electromagnetic version of a two stroke internal

combustion engine.

The modern DC motor was invented by accident in 1873, when Zénobe Gramme connected

a spinning dynamo to a second similar unit, driving it as a motor.

The classic DC motor has a rotating armature in the form of an electromagnet. A rotary

switch called a commutator reverses the direction of the electric current twice every cycle,

to flow through the armature so that the poles of the electromagnet push and pull against

the permanent magnets on the outside of the motor. As the poles of the armature

electromagnet pass the poles of the permanent magnets, the commutator reverses the

polarity of the armature electromagnet. During that instant of switching polarity, inertia

keeps the classical motor going in the proper direction. (See the diagrams below.)

A simple DC electric motor. When the coil is powered, a magnetic field is generated around

the armature. The left side of the armature is pushed away from the left magnet and drawn

toward the right, causing rotation.

The armature continues to rotate.

When the armature becomes horizontally aligned, the commutator reverses the direction of

current through the coil, reversing the magnetic field. The process then repeats.

Electromagnet

XI.WORKING OF ELECTROMAGNETIC DISC BRAKE

2. General Principle of Electromagnetic Brakes

2.1. Introduction Electromagnetic brakes have been used as supplementary retardation equipment

in addition to the regular friction brakes on heavy vehicles. We outline the general principles of

regular brakes and several alternative retardation techniques in this section. The working principle

and characteristics of electromagnetic brakes are then highlighted.

2.2. General Principle of Brake System The principle of braking in road vehicles involves the

conversion of kinetic energy into thermal energy (heat). When stepping on the brakes, the driver

commands a stopping force several times as powerful as the force that puts the car in motion and

dissipates the associated kinetic energy as heat. Brakes must be able to arrest the speed of a vehicle

in a short periods of time regardless how fast the speed is. As a result, the brakes are required to

have the ability to generating high torque and absorbing energy at extremely high rates for short

periods of time. Brakes may be applied for a prolonged periods of time in some applications such as

a heavy vehicle descending a long gradient at high speed. Brakes have to have the mechanism to

keep the heat absorption capability for prolonged periods of time.

2.3. Conventional Friction Brake 5 The conventional friction brake system is composed of the

following basic components: the “master cylinder” which is located under the hood is directly

connected to the brake pedal, and converts the drivers’ foot pressure into hydraulic pressure. Steel

“brake hoses” connect the master cylinder to the “slave cylinders” located at each wheel. Brake

fluid, specially designed to work in extreme temperature conditions, fills the system. “Shoes” or

“pads” are pushed by the slave cylinders to contact the “drums” or “rotors,” thus causing drag,

which slows the car. Two major kinds of friction brakes are disc brakes and drum brakes. Disc brakes

use a clamping action to produce friction between the “rotor” and the “pads” mounted in the

“caliper” attached to the suspension members (see Figure 2.1).

Disc brakes work using the same basic principle as the brakes on a bicycle: as the caliper pinches the

wheel with pads on both sides, it slows the vehicle (Limpert 1992). Drum brakes consist of a heavy

flat-topped cylinder, which is sandwiched between the wheel rim and the wheel hub (see Figure

2.2). The inside surface of the drum is acted upon by the linings of the brake shoes. When the brakes

are applied, the brake shoes are forced into contact with the inside surface of the brake drum to

slow the rotation of the wheels (Limpert 1992). Air brakes use standard hydraulic brake system

components such as braking lines, wheel cylinders and a slave cylinder similar to a master cylinder to

transmit the air-pressure-produced braking energy to the wheel brakes. Air brakes are used

frequently when greater braking capacity is required.

2.4. “Brake Fading” Effect 6 The conventional friction brake can absorb and convert enormous

energy values (25h.p. without self-destruction for an 5-axle truck, Reverdin 1974), but only if the

temperature rise of the friction contact materials is controlled. This high energy conversion

therefore demands an appropriate rate of heat dissipation if a reasonable temperature and

performance stability are to be maintained. Unfortunately, design, construction, and location

features all severely limit the heat dissipation function of the friction brake to short and intermittent

periods of application. This could lead to a ‘brake fade’ problem (reduction of the coefficient of

friction, less friction force generated) due to the high temperature caused by heavy brake demands.

The main reasons why conventional friction brakes fail to dissipate heat rapidly are as follows: - poor

ventilation due to encapsulation in the road wheels, - diameter restriction due to tire dimensions, -

width restrictions imposed by the vehicle spring designer; - problems of drum distortion at widely

varying temperatures. It is common for friction-brake drums to exceed 500 °C surface temperatures

when subject to heavy braking demands, and at temperatures of this order, a reduction in the

coefficient of friction (‘brake fade’) suddenly occurs (Grimm, 1985). The potential hazard of tire

deterioration and bursts is perhaps also serious due to the close proximity of overheated brake

drums to the inner diameter of the tire.

2.5. Retarders Retarders are means of of overcoming the above problems by augmenting a vehicle’s

foundation brakes with a device capable of opposing vehicle motion at relatively low levels of power

dissipation for long periods. There are several retarder technologies currently available. Two major

kinds are the hydrokinetic brake and the exhaust brake. Hydrokinetic brake 7 uses fluid as the

working medium to oppose rotary motion and absorb energy (Packer 1974). Hydrodynamic brakes

are often built into hydrodynamic transmissions (Foster, 1974). Exhaust brakes use a valve which is

fitted into the exhaust pipe between the exhaust manifold and silencer. When this valve is closed air

is compressed against it through the open exhaust valve by the piston rising on the exhaust stroke.

In that way the engine becomes a low pressure single stage compressor driven by the vehicle’s

momentum, resulting in a retarding effect being transmitted through the transmission to the driving

road wheels. The power-producing engine is converted into a powerabsorbing air compressor

(Smith, 1974). This approach could put a lot of stress on the cylinder and exhaust system. So it may

require extra engineering efforts to implement this system. As a brake applied to the engine, exhaust

brakes can only absorb as much power as the engine can deliver. But the power absorbed in braking

is usually greater than the power absorbed in driving. Compared with these retarders,

electromagnetic brakes have greater power capability, simplicity of installation and controllability.

2.6. General Principle and Advantage of Electromagnetic Brakes (retarders)

2.6.1. Installation Location Electromagnetic brakes work in a relatively cool condition and satisfy all

the energy requirements of braking at high speeds, completely without the use of friction. Due to its

specific installation location (transmission line of rigid vehicles), electromagnetic brakes have better

heat dissipation capability to avoid problems that friction brakes face as we mentioned before.

Typically, electromagnetic brakes have been mounted in the transmission line of vehicles, as shown

in figure 2.2. The propeller shaft is divided and fitted with a sliding universal joint and is connected to

the coupling flange on the brake. 8 The brake is fitted into the chassis of the vehicle by means of

anti-vibration mounting. The practical location of the retarder within the vehicle prevents the direct

impingement of air on the retarder caused by the motion of the vehicle. Any air flow movement

within the chassis of the vehicle is found to have a relatively insignificant effect on the air flow

around tire areas and hence on the temperature of both front and rear discs. So the application of

the retarder does not affect the temperature of the regular brakes. In that way, the retarders help to

extend the life span of the regular brakes and keep the regular brakes cool for emergency situation.

2.6.2. Working Principle The working principle of the electric retarder is based on the creation of

eddy currents within a metal disc rotating between two electromagnets, which sets up a force

opposing the rotation of the disc (see figure 2.3). If the electromagnet is not energized, the rotation

of the disc is free and accelerates uniformly under the action of the weight to which its shaft is

connected. When the electromagnet is energized, the rotation of the disc is retarded and the energy

absorbed appears as heating of the disc. If the current exciting the electromagnet is varied by a

rheostat, the braking torque varies in direct proportion to the value of the current. It was the

Frenchman Raoul Sarazin who made the first vehicle application of eddy current brakes. The

development of this invention began when the French company Telma, associated with Raoul

Sarazin, developed and marketed several generations of electric brakes based on the functioning

principles described above (Reverdin, 1974). A typical retarder consists of stator and rotor. The

stator holds 16 induction coils, energized separately in groups of four. The coils are made up of

varnished aluminum wire mounded in epoxy resin. The stator assembly is 9 supported resiliently

through anti-vibration mountings on the chassis frame of the vehicle. The rotor is made up of two

discs, which provide the braking force when subject to the electromagnetic influence when the coils

are excited. Careful design of the fins, which are integral to the disc, permit independent cooling of

the arrangement. 2.6.3. Electric Control System The electric wiring diagram of the installation is

shown in figure 2.4. The energization of the retarder is operated by a hand control mounted on the

steering column of the vehicle. This control has five positions: the first is ‘off’, and the four remaining

positions increase the braking power in sequence. This hand-control system can be replaced by an

automatic type that can operate mechanically through the brake pedal. In this case, the contacts are

switched on successively over the slack movement of the brake pedal. The use of an automatic

control must be coupled with a cut-off system operating at very low vehicle speed in order to

prevent energization of the retarder while the vehicle is stationary with the driver maintaining

pressure on the brake pedal. Both the manual control and the automatic control activate four

solenoid contractors in the relay box, which in turn close the four groups of coil circuits within the

electric brake at either 24 volts or 12 volts, as appropriate (Reverdin 1974 and Omega Technologies).

2.6.4. Characteristic of Electromagnetic Brakes

It was found that electromagnetic brakes can develop a negative power which represents nearly

twice the maximum power output of a typical engine, and at least three times the braking power of

an exhaust brake (Reverdin 1974). These performance of electromagnetic brakes make them much

more competitive candidate for alternative retardation equipments compared with other retarders.

By using the electromagnetic brake as supplementary 10 retardation equipment, the friction brakes

can be used less frequently, and therefore practically never reach high temperatures. The brake

linings would last considerably longer before requiring maintenance, and the potentially “brake

fade” problem could be avoided. In research conducted by a truck manufacturer, it was proved that

the electromagnetic brake assumed 80 percent of the duty which would otherwise have been

demanded of the regular service brake (Reverdin 1974). Furthermore, the electromagnetic brake

prevents the dangers that can arise from the prolonged use of brakes beyond their capability to

dissipate heat. This is most likely to occur while a vehicle descending a long gradient at high speed.

In a study with a vehicle with 5 axles and weighing 40 tons powered by an engine of 310 b.h.p

traveling down a gradient of 6 percent at a steady speed between 35 and 40 m.p.h, it can be

calculated that the braking power necessary to maintain this speed is the order of 450 h.p. The

braking effect of the engine even with a fitted exhaust brake is approximately 150 h.p. The brakes,

therefore, would have to absorb 300 h.p, meaning that each brake in the 5 axles must absorb 30 h.p,

which is beyond the limit of 25 h.p. that a friction brake can normally absorb without selfdestruction.

The electromagnetic brake is well suited to such conditions since it will independently absorb more

than 300 h.p (Reverdin 1974). It therefore can exceed the requirements of continuous uninterrupted

braking, leaving the friction brakes cool and ready for emergency braking in total safety. The

installation of an electromagnetic brake is not very difficult if there is enough space between the

gearbox and the rear axle. It does not need a subsidiary cooling system. It does not rely on the

efficiency of engine components for its use, as do exhaust and hydrokinetic brakes. The

electromagnetic brake also has better controllability. The exhaust brake is an on/off device and

hydrokinetic brakes have very complex control system. The electromagnetic brake control system is

an electric switching system which gives it superior controllability. 11 From the foregoing, it is

apparent that the electromagnetic brake is an attractive complement to the safe braking of heavy

vehicles. 2.6.5. Thermal Dynamics Thermal stability of the electromagnetic brakes is achieved by

means of the convection and radiation of the heat energy at high temperature. The major part of the

heat energy is imparted to the ventilationg air which is circulating vigorously through the fan of the

heated disc. The value of the energy dissipated by the fan can be calculated by the following

expression: Q = MCpDq (2.1) where: M = Mass of air circulated; Cp = Calorific value of air; Dq =

Difference in temperature between the air entering and the air leaving the fan; The electromagnetic

brakes has excellent heat dissipation efficiency owing to the high temperature of the surface of the

disc which is being cooled and also because the flow of air through the centrifugal fan is very rapid.

Therefore, the curie temperature of the disc material could never been reached (Reverdin 1974).

The practical location of the electromagnetic brakes prevents the direct impingement of air on the

brakes caused by the motion of the vehicle. Any air flow movement within the chassis of the vehicle

is found to have a relatively 12 insignificant effect on the air flow and hence temperature of both

front and rear discs. Due to its special mounting location and heat dissipation mechanism,

electromagnetic brakes have better thermal dynamic performance than regular friction brakes.

2.7. Summary

With all the advantages of electromagnetic brakes over friction brakes, they have been widely used

on heavy vehicles where the ‘brake fading’ problem is serious. The same concept is being developed

for application on lighter vehicles. 13 Figure

The electromagnet is energized by the AC supply where the magnetic field produced is used to

provide the braking mechanism. When the electromagnet is not energized, the rotation of the disc is

free and accelerates uniformly under the action of weight to which the shaft is connected. When the

electromagnet is energized, magnetic field is produced thereby applying brake by retarding the

rotation of the disc and the energy absorbed is appeared as heating of the disc. So when the

armature is attracted to the field the stopping torque is transferred into the field housing and into

the machine frame decelerating the load. The AC motor makes the disc to rotate through the shaft

by means of pulleys connected to the shaft.

A. Engagement Time There are actually two engagement times to consider in an

electromagnetic brake. The first one is the time it takes for a coil to develop a magnetic field,

strong enough to pull in an armature. The second one is air gap, which is the space between

the armature and the coil shell. CAD systems can automatically calculate component inertia,

but the key to sizing a brake is calculating how much inertia is reflected back to the brake. To

do this, engineers use the formula: T = (WK2 × ΔN) / (308 × t) Where T = required torque in

lb-ft, WK2 = total inertia in lb-ft2, ΔN = change in the rotational speed in rpm, and t = time

during which the acceleration or deceleration must take place.

XIII. ADVANTAGES Problems of drum distortion at widely varying temperatures. Which is common

for friction-brake drums to exceed 500 °C surface temperatures when subject to heavy braking

demands, and at temperatures of this order, a reduction in the coefficient of friction (‘brake fade’)

suddenly occurs. This is reduced significantly in electromagnetic disk brake systems.

Potential hazard of tire deterioration and bursts due to friction is eliminated.

There is no need to change brake oils regularly.

There is no oil leakage

. The practical location of the retarder within the vehicle prevents the direct impingement of air on

the retarder

caused by the motion of the vehicle. The retarders help to extend the life span of the regular

brakes and keep the regular brakes cool for emergency

situation. The electromagnetic brakes have excellent heat dissipation efficiency owing to the high

temperature of the

surface of the disc which is being cooled. Due to its special mounting location and heat dissipation

mechanism, electromagnetic brakes have bette

r thermal dynamic performance than regular friction brakes. Burnishing is the wearing or mating of

opposing surfaces .This is reduced significantly here

. In the future, there may be shortage of crude oil, hence by-products such as brake oils will be in

much demand

. EMBs will overcome this problem. Electromagnetic brake systems will reduce maintenance cost

The problem of brake fluid vaporization and freezing is eliminated

XIV. DISADVANTAGES

Dependence on battery power to energize the brake system drains down the battery much faster

. Due to residual magnetism present in electromagnets, the brake shoe takes time to come back to

its origina

position. A special spring mechanism needs to be provided for the quick return of the brake shoe.

XV. CONCLUSION

With all the advantages of electromagnetic brakes over friction brakes, they have been widely used

on heavy vehicles where the ‘brake fading’ problem exists. The same concept is being developed for

application on lighter vehicles. The concept designed by us is just a prototype and needs to be

developed more because of the above mentioned disadvantages. These electromagnetic brakes can

be used as an auxiliary braking system along with the friction braking system to avoid overheating

and brake failure. ABS usage can be neglected by simply using a micro controlled electromagnetic

disk brake system .These find vast applications in heavy vehicles where high heat dissipation is

required. In rail coaches it can used in combination of disc brake to bring the trains moving in high

speed. When these brakes are combined it increases the life of brake and act like fully loaded

brakes. These electromagnetic brakes can be used in wet conditions which eliminate the anti-

skidding equipment, and cost of these brake are cheaper than the other types. Hence the braking

force produced in this is less than the disc brakes if can be used as a secondary or emergency braking

system in the automobiles.

REFERENCES

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GlasersAnnalen, Nr 12, S. 613-617, 1997.

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higher speeds, its basic characteristic, its improvement and somerelated problem,’’ Japanese

Railway Engineering, Vol. 108, 1988. [4]Mcconnell, H.M., ‘‘Eddy-current phenomena in

ferromagnetic material,’’ AIEE Transactions,Vol. 73, part I, pp. 226–234, July, 1954. [5]Kesavamurthy,

N., ‘‘Eddy-current in solid iron due to alternating magnetic flux,’’ TheInstitution of Engineers

Monograph, No. 339U, pp. 207– 213, June, 1959, [6] Biro, O. and Preis, K., ‘‘Finite element analysis

of 3-D eddy current,’’ IEEE Transactions onMagnetics, Vol. 26, No. 2, pp. 418–423, Mar, 1990.

[7]Heald, M.A., ‘‘Magnetic braking: improved theory,’’ American Journal of Physics, Vol. 56,No. 6, pp.

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