What is Eddy Current?
We know that whenever the magnetic flux passing through a coil changes then it will generate electromotive force and current. The same question arises when we want to know what happens when the magnetic flux is passed through a solid coil, will it also be able to induce emf and current? And the answer is yes. The current induced by the conducting body is known as the eddy current. What is eddy current is, basically eddy current is the current induced in any solid conducting body when the magnetic flux is passed through it.
Eddy Current Definition
Eddy current is defined as the induced current in a conducting coil when a magnetic flux is allowed to flow through it or in other words, it is the induced current in a conducting body due to a change in magnetic flux. The eddy current definition can be explained as an important nature of a solid conducting body.
Eddy Current Meaning
Now, let us have a look at the eddy current meaning and what is eddy current in detail. According to the eddy current definition we understood that it is the current generated or induced as a result of the change in magnetic flux.
Now, what is eddy current meaning, or what do you mean by eddy current? Whenever the conductor encounters a change in the magnetic flux, then the free electrons present in the conductor will experience a magnetic force. As a result of the effect of the magnetic force on the free electrons, these free electrons will move in the form of small loops or eddies, the electric current generated due to the motion of free electrons in the form of eddies is known as the eddy current. The generation of eddy current follows Faraday’s law of magnetic induction.
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The magnitude of induced eddy current can be calculated using Faraday’s law of magnetic induction. According to the second law of Faraday’s laws of magnetic induction, we know that induced emf is written as, rate of change of magnetic flux with respect to time will give rise to induced emf in the conductor, mathematically we get:
Emf = E = -dΦ/dt
Φ -The magnetic flux passed through the conductor
The negative sign is corresponding to the direction of the magnetic flux and is determined by the Lenz law.
Now, the current induced in a conductor whose resistance is R can be calculated by using ohm’s law, thus we get:
I= e/R
Where,
e-The emf induced in the good conductor as a result of the change in magnetic flux
R-The resistance of the conductor
Substituting the value of induced emf in the above equation we get,
I = - dΦ/dt/R = -1/RdΦ/dt
Equation (2) is known as the expression for induced current. And the direction of induced current can be estimated by the Lenz law.
Uses of Eddy Current
To understand what is eddy current it is necessary to understand Faraday’s electromagnetic induction. In rapidly changing magnetic fields, due to the production of heavy emf, large eddy currents are induced. Eddy currents are capable of producing temperature as a result of this heavy temperature. Thus a coil-over a constituent metal placed in a field of the highly oscillating magnetic field will produce a high-frequency AC. The temperature produced as a result of a heavy eddy current is enough to melt the metal. This is used to extract metals from ores.
Here are some of the uses of the eddy current:
Automobile speedometers: Eddy currents are used to know the speed of a vehicle. A speedometer consists of a rotating magnet, which rotates based on the speed of the vehicle, producing Eddy Current in the drum. When the drum turns in the direction of the rotating magnet, the pointer attached to the drum indicates the speed of the vehicle.
Magnetic brakes: When you press the brakes, the metal wheels get exposed to a magnetic field which generates eddy currents in the wheels. The interaction between the eddy currents and the applied magnetic field slows the wheels down.
Energy meters: A metal disc in the electric power meter rotates due to eddy currents. The magnetic field induces the electric currents in the disc by alternating varying currents in a coil.
In induction furnaces: Eddy current is used for a uniform increase of temperatures in induction furnaces. Large eddy currents start flowing in rapidly changing magnetic fields, due to a large emf being produced. This produces high temperatures. A coil is wound over a constituent metal, placed in a highly oscillating magnetic field, produced by high frequency.
Free-fall devices (detecting coins in vending machines): Eddy current helps detect counterfeit coins and notes in vending machines. Eddy currents slow down the speed of the coins as they move past a stationary magnet.
Galvanometers (a device which measures small electrical currents): Eddy current is set up in the galvanometer by an oscillating coil, and its resistive forces oppose further oscillations of the coil. This enables the coil to quickly attain its equilibrium position.
Proximity/displacement sensors
Induction alternating current (AC) motors
Did You Know?
The eddy current testing is nondestructive testing. And eddy current testing is an important part of our everyday life. Not everyone has heard of eddy current testing, but these methods touch all of our lives, perhaps even as a routine. This is because the eddy current techniques are used in a wide range of industries such as those industries where component failure could cause devastating damage and loss.
For an instant, the eddy current testing is used to inspect tubing and other structures for applications such as pipelines of gas and oils, nuclear reactors, chemical manufacturing, and municipal water systems. Compact and portable eddy current testing equipment is used for on-site inspections in the field, such as looking for cracks in bridges and in airplane components from wings to the landing gear. That makes ECT, and other methods of nondestructive testing, highly important to public safety playing a role in helping to prevent destructive events like pipeline breaks, bridge collapses, and plane crashes.
FAQs on Eddy Current
1. What are eddy currents and what is the primary cause of their generation?
Eddy currents, also known as Foucault currents, are loops of electric current induced within bulk conductors by a changing magnetic field in the conductor, according to Faraday's law of induction. The primary cause is the change in magnetic flux passing through a conductive material. When the magnetic field changes, it creates a changing magnetic flux, which in turn induces an electromotive force (EMF) and drives the flow of these circulating currents within the body of the conductor itself.
2. What is the underlying principle that governs the direction and magnitude of eddy currents?
The principles governing eddy currents are fundamental laws of electromagnetism:
- Magnitude: The magnitude of the induced EMF, and thus the strength of the eddy currents, is determined by Faraday's Law of Induction. It states that the induced EMF is directly proportional to the rate of change of magnetic flux (E = -dΦ/dt).
- Direction: The direction of the eddy current loops is determined by Lenz's Law. This law states that the induced current will flow in a direction that creates a magnetic field opposing the change in magnetic flux that produced it.
3. What are some important real-world examples and applications of eddy currents?
Eddy currents have several practical applications where their effects are used advantageously. Key examples include:
- Magnetic Braking: Used in high-speed trains and roller coasters, where strong electromagnets induce eddy currents in the metal rails or wheels, creating a drag force that slows the vehicle down without physical contact.
- Induction Furnaces: High-frequency alternating current in a coil produces rapidly changing magnetic fields, inducing large eddy currents in a piece of metal. The heat generated by these currents (I²R heating) is sufficient to melt the metal.
- Automobile Speedometers: A rotating magnet connected to the vehicle's driveshaft induces eddy currents in an aluminium drum, which causes the drum to rotate against a spring. The attached pointer indicates the vehicle's speed.
- Energy Meters: The shiny metal disc in older analog electricity meters rotates due to eddy currents induced by magnetic fields from the household current, measuring energy consumption.
4. Why are eddy currents often considered undesirable in devices like transformers?
In many AC devices like transformers and motors, eddy currents are a major source of energy loss. As these currents circulate through the resistance of the conductive core material, they dissipate energy in the form of heat (known as I²R losses). This unwanted heating reduces the overall efficiency of the transformer because a portion of the electrical energy is converted into waste heat instead of being transferred from the primary to the secondary coil. This can also lead to overheating and potential damage to the device.
5. How is the core of a transformer designed to minimise eddy current loss?
To minimise energy loss from eddy currents, the soft iron core of a transformer is not made from a solid block of metal. Instead, it is constructed from thin, insulated sheets of iron or steel called laminations. These laminations are stacked together, with each sheet electrically insulated from the next. This design breaks up the continuous conductive path, forcing the eddy currents into very small, restricted loops within each lamination. This dramatically increases the overall resistance to the flow of large eddy currents, significantly reducing heat loss and improving the transformer's efficiency.
6. Explain how eddy currents are used to create non-destructive testing (NDT) systems.
In non-destructive testing, a probe containing a coil with an alternating current is placed near the surface of a conductive material. This induces eddy currents in the material. These currents, in turn, generate their own magnetic field that opposes the original field. If there is a crack, flaw, or change in material thickness, it will disrupt the path of the eddy currents. This disruption changes the impedance of the probe's coil, which can be measured electronically. By analysing these changes, inspectors can detect surface and near-surface defects without damaging the component.
7. Are eddy currents always circular? Explain the nature of their path in a conductor.
No, eddy currents are not always perfectly circular. The term 'eddy' is an analogy to whirlpools in a fluid. Their path is more accurately described as closed circulating loops within the plane perpendicular to the direction of the changing magnetic field. The precise shape of these loops depends on the geometry of the conductor and the specific area where the magnetic flux is changing. In a large, flat plate, they can form broad, swirling patterns, while in more constrained geometries, their paths will conform to the boundaries of the conductor.
8. How does the electrical conductivity of a material affect the strength of eddy currents?
The strength of eddy currents is directly related to the electrical conductivity of the material. For a given induced EMF (determined by the changing magnetic flux), a material with higher conductivity (like copper or aluminium) will have lower electrical resistance. According to Ohm's Law (I = V/R), lower resistance allows a larger current to flow. Therefore, materials with high conductivity will support stronger eddy currents, leading to more significant effects, such as greater heating in an induction furnace or a stronger braking force in a magnetic brake.
