Download eddy current brake in trains

Seminar Report 2004
Eddy Current Brake
ABSTRACT
This is an electric braking system which works on the principle that eddy
current produced in it opposes the driving torque. This opposing torque is used to
brake the automobiles. Mainly this system is purely based on Faraday’s laws of
electromagnetic induction and Lenz’s law.
For operating this a control switch is provided on the steering column in a
position for easy manual. The skidding and complexity of mechanical braking system
can be minimized by this system. Also the wear and tear of the vehicles can be
reduced.
Since researches are going on to eliminate some of the disadvantages of this
system, we can accept it to be the norm one in a few years of time.
Dept. of Electrical & Electronics
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M.E.S.C.E, Kuttippuram
Seminar Report 2004
Eddy Current Brake
CONTENTS
1.
INTRODUCTION
2.
PRINCIPLE OF OPERATION
3.
CONSTRUCTION
4.
WORKING
5.
EDDY CURRENT BRAKE IN TRAINS
6.
BRAKING POWER CALCULATIONS
7.
EDDY CURRENT BRAKE REQUIREMENTS
8.
MOUNTING AND INSTALLATION
9.
ADVANTAGES
10.
DISADVANTAGES
11.
APPLICATIONS
12.
CONCLUSION
13.
REFERENCES
Dept. of Electrical & Electronics
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M.E.S.C.E, Kuttippuram
Seminar Report 2004
Eddy Current Brake
INTRODUCTION
Many of the ordinary brakes, which are being used now days stop the vehicle
by means of mechanical blocking. This causes skidding and wear and tear of the
vehicle. And if the speed of the vehicle is very high, the brake cannot provide that
much high braking force and it will cause problems. These drawbacks of ordinary
brakes can be overcome by a simple and effective mechanism of braking system ‘The
eddy current brake’. It is an abrasion-free method for braking of vehicles including
trains. It makes use of the opposing tendency of eddy current
Eddy current is the swirling current produced in a conductor, which is
subjected to a change in magnetic field. Because of the tendency of eddy currents to
oppose, eddy currents cause energy to be lost. More accurately, eddy currents
transform more useful forms of energy such as kinetic energy into heat, which is
much less useful. In many applications, the loss of useful energy is not particularly
desirable. But there are some practical applications. Such an application is the eddy
current brake.
Dept. of Electrical & Electronics
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M.E.S.C.E, Kuttippuram
Seminar Report 2004
Eddy Current Brake
PRINCIPLE OF OPERATIONS
Eddy current brake works according to Faraday’s law of electromagnetic
induction. According to this law, whenever a conductor cuts magnetic lines of forces,
an emf is induced in the conductor, the magnitude of which is proportional to the
strength of magnetic field and the speed of the conductor. If the conductor is a disc,
there will be circulatory currents i.e. eddy currents in the disc. According to Lenz’s
law, the direction of the current is in such a way as to oppose the cause, i.e. movement
of the disc.
Essentially the eddy current brake consists of two parts, a stationary magnetic
field system and a solid rotating part, which include a metal disc. During braking, the
metal disc is exposed to a magnetic field from an electromagnet, generating eddy
currents in the disc. The magnetic interaction between the applied field and the eddy
currents slow down the rotating disc. Thus the wheels of the vehicle also slow down
since the wheels are directly coupled to the disc of the eddy current brake, thus
producing smooth stopping motion.
Dept. of Electrical & Electronics
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M.E.S.C.E, Kuttippuram
Seminar Report 2004
Eddy Current Brake
EDDY CURRENT INDUCED IN A CONDUCTOR
Dept. of Electrical & Electronics
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M.E.S.C.E, Kuttippuram
Seminar Report 2004
Eddy Current Brake
CONSTRUCTION
Essentially an eddy current brake consists of two members, a
stationary magnetic field system and a solid rotary member, generally of mild steel,
which is sometimes referred to as the secondary because the eddy currents are
induced in it. Two members are separated by a short air gap, they’re being no contact
between the two for the purpose of torque transmission. Consequently there is no
wear as in friction brake.
Stator consists of pole core, pole shoe, and field winding. The field
winding is wounded on the pole core. Pole core and pole shoes are made of east steel
laminations and fixed to the state of frames by means of screw or bolts. Copper and
aluminium is used for winding material the arrangement is shown in fig. 1. This
system consists of two parts.
1.
Stator
2.
Rotor
Stator:It is supported frame members of the vehicle chassis. It has introduced
magnetic poles energized by windings. Current is supplied to the winding from the
battery.
Rotor:It is a rotating disc, which is fitted on the line of crankshaft with small air gap to
stator. When disc rotates a flux change occur in the section of the disc passing the
poles of stator. Due to the flux change there is a circulatory or eddy current in the
Dept. of Electrical & Electronics
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M.E.S.C.E, Kuttippuram
Seminar Report 2004
Eddy Current Brake
disc around the magnetic lines of force. The effect of this eddy current induces ‘N’
and ‘S’ poles at the surface of the disc. Then there will be a ‘drag’ or braking effect
in between eddy current
induced
poles and magnetic
poles in the stator. By
changing current from the battery we can change the braking force. In this breaking
system kinetic energy of the vehicle is converted to heat and this heat is dissipated
through the rotating disc.
Total resistance of field winding
R = L/A
where,
L
=
total length of field winding in meter.

=
Resistivity of the wire in ohm meter
A
=
the area of cross section of field winding in m2
Total no: of terms = total length /mean length of one term
The rotor is a rotating disc on shaft, which is placed very near to the stator
with small air gap (1 mm to 2 mm). Rotating disc may be one or both side of stator.
The two units have common ring member, poles cores on which winding are
provided being fixed to ring number. If a malleable casting is employed, then the
pole core could be cast integrally with the right. After fitting the windings on the
cores, poles shoes are fitted to provide pole faces of appropriate shape and area. The
rotor disc should be provided with properly designed fins for faster heat removal.
The magnetic circuits of the two units are substantially the same, non-undue and
thrust would be imposed on the motor bearings. Slight axial displacement of rotor
could however, cause quite appreciable discrepancy, the air gap of two units. The
effect would be to increase the magnetic pull in one air gap and diminish it on other
which could give to rise to excessive and thrust on rotor bearing to overcome the
inherent defect, the air gaps of both units could be put in series by making the central
Dept. of Electrical & Electronics
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M.E.S.C.E, Kuttippuram
Seminar Report 2004
Eddy Current Brake
number non magnetic and providing a continues pole core for each pair axially
opposite poles. This modification could possibly reduce the length of the combined
pole course or permits a larger winding length.
The maximum diameter of the eddy current brake is decided by
1.
The spacing of vehicle chassis frame.
2.
Vehicle floor clearance
Dept. of Electrical & Electronics
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M.E.S.C.E, Kuttippuram
Seminar Report 2004
Eddy Current Brake
WORKING
When the vehicle is moving, the rotor disc of eddy current brake which is
coupled to the wheels of the vehicle rotates, in close proximity to stationary magnetic
poles. When we want to brake the vehicle, a control switch is put on which is placed
on the steering column in a position for easy operation.
When the control switch is operated, current flows from a battery to the field
winding, thus energizing the magnet. Then the rotating disc will cut the magnetic
field. When the disc cuts the magnetic field, flux changes occur in the disc which is
proportional to the strength of the magnetic field. The current will flow back to the
zero field areas of the metal plate and thus create a closed current loop like a whirl or
eddy. A flow of current always means there is a magnetic field as well. Due to Lenz’s
law, the magnetic field produced by the eddy currents works against the movement
direction. Thus instead of mechanical friction, a magnetic friction is created. In
consequence, the disc will experience a “drag” or the braking effect, and thus the disc
stops rotation. The wheels of the vehicle, which is directly coupled to the disc, also
stop rotation. Faster the wheels are spinning, stronger the effect, meaning that as the
vehicle slows, the braking force is reduced producing a smooth stopping action.
The control switch can be set at different positions for controlling the
excitation current to several set values in order to regulate the magnetic flux and
consequently the magnitude of braking force. i.e. if the speed of the vehicle is lpw, a
low braking force is required to stop the vehicle. So the control switch is set at the
lowest position so that a low current will be supplied to the field winding. Then the
magnetic field produced will be of low strength, so that a required low braking force
is produced.
Dept. of Electrical & Electronics
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M.E.S.C.E, Kuttippuram
Seminar Report 2004
Eddy Current Brake
When the control switch is operated during the standby position of the
vehicle, the magnet will be energized and magnetic field is created. But since the
wheels are not moving, magnetic lines of force are not cut by it, and the brake will not
work. However, a warning lamp is provided on the instrument
panel to indicate
whether the brake is energized. This provides a safe guard for the driver against
leaving the unit energized.
When control switch is put in any one of the operating positions, the
corresponding conductor in the contractor box is energized and current flows from the
battery to the field winding to the contractor box. This current magnetizes the poles
in stator, which placed very near to the rotor. When rotor rotates it will cut magnetic
lines and eddy current will set up in the rotor. The magnetic field of this eddy current
produces a breaking force or torque in the opposite direction of rotation disc. This
kinetic energy of rotor is converted as heat energy and dissipated from rotating disc
to surrounding atmosphere. Current in the field can change by changing the position
of the controls switch. Thus we can change the strength of the braking force.
Dept. of Electrical & Electronics
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M.E.S.C.E, Kuttippuram
Seminar Report 2004
Eddy Current Brake
EDDY CURRENT BRAKE IN TRAINS
In the case of trains, the part in which the eddy current is induced is rail. The
brake shoe is enclosed in a coil, forming an electromagnet. When the magnet is
energized, eddy currents are induced in the rail by means of electromagnetic
induction, thereby producing braking action.
EDDY CURRENT BRAKE IN TRAINS
Dept. of Electrical & Electronics
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M.E.S.C.E, Kuttippuram
Seminar Report 2004
Eddy Current Brake
DISTRIBUTION OF MAGNETIC FLUX DENSITY
Dept. of Electrical & Electronics
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M.E.S.C.E, Kuttippuram
Seminar Report 2004
Eddy Current Brake
TYPES OF EDDY CURRENT BRAKES
There are two types of eddy current brakes according to the method of
excitation.
1.Electrically excited eddy current brake
2.Permanent magnet eddy current brake
1.ELECTRICALLY EXCITED EDDY CURRENT BRAKE
Electrically excited eddy current brakes are abruption-free method for
braking. In high-speed trains they offer a good alternative to the mechanical rail
brakes which are being used now a days. During braking, the brake comes in contact
with the rail, and the magnetic poles of brakes are energized by a winding supplied.
Magnetic poles of brakes are energized by a winding supplied with current from the
battery. Then the magnetic flux is distributed over the rail. The eddy currents are
generated in the rail, producing an electromagnetic braking force. This types of
braking need an additional safety power supply when there are breakdowns in the
electrical power supply.
Dept. of Electrical & Electronics
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M.E.S.C.E, Kuttippuram
Seminar Report 2004
Eddy Current Brake
ELECTRICALLY EXCITED EDDY CURRENT BRAKE
Dept. of Electrical & Electronics
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M.E.S.C.E, Kuttippuram
Seminar Report 2004
Eddy Current Brake
2. PERMANENT MAGNET EDDY CURRENT BRAKE
Recently, permanent magnet eddy current brakes have been developed for
subways, trams and local trains. These brakes need a mechanical actuator to turn the
magnets jn an on and off position. The main advantage of this type of brake is safety.
i.e. it does not need electrical power supply to energize the magnet.
Dept. of Electrical & Electronics
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M.E.S.C.E, Kuttippuram
Seminar Report 2004
Eddy Current Brake
BRAKING POWER CALCULATIONS
Sophisticated calculation methods for the determination of braking forces of
eddy current brakes are important for the design of the brakes.
For a simple eddy current brake employing a thin non-magnetic disc as
copper the drag or braking force on the disc
where
H = Magnetic field strength in Webers.
A = Pole force area in cm2.
V = Velocity in cm/sec of mean radius of disc under the poles.
T = disc thickness in cm.
 = specific resistance of disc material at its operating temperature in micro
ohms/cm3.
Torque = F * R Nm
Where R = mean pole radius in meter.
Power P = 2  NT/60 watts.
Dept. of Electrical & Electronics
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M.E.S.C.E, Kuttippuram
Seminar Report 2004
Eddy Current Brake
EDDY CURRENT BRAKE REQUIREMENTS
An eddy current brake is an energy converter functions is to convert the
kinetic energy of a vehicle into heat and dissipate it such a rate to maintain the
temperature of unit with in reasonable limits under maximum and prolonged braking
conditions. The energy absorbed by the brake is transformed into heat by the currents
induced in the motor, and this is heat manly dissipated in surrounding air through the
medium of suitable designed fins the rotating member.
In mountains area, continues braking force is needed for a long time (say
about half an hour), at this condition, eddy current braking is more suitable to function
without over heating. The use of these retarders is by no means limited to mountain
on country. They can be advantageously employed on public service vehicles on city
routes without frequent stops. But in this braking system there with out be any
braking force in the vehicle is rest. So the eddy current brake is used as an auxiliary
heavy-duty retarder. By using auxiliary retarder very smooth retardation is assumed
and likely hood of skidding on slippery roads surfaces is minimized. Smooth braking
action cuts down tier were and since brake is used as an auxiliary heavy-duty retarder.
By using auxiliary retarder very smooth retardation is assumed and likely hood of
skidding on slippery roads surfaces is minimized. Smooth braking action cuts down
tier were and since the conventional brakes are relieved of heavy duty being required
only to bring vehicles to rest.
Dept. of Electrical & Electronics
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M.E.S.C.E, Kuttippuram
Seminar Report 2004
Eddy Current Brake
MOUNTING AND INSTALLATION
A typical mounting of an eddy current brake consists of two discs in which
pole salient type, supported between the frame numbers of a vehicle chassis. Rotor is
coupled to road wheels being often mounted on a shaft that is interposed between the
gearbox and propeller shaft and stator is mounted on the frame of the vehicle.
The driver who can select one or four excitation settings according to the
breaking effect required mounts a control switch on a steering column in a position
for easy operation. In the operative positions of this switch 1, 2, 3, 4 contractors are
energized to supply current to the excitation windings of the retarder. Warning lamp is
also provided on the instrument panel to indicate when the retarder is energized. This
provides a safe guard for the driver against leaving the unit energized when the
vehicle is stationary.
Dept. of Electrical & Electronics
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M.E.S.C.E, Kuttippuram
Seminar Report 2004
Eddy Current Brake
ADVANTAGES
 Less maintenance
 Wide range of braking force available within the temperature limit
 Prolonged braking is possible
 Long life
 Less strain to the operation
 Smooth retardation, which cuts down, the tire wears
Eddy current brakes offer smooth retardation of vehicles without skidding. It
is totally free of wear and tear. So it has long life compared to ordinary brakes. These
need less maintenance.
In mountain areas continuous braking is needed for a long time. At this
condition, eddy current braking is more suitable to function without overheating.
By changing the excitation current to the field winding, i.e. by adjusting the
position of the control switch, we can vary the braking force to required range. The
operation of eddy current brake is very simple. i.e. the control switch is a soft switch
which can be operated without any strain.
Eddy current brakes works even under the toughest environmental conditions.
For example, in larger water slides and water coasters where the humidity
would
immediately result in reduction of friction and thus impair the effectiveness of
ordinary brakes. It works even in highly corrosive environments and heavily
contaminated areas.
Dept. of Electrical & Electronics
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M.E.S.C.E, Kuttippuram
Seminar Report 2004
Eddy Current Brake
DISADVANTAGES
 No breaking force at rest
 Need of electric power
The main disadvantage of the edyy current brake is that it needs electric power
to work. Researches are going on to overcome this disadvantage by making the brake
regenerative i.e. by converting the kinetic energy of the vehicle into electric energy
and storing it back into the battery.
Dept. of Electrical & Electronics
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M.E.S.C.E, Kuttippuram
Seminar Report 2004
Eddy Current Brake
APPLICATIONS
 For additional safety on long decants in mountain area
 For high speed passenger and goods vehicle
Eddy current brakes are best substitutes for ordinary brakes, which are being
used nowadays in road vehicles even in trains, because of their jerk-free operation. In
mountain areas where continuous braking force is needed, for a long time, the eddy
current braking is very much useful for working without overheating. Eddy current
brakes are very much useful for high-speed passengers and good vehicles. It can also
be used to slow down the trolleys of faster roller coasters.
Dept. of Electrical & Electronics
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M.E.S.C.E, Kuttippuram
Seminar Report 2004
Eddy Current Brake
CONCLUSION
Eddy current brakes are the best choice when demands for reliability and
safety are the highest. They work even in the toughest environmental conditions. Even
the strike of lightning will not result in the loss of the braking force.
Eddy current braking system is not popular now a days. But we hope that the
eddy current braking system which is simpler and more effective will take the place of
the ordinary braking system and we can do expect it to be the norm one in few years
of time.
Dept. of Electrical & Electronics
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M.E.S.C.E, Kuttippuram
Seminar Report 2004
Eddy Current Brake
REFERENCES
1. Manual 1 Gonzalez, Volume 25,Issue 4,July 2004
2. IEEE Transactions on magnetics, Volume 34, Issue 4, July 1998
3. Analysis of eddy current brake for high-speed railway by
Wang.P.J.&
Chiuch.S.J.
4. Automobile electrical equipments by Young &Griffith.
5. Automatic engineering by Kripal Sing.
6. Clutches and brakes by William.C.Orthwin.
7. www.reelectromagneticbrakes.com
8. www.lineareddycurrentbrakes.com
Dept. of Electrical & Electronics
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M.E.S.C.E, Kuttippuram