Chapter 1

Introduction

1

1.0 Introduction

The key objective of this project is to introduce and prepare a working model of eddy

current brakes. The problems faced in conventional frictional brakes; i.e. fading,

overheating, very short life span etc. precedes the motivation of the work, presented in

the report, followed by the statement of problem and objectives.

1.1 Brakes

A brake is a device which inhibits motion. 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. Other methods convert kinetic energy into potential

energy in such stored forms as pressurized air or pressurized oil. Still other braking

methods even transform kinetic energy into different forms, for example by transferring

the energy to a rotating flywheel.

Brakes are generally applied to rotating axles or wheels, but may also take other forms

such as the surface of a moving fluid (flaps deployed into water or air). Some vehicles

use a combination of braking mechanisms, such as drag racing cars with both wheel

brakes and a parachute, or airplanes with both wheel brakes and drag flaps raised into the

air during landing.

Since kinetic energy increases quadratically with velocity (K = mv2 / 2), an object

traveling at 10 kilometers per second has 100 times as much energy as one traveling at 1

kilometer per second, and consequently the theoretical braking distance, when braking at

the traction limit, is 100 times as long. In practice, fast vehicles usually have significant

air drag, and energy lost to air drag rises quickly with speed.

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Friction brakes on automobiles store braking heat in the drum brake or disc brake while

braking then conduct it to the air gradually. When traveling downhill some vehicles can

use their engines to brake.

When the brake pedal is pushed the caliper containing piston pushes the pad towards the

brake disc which slows the wheel down. On the brake drum it is similar as the cylinder

pushes the brake shoes towards the drum which also slows the wheel down.

1.2 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 spots 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. 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.

1.3 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,”

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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 “rotors” and the “pads”

mounted in the “caliper” attached to the suspension members. 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.

Drum brakes consist of a heavy flat-topped cylinder, which is sandwiched between the

wheel rim and the wheel hub. 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.

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.

1.4 Brake Fading Effect

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,

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- Diameter restriction due to tire dimensions,

- 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.

5

Chapter 2

Literature Survey

6

2.1 Introduction

This chapter pertains to the literature survey on magnetic properties of materials and

there classification.

2.2 Literature on Magnetic Properties

The basic principal of electromagnet is as follows:

Oersted found that a magnetic field is established around a current carrying conductor.

Magnetic field exists as long as there is current in the wire. The direction of magnetic

field was found to be changed when direction of current was reversed.

Conclusion a moving charge produces electric as well as magnetic field.

We are using toroid as electromagnet, a toroid can be considered as a ring shaped closed

solenoid. Hence it is like an endless cylindrical solenoid ( a cylindrical coil of many

tightly wound turns of insulated wire with generally diameter of the coil smaller than its

length is called a solenoid.

FIG. 2.1 TOROID

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Magnetic field generated by toroid at distance r from the central periphery of the core,

B= (µ*N*i)/(2Пr)

Where,

B, is the magnetic field generated

µ, is the absolute permeability of air

i, is the current supplied in wire and

N, is the no. of turns or loops of wire on core

2.2.1 Magnetic Flux, Φ

The number of magnetic lines of force passing normally through a surface is defined as

magnetic flux. Its SI unit is Weber (wb).

2.2.2 Magnetic Flux Density, B

When a piece of a magnetic substance is placed in an external magnetic field the

substance becomes magnetized. The number of magnetic lines of induction inside a

magnetized substance crossing unit area normal to their direction is called magnetic

induction or magnetic flux density. Its SI unit is Tesla or wb/m2 or N/amp-m.

2.2.3 Magnetic Permeability

It is the degree or extent to which magnetic lines of force can enter a substance and is

denoted by µ.

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2.2.4 Magnetic Materials

They are classified in three categories:

1. Diamagnetic materials:

Diamagnetism is the intrinsic property of every material and it is generated due to

mutual interaction between the applied magnetic field and orbital motion of

electrons. Examples: Bismuth, Cu, H2O, gold.

2. Paramagnetic materials:

In these substances the inner orbits of atoms are incomplete. The electron spins

are uncoupled, consequently on applying a magnetic field the magnetic moment

generated due to spin motion align in the direction of magnetic field and induces

magnetic moment in its direction due to which the material gets feebly

magnetized. In these materials the electron no. are odd. At high enough

temperatures, all strong magnetic materials become paramagnetic. Examples:

aluminum, potassium.

3. Ferromagnetic material:

In these materials, permanent atomic magnetic moments have strong tendency to

align themselves even without any external field. Examples: iron, cobalt, nickel.

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FIG 2.2 ATOMIC MOMENTS OF FERROMAGNETIC MATERIAL

2.2.5 Skin Effect

Skin effect is the tendency of an alternating electric current (AC) to distribute itself

within a conductor so that the current density near the surface of the conductor is greater

than that at its core. That is, the electric current tends to flow at the "skin" of the

conductor, at an average depth called the skin depth. The skin effect causes the effective

resistance of the conductor to increase with the frequency of the current because much of

the conductor carries little current. Skin effect is due to eddy currents set up by the AC

current. At 60 Hz in copper, skin depth is about 8.5 mm. At high frequencies skin depth

is much smaller.

Because we are using DC current in our model there will be no skin effect; but it will

exist when we want to take current from the alternator.

2.2.6 Hysteresis

Hysteresis refers to systems that have the effects of the current input (or stimulus) to the

system are experienced with a certain delay in time. Such a system may exhibit path

dependence, or "rate-independent memory". Hysteresis phenomena occur in magnetic

materials, ferromagnetic materials and ferroelectric materials, as well as in the elastic,

electric, and magnetic behavior of materials, in which a lag occurs between the

application and the removal of a force or field and its subsequent effect. Electric

hysteresis occurs when applying a varying electric field, and elastic hysteresis occurs in

response to a varying force.Many physical systems naturally exhibit hysteresis.

A piece of iron that is brought into a magnetic field retains some magnetization, even

after the external magnetic field is removed.Once magnetized, the iron will stay

magnetized indefinitely. To demagnetize the iron, it would be necessary to apply a

magnetic field in the opposite direction. This is the effect that provides the element of

memory in a hard disk drive .

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2.3 Eddy Current Description

Only one half of an electromagnet’s interaction with the disk is analyzed because both

sides of the magnet are symmetric. Additionally, any effect that the proximity of the

edge of the disk has on the strength of the forces produced is assumed to be negligible by

maintaining a small distance between the edge of the electromagnets and the inner/outer

edge of the disk. As by Faraday’s Law, , where is the magnetic

flux, t is time, and V is voltage. Additionally, using the basic relationship of

, it is assumed that the current induced in each differential piece will be

proportional to the induced voltage divided by the resistance of said differential piece.

Thus, and the radial flowing current is calculated in each element.

As by the equation , where in this case I is the induced differential current, L is

the length of the element in the direction of radial current flow, and B is taken to be the

average strength of the magnetic field over each differential element. The resulting

quantity by multiplying average braking or eddy current force acting on shaft with its

radius is the total torque exerted on the disk.

2.4 Various methods of producing induced e.m.f.

The magnetic flux can be changed by changing B, Ɵ or A .

Hence, there are three methods of producing induced e.m.f.

1. By changing the magnitude of the magnetic field B.

2. By changing the area A.

3. By changing the relative orientation of the surface area and the magnetic field (Ɵ).

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2.5 Applications of Eddy Currents

Working of induction furnace is based on the heating effects of Eddy Currents.

Induction motors

In Electromagnetic brakes

In speedometers

Electromagnetic shielding

In Paddle Machine Brake

In Turbine Brake

Voltage is induced when a magnet moves towards or away from a coil, inducing a current

in the coil. Faster the magnet’s motion, the greater the induced current.

FIG 3.3 INDUCTION OF EDDY CURRENT

The induced voltage in a coil is proportional to the product of the number of loops and

rate at which the magnetic field changes within the loops.

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Chapter 3

General Principle of Eddy Current Brakes

13

3.1 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. 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 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.

3.2 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 performances of

electromagnetic brakes make them much more competitive candidate for alternative

retardation equipments compared with other retarders. By using the electromagnetic

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brake as supplementary 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 bhp traveling down a

gradient of 6 percent at a steady speed between 35 and 40mph, it can be calculated that

the braking power necessary to maintain this speed is the order of 450hp. The braking

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

therefore, would have to absorb 300hp, 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 self-destruction. The electromagnetic brake is well suited to such

conditions since it will independently absorb more than 300h.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.

From the foregoing, it is apparent that the electromagnetic brake is an attractive

complement to the safe braking of heavy vehicles.

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3.3 Eddy currents

Whenever there is a change in magnetic flux in any magnetic field a back e.m.f is

produced in the object causing that change which results production of a current in such a

way that it resists the cause of change of magnetic flux; this current is known as Eddy

current.

3.4 Eddy Current Generation

Eddy Currents are induced current that exist in a solid. A changing magnetic flux over an

area of the solid will produce an Eddy Current which will create a magnetic field

opposing the field producing the Eddy Currents. The opposition of this generated

magnetic field is dependent on the changing area. As the area of flux increases the Eddy

Current generation is in a “negative” direction. With a decreasing area exposed to the

flux the generated Eddy Currents will act in the opposite, “positive”.

FIGURE 2.2 EDDY CURRENT GENERATION DIAGRAM

The figure shows one plate at two instances in time. The first instant models the plate just

entering the magnetic field directed into the page. The swirl indicated on the plate

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illustrates the direction of the Eddy Current. The Eddy Current in position one has a

“negative” direction or counter clockwise direction. The second position shows the Eddy

current swirl going in the “positive” direction, or clockwise, as the plate has a decreasing

area passing through the flux. Essentially, at the middle of the field there is no Eddy

Current generation and also acts as the point in which the Eddy Current generation

changes direction. The diagram also shows a force, , which represents the force

created by the Eddy Currents that are generated. The force created by the Eddy Currents

will always oppose the direction of motion. The magnetic field generated by the Eddy

Currents will oppose one another in position one of figure 1, and attract each other as the

area is decreasing, thereby creating a force that always opposes the direction of the

plate’s motion. The force produce by the Eddy Current generation is proportional to the

conductivity of the material, the speed of plate or the rate of change of flux and the

magnitude of the magnetic field, B.

3.5 Types of Eddy Current Brakes

Electromagnetic brakes are similar to electrical motors; non-ferromagnetic metal discs

(rotors) are connected to a rotating coil, and a magnetic field between the rotor and the

coil creates a resistance used to generate electricity or heat. When electromagnets are

used, control of the braking action is made possible by varying the strength of the

magnetic field. A braking force is possible when electric current is passed through the

electromagnets. The movement of the metal through the magnetic field of the

electromagnets creates eddy currents in the discs. These eddy currents generate an

opposing magnetic field, which then resists the rotation of the discs, providing braking

force. The net result is to convert the motion of the rotors into heat in the rotors.

‘

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3.5.1 Linear eddy current brakes

It consists of a magnetic yoke with electrical coils which are being magnetized

alternately. This magnet does not touch the rail (held at approx 7 mm). When the magnet

is moved along the rail, it generates a non-stationary magnetic field which generates

electrical tension and causes eddy currents.

These disturb the magnetic field in such a way that the magnetic force is diverted to the

opposite of the direction of the movement. The braking energy of the vehicle is converted

in eddy current losses which lead to a warming of the rail.

FIG3.3 LINEAR EDDY CURRENT BRAKES IN ICE 3

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3.5.2 Circular eddy current brakes

When electromagnets are used, control of the braking action is made possible by varying

the strength of the magnetic field. A braking force is possible when electric current is

passed through the electromagnets. The movement of the metal through the magnetic

field of the electromagnets creates eddy currents in the discs.

These eddy currents generate an opposing magnetic field, which then resists the rotation

of the discs, providing braking force. The net result is to convert the motion of the rotors

into heat in rotors.

FIG2. 4 CIRUCULAR EDDY CURRENT BRAKES

Eddy current brakes at the Intamin roller coaster Goliath in Walibi World (Netherlands)

The first train in commercial circulation to use such a braking is the ICE 3.Modern roller

coasters use this type of braking, but utilize permanent magnets instead of

electromagnets, and require no electricity. However, their braking strength cannot be

adjusted.

Radial Clearance Eddy-current Retarders are specifically designed to be used in long-

distance sigh seeing cars between 8 meters and 10 meters. They are light in weigh, and

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easy in installation and maintenance. With a small moment of inertia of rotor and low

electric power consumption, they won’t increase the burden of the cars. Output torque

matches the type of the car, so they have the obvious effects of slowing down. At the

same time, they can effectively reduce the friction of service braking system, prevent the

overheating of wheel boss, and avoid the flat tire.

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Chapter 4

Equipments Used

21

4.1 Selection of Equipment

List of equipment required:

1. Prime mover to give angular moment to shaft

2. Electromagnet to create magnetic field around shaft perpendicular to movement

of shaft

3. Battery; source of electrical power input to brake

4. Tachometer

We have used D.C. motor as prime mover.

4.1.1 Prime Mover

We needed a prime mover to rotate the shaft and flywheel at high speed around 1000rpm

or more; because the induced eddy current is proportional to the rate of change of flux

which increases with speed of shaft.

Power 1.6hp @ 2800 rpm

Voltage 12 Volts DC

FIG 4.1 PRIME MOVER

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4.1.2 Electromagnet

We required producing the magnetic field around the shaft; therefore we are using

electromagnet to get magnetic field around shaft only when desired to stop the shaft. We

are using 24 volt DC current supply through battery used in trucks. Keeping the voltage

same we have to draw more current to achieve powerful magnetic field.

It can be achieved by using a thick wire or wire of AWG 20 or AWG 22 grade to reduce

the resistance and increase the current drawn.

No of turns 500

Wire used AWG 36 Copper

External radii mm

Internal radii mm

Length mm

FIG 4.3 A ELECTROMAGNET

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The electromagnets will be built and assembled by our team. We will use standard

coated copper wire AWG gauge 36 coiled around a ferrous metal core. Coating the

copper wire will prevent corrosion and increase the life of the electromagnets and

maintain the efficiency of the overall braking system. The number of turns of copper

around our ferrous material will determine the strength of the induced magnetic field.

FIGURE 4.4 MAGNETIC FIELD LINES OF COIL PAIRS

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Chapter 5

Methodology

25

5.1 Methodology

The procedure followed by our group is as follows:

1. To prepare a rigid enough frame to support the device with minimum vibrations

possible

2. To mount the motor on it

3. Preparing electromagnet

4. To mount the electromagnet on it

5. To mount the whole assembly on frame.

5.2 Calculations

26

Chapter 6

Results and Discussion

27

6.1 Result

1. The maximum speed of shaft is 1000rpm (approx).

2. Reduction in speed after application of brakes from 1000rpm to 700rpm in

2.83seconds.

Percentage reduction in speed= 30%

3. Reduction of speed after 700rpm is not considerable or very small because of very

small rate of change of flux.

6.2 Advantages

1. The device should be used in heavy automobiles as an accessory.

2. It is highly suitable at high speed.

3. It works on electricity and consumes very small amount of power for a tiny time

period.

4. Can be easily controlled and resettable.

5. Very light weight and low maintenance.

6. Consumes small space therefore installation is easy.

7. Running cost is small.

6.3 Disadvantages

1 Higher initial cost.

2 Very large amount of heat generation.

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6.3 Conclusion

1. The eddy current brakes can be used as an accessory in heavy automobiles with

conventional friction brakes; because it is the remedy of problems faced by

conventional brakes like fading, skidding, high maintenance requirement, low

reliability, requirement of servo mechanisms, breaking, higher weights etc.

2. This device is easy to install an cost incurred is small so can be used in the

automobiles manufactured.

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6.4 Future Enhancement

1. The eddy current increases with decrease in resistivity of material. Therefore;

there is scope of applying copper wire windings of AWG 20 or less to get highly

conductive surface and minimum resistance possible to increase the eddy current

induced.

2. The magnetic field induced by electromagnet is not too large and can be increased

by supplying higher current. The stator is rated for 61amp at 12 volt and the

supply of dc input is very small to permissible limit.

So there is scope to enhance the input signal of electricity by applying amplifiers.

3. Speed of shaft can be increased by providing a gear arrangement instead of chain

sprocket assembly of high gear ratios to get higher speeds.

4. Frame should be grounded to solve vibration problem of frame and to make it

rigid.

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REFERENCES

Websites:

1. www.freepatentonline.com

2. www.wikipedia.com

3. www.telmaretarders.com

4. Prof. Peter R. Saulson “Electrical Power, Magnetism and Electromagnetic

Motors” aulson@physics.syr.edu

http://physics.syr.edu/courses/PHY101/Physics 263-4

5. Prof. S.K. Sahdev and Prof. R.K. Chaturvedi Dhanpat Rai & Co. edition

1988 Page(2.1-2.49)

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