What are Eddy Currents?
Eddy currents circulate a conductor like eddies swimming across an ocean surface. They originate from a conductor in response to changes in an external magnetic field acting on it. They are formed in close loops running parallel or perpendicular to the direction of the magnetic field and are a suitable metric for determining good conductors (such as copper or iron).
If a piece of the magnet is dropped through a conductor, the conductor experiences a change in its magnetic field. This change induces currents that circle the conductor in the shape of eddies, and though it lasts only a moment, the currents create a new magnetic field. The new field acts opposite the direction of the conductor’s magnetic field, resisting the motion of the magnet around the elector. This effect is useful in slowing the motion of different conductors and has been coined the Eddy Current Brake.
Eddy Currents: How They Work
Eddy currents are also known as Foucault currents, owing to the French Scientist Léon Foucault, who proposed that a reaction could occur between a magnet and a conductor to create a field that affected the conductor’s behavior. François Arago, a mathematician and the then Prime Minister of France, is credited for this genius observation, noting in 1824 that any material that could conduct electricity could also be magnetized. Three decades later, Foucault confirmed this, leveraging the Lenz law introduced by Heinrich Lenz.
The Lenz law shows the relationship between the direction of a magnetically induced current and the conductor’s magnetic field. It states that the direction of a current induced by a magnet will move such that the magnetic field it produces will oppose the magnetic field of the conductor that produced it.
This law explains the behavior of a magnet falling slowly through a copper tube, as the magnet causes a drag that slows the motion of the magnet.
Eddy currents aren’t limited to the action of magnets passing through a conductor. A conductor in proximity to a magnet can also induce eddy currents. Another well-known example is the breaking of a magnet into two separate pieces. Suppose a spinning conductor disc is inserted between the two magnets. In that case, the velocity of the spin will reduce as the magnetic field induced by its eddy currents act opposite its magnetic field, causing a drag.
In this above example, the eddy currents produced are of two natures: clockwise and counter-clockwise. Counter-clockwise currents are produced when the conducting material comes in contact with the magnet’s strongest point. The currents produce their magnetic field in an upward direction.
At the same time, the material travels through the magnet and reaches the end of the magnet, producing clockwise eddy currents whose magnetic fields point downward. The simultaneous actions of the individual magnetic fields against the conductor’s field result in the drag that gives the braking effect. Note that for this to work, the conductor must be in motion. Thus eddy braking is not suitable for conductors in a stationary position.
Can Neodymium Magnets Induce Eddy Currents?
Neodymium magnets induce eddy currents in proximity to a good conductor. Neodymium magnets are nonferrous magnets and are compatible with metallic conductors. Their exemplary magnetic strength enhances the magnetic field produced by their induced eddy currents.
Neodymium magnets may be ineffective against nonmetallic materials. Thus, when the magnets are required to act as eddy current separators, they cannot be used if the materials in question are nonmetallic, such as fabric or plastic.
Calculating Eddy Currents
The force of eddy currents isn’t easily calculated. Different magnets will give different forces, and these forces are equally dependent on:
- The velocity of the magnet traveling through the conductor. The faster the magnet, the more force it exerts (which affects the force of the currents it induces).
- The geometry of the conductor. A thin disc of the conductor will induce fewer currents than a thicker disc. Likewise, thicker tubes will naturally slow the speed of a magnet than their thinner counterparts.
- The strength of the magnet determines the strength of the magnetic field.
- The magnitude of the change in magnetic field strength and the proximity of the magnet to the conductor change.
Though eddy currents are relative to the materials involved in the reaction, the loss in eddy currents can be calculated. This measures the changes in the eddy currents of a conductor over a period. The flux density, eddy coefficient, material volume, and conductor thickness must be provided. To explore this formula, click here.
Can Eddy Currents be Minimized?
Eddy currents generate heat, as their electrical energy is easily converted to heat. This makes them undesirable in some situations. To reduce eddy currents:
- Laminate the core of the metal. The laminating object is usually an insulator that separates the core; their planes are set up perpendicular to the direction of the eddy currents, negating the intensity of the current. This reduces the electrical energy generated in the form of heat.
- They are using poor conductors. Eddy currents are only possible between magnets and conductors. Using magnetic materials that resist the passage of electric currents will nullify any induced currents. Materials made from iron or nickel are suitable, as they show high resistivity to electrons compared to copper (thus, copper is fitting material for electrical work).
Uses of Eddy Currents
We discussed earlier that eddy currents cause a braking effect. Let’s take a further look at this and other applications of eddy currents in engineering and technology:
- Braking Mechanism in Trains. Eddy currents bring a train from accelerating along a rail to stopping over a short course. How does this work? Trains have metallic wheels and run on metallic rails. When the brake is pulled, the wheels come in contact with a magnetic field (exerted by magnets between the rails). This induces eddy currents, creating new magnetic fields that slow the wheel’s speed sliding along the track. As the train slows, the wheels spin and the force reduces until the wheels stop moving. This is a significant application of eddy currents.
- Induction Furnaces. In these furnaces, the electric force released generates significant eddy currents, which create large magnetic fields. This elevates the temperature of the furnace, and the higher the temperature, the more eddy currents are produced. The loop is responsible for the very high temperatures of most induction furnaces, making them suitable for reactions that occur at high temperatures, such as the extraction of metals from their ores.
- Eddy currents can revive a galvanometer having a delayed rest time. If, after use, the needle of a galvanometer doesn’t immediately come to rest at its equilibrium point (the zero mark), a coil can be waved over a nonmagnetic metal frame close to the galvanometer. This eliminates the lag and allows the recording of the reading taken by the galvanometer. The eddy currents created by the coil alter the movement of the needle and bring it to its equilibrium point.
- Eddy currents convert electric energy to heat. Induction-based cookers maximize this ability. The cooker’s tops are made to absorb both heat and electricity. Within these tops are copper coils capable of generating a magnetic field. When you power your cooker, alternating current passes through the coil and creates magnetic fields. These fields induce eddy currents that generate heat which warms the tops and utensils placed on the cooker tops. The procedure is repeated until the utensil is sufficiently heated or the power source is removed. This principle is relevant in induction-powered welding machines too.
- Testing of Materials. Because eddy currents are not observed with poor conductors, they can be used to determine the integrity of a material. Likewise, they can show defects present in a magnetic material. For large metallic materials such as airplanes, irregular eddy current readings prove discrepancies in the quality of the metal in the plane’s body.
While eddy currents are suitable for heat-based devices, they cause inconveniences for devices like transformers and generators. In these cases, the heat produced is considered a waste and is diverted from the devices. This also makes eddy current reduction an important part of applying eddy currents.
Processes involving eddy currents often require experts in electromagnetism and magnetism supervision and input. The force of an eddy current acting on a conductor cannot be immediately recorded; it’s hard to determine the exact factors that make eddy currents applicable to any situation.
While eddy currents are effective for several industrial processes, you should always involve trained hands. Are they interested in more insights into the behavior of magnets, magnetism, and other magnet-related subjects?
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