What is an Eddy Current : Theory, Uses & Drawbacks

Before knowing a clear scenario on eddy current, let us start to know its history, how it was developed, and what are its enhancement. So, the first scientist to look into the concept of this current was Arago in the year 1786 – 1853. Whereas in the period between 1819 – 1868, Foucault gained credits in the discovery of eddy current. And the first utilization of eddy current takes place for non-destructive analyzation that happened in the year 1879 when Hughes implemented the concepts of conducting metallurgical categorizing experiments. Now, the article gives a clear explanation of Eddy Current, Its principle, mathematical equations, uses, drawbacks, and applications.

What is Eddy Current?

These are also called Foucault’s currents where these flow around the conductors in the form of rotating swirls in streams. These are simulated by varying the magnetic fields and movement in closed rings, which are in a vertical position to the magnetic field’s plane. Eddy currents can be generated when there is conductor movement across the magnetic field or when there is variation in the magnetic field that encloses the fixed conductor.

This means that anything that outcomes in the conductor faces a changeover either in the magnetic fields direction or intensity and this delivers these circulating currents. The size of this current has direct proportion to the magnetic field size, loop cross-sectional area, and amount of change in the flux and has an inverse proportional rate to the conductor’s resistivity. This is the main eddy current principle.

Theory

This section explains the eddy current theory and how it can be understood.

Through Lenz law, this current produces a magnetic field that contradicts the variation in the magnetic field, which was created by it, and so eddy currents respond back on the magnetic field cause. As an instance, an adjacent conductive edge will impose a dragging pressure on a movable magnet that differs with its movement, because these currents are stimulated in the surface of a magnetic field which is movable.

This phenomenon is applicable in eddy current brakes that are utilized to resist revolving power equipment in a speedy manner when they are OFF. The flow of current across the conductor’s resistance even disperses energy as heat. So, this current is the crucial reason for energy loss in the AC power-driven devices which are generators, inductors, and others.  In order to minimize this, there needs to be a specific construction like ferrite cores or shielded magnetic cores that have to be done.

When a copper coil or in general electrical conductors are located in a circuit where there is the passage of AC current, the magnetic field is generated across the coil and this is dependent on the self-inductance theory. And the right-hand thumb rule defines the magnetic field path. The resultant magnetic field strength is based on the excitation current of the coil and AC frequency level. When the coil lies in the vicinity of the metal surface, then there will be the induction of the substance.

When the coil lies in the location on the sample having a deficiency, then there happens interruption in the eddy current flow which results in variation in the density and directions. A corresponding variation in the strength of the secondary magnetic field triggers changes in system balance which is noted down as coil’s impedance. The contemporary changes in the eddy current technology consist of pulsed current, eddy current array, and few others.

Eddy Current Loss

This is one more crucial topic to be discussed.

Eddy currents are generated when a conductor undergoes varying magnetic fields. As these eddy currents are ideal and not functional, these impose a loss in the magnetic substance and are known as Eddy Current Losses. In the same way as hysteresis losses, eddy current losses also enhance the magnetic substance temperature. These losses are collectively named as magnetic/core/iron losses.

Let us consider eddy current loss in a transformer.

The magnetic flow in the inner section of the transformer’s core stimulates emf in the core based on Lenz and Faraday’s laws which allows the flow of current into the core. The eddy current loss formula is given by

Eddy current loss = k_ef²B_m²τ²

In the above mathematical expression of eddy current loss,

‘k_e’ represents a constant value that is based on the size and has an inverse relation to the material’s resistivity.

‘f’ represents the frequency range of the excitation material

‘B_m’ corresponds to the maximum value of the magnetic field and

τ  represents the material’s thickness

So as to minimize these current losses the core section in the transformer is developed by assembling thin sheets termed as laminations collected and every individual plate is shielded or polished. With this varnishing, the eddy current movement is restricted to a very minimal level of the cross-section area of every individual plate and shielded from the other plates. Because of this, the flow direction of the current reaches a small value.

In order to minimize the impact of eddy current losses, there are mainly two approaches.

Minimizing the magnitude levels of the current – The magnitude level of the eddy current can be minimized by dividing the solid core into slim sheets which are called laminations, where these are in a parallel direction to the magnetic field.

Every individual lamination is covered from the other end using a slim surface of either oxide film or by varnishing. Through the core lamination, the cross-sectional areas get minimized and so the stimulated electromotive force also gets minimized. As because the cross-sectional area is minimal where the current flow is there, the resistivity levels get enhanced.

The loss that happened by this current can also be minimized by the implementation of a magnetic substance which has an enhanced value of resistivity such as silicon steel.

Braking System

Eddy current braking system is also termed electric/induction braking. This is an instrument utilized for either stopping or slowing down the moving substance by dispersing kinetic energy in the form of heat. In contrast to general friction brake systems, the dragging pressure in the current brake is an EMF between the magnet and the adjacent thing that is in relative movement because of simulation in the conductor simulation in the eddy current via EMF.

Advantages of Disadvantages

Now, consider the benefits and drawbacks behind this concept.

Advantages of Eddy Current

• This approach is mainly applicable to the analysis procedure
• This is the contactless analysis procedure that shows no impact on the work
• The analyzation is completely sped and gives precise results
• The coating surface is easily analyzed that is used on multiple products
• It is even employed in a speedometer device and also in the induction furnace procedure.

Disadvantages of Eddy Current

• Because of this process, there will be magnetic flux leakage
• Extensive heat loss takes place because of cyclic currents because of the magnetic circuit friction. With this electrical energy gets wasted as a form of heat

Applications of Eddy Current

• Implemented in trains those have eddy current brakes
• Used to offer damping torque in PMMC devices
• Utilized in electrical devices such as induction type energy meters
• These are employed to know damages in the metal sections.

This is all the detailed concept. This article has provided concepts of eddy current, principle, theory, formula, loss of eddy current, and applications.