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Eddy Currents
Eddy currents are an electric phenomenon that is produced when a conductor (metal) passes through a variable magnetic field. The relative movement causes a circulation of electrons, or induced current within the conductor. These circular Eddy currents create electromagnets with magnetic fields that oppose the effect of the applied magnetic field.
The Eddy currents and the generated contrary fields will be stronger
- when the applied magnetic field is stronger, or
- with higher conductivity of the conductor, or
- with higher relative speed of movement.
For a practical demonstration of the Eddy Currents, cylindrical magnets are used that fall vertically in a copper or aluminium tube. It can be experimentally proven that the force that opposes the weight is proportional to the speed of the magnet. The experience is illustrated in the attached drawing:
Let's suppose that the cylindrical magnet descends with its North pole (colour red) in front and its South pole (colour blue) behind. On a magnet, the magnetic field lines are outgoing at the North pole and incoming at the South pole.
During the descent of the magnet, the flow from the magnetic field increases in the region near the magnet's North pole. An induced current originates in the tube, Eddy Current, which opposes the increased flow, in the direction that is indicated in the first figure.
In the following figure, the equivalency between currents (spirals or solenoids) and magnets is shown, in such a way that the induced current ahead of the North pole equals a magnet of opposing polarity, by which they repel each other. However, the induced current behind the magnet has the same polarity by which they attract each other. Both currents generate a force (f) that stops the falling movement of the magnet.
The same principle of the Eddy Currents is applied to the dynamic brakes of trucks, a type of brake that most heavy vehicles have nowadays. Its biggest advantage is that they work without contact and therefore they don't wear out. In these devices, some supporting discs for the drive shaft rotate around some electromagnets powered by a battery. In order to brake, a current is sent to the electromagnets. The faster the vehicle goes, the faster the rotation of the discs is between the electromagnets and the braking is more effective; which is why they are ideal for going downhill. On the other hand, their effectiveness diminishes when the speed is low and disappears when stopping, which is why mechanical brakes also have to be installed for the slow motion.
An industrial application of the Eddy Currents can be found in motor-variators. The use of motor-variators is a typical application, when high performance and power are demanded by the speed variation of an electric motor, since it's the most robust and reliable speed variation in alternative current on the market. Motor-variators are equipment with magnetic coupling by Eddy Current, with a total transmission of the torque from the motor to the portal axle, maintaining the torque even in the ignition of the motor. This transmission is carried out completely exempt from gears and friction. For this reason it is used in the drilling of oil wells, for example. If the axle that is drilling gets jammed when reaching a harder stratum, the motor will not be damaged since there is no direct connection through gears.
The Eddy Currents are those that are generated by induction in the metallic pieces that cross the inductor drum of a Non-Iron Metal Separator by Eddy Currents, provoking a force of opposing repulsion for the purposes of the inductor drum and achieving a forward movement, separating itself from the rest of the materials that don't have influence and fall while following a natural parabolic path.
