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.
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.
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.
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
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 =
Lr/A
where,
L = total
length of field winding in meter.
r = 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
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
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.
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.
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.
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
ELECTRICALLY EXCITED EDDY CURRENT BRAKE
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.
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.
r = 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 p NT/60 watts.
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.
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.
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.
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.
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.
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.
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