21-02-2010, 11:22 AM
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EDDY CURRENT BRAKES
EDDY CURRENT:
It is a swirling current set up in a conductor in response to a changing magnetic field.
By Lenz's law, the current swirls in such a way as to create a magnetic field opposing the change
Eddy Current Brakes
It slow an object by creating eddy currents through emi which create resistance, and in turn either heat or electricity.
Braking action is made by varying the strength of the magnetic field.
¢ A braking force is possible when electric current is passed through the electromagnets.
THEORY OF OPERATION
Eddy current brakes develop torque by the direct magnetic linking of the rotor to the stator.
This linking generates eddy currents in the driven rotor.
Eddy current brakes must have a slip between the rotor and the stator to generate torque.
An eddy current brake having an electromagnetic pole and the rotor is positioned in close proximity to the stator with an air gap between them
The stator comprises steel pole pieces with hollow cores that establish a magnetic circuit for a magnetic flux field.
The pole pieces have machine-wound electrical windings.
The windings are fastened with pole caps.
The hollow core reduces the weight and material of the stator without significantly adversely affecting the braking capacity.
The pole caps reduce the magnetic saturation and increases the overall brake torque output.
WORKING PRINCIPLE
Electromagnets produce magnetic field from supplied current
Change of magnetic flux (with time) induces eddy currents in conductor (disc)
Eddy Currents produce another magnetic field opposing first field
Opposing magnetic fields create force that reduces velocity
CONSTRUCTIONAL DETAILS
Components
Electromagnets
Cast Iron Core
Conducting (Copper) Wire
Mounting bolts
Disc
Mild steel
Machined from plates
It 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 aluminum is used as winding materials.
CLASSIFICATION OF EDDY CURRENT
BRAKES
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.
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 the rotors.
Advantages. . .
It uses electromagnetic force and not mechanical friction
Non-mechanical (no moving parts, no friction)
Fully resettable
Can be activated at will via electrical signal
Low maintenance
Operates at any rotational speed
Light weight
Disadvantages. . .
Braking force diminishes as speed diminishes with no ability to hold the load in position at standstill.
That could be considered to be a safety issue, but it really means that friction braking may need to be used as well.
Eddy-current brakes can only be used where the infrastructure has been modified to accept them.
APPLICATIONS
It is used as a stopping mechanism in trains.
It is also used in the smooth breaking and functioning of roller coasters and such fast moving machines.
CONCLUSION
¢ 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. If the speed of the vehicle is very high, it 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
