eddy current brake
TRANSCRIPT
Chapter 1
Introduction
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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.
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Chapter 2
Literature Survey
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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
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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
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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
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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
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Chapter 6
Results and Discussion
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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” [email protected]
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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