Get clarity on Hysteresis loss and Eddy Current loss

25 Mar, 2025

If you’ve studied magnetic circuits, you’ve probably encountered the terms "hysteresis loss" and "eddy current loss" multiple times. However, understanding what they truly are and why they occur isn’t always straightforward. This article aims to clarify these concepts

Before diving into specific losses, let’s first define what a 'loss' means in the context of magnetic circuits

Losses in magnetic circuits primarily refer to energy dissipation within the magnetic material when it is subjected to changing magnetic fields

Core Losses (or Iron Losses): These losses occur in the iron core of electrical machines. They are further divided into: Hysteresis Loss and Eddy Current Loss.

Hysteresis loss

• Hysteresis literally means "lagging behind".

• Occurs in ferromagnetic materials${}^{[1]}$, such as iron cores used in transformers.

• It is a form of energy loss that occurs as heat within the magnetic material.

  Origin of Loss:

• In a ferromagnetic material the magnetic domains are randomly oriented in the absence of any external magnetic field. When a ferromagnetic material is subjected to an alternating magnetic field (H), the magnetic domains within the material align with the field.

• When the field is removed, not all domains return to their original random orientation, resulting in a residual magnetic flux density.

• To reduce the magnetic flux density to zero, a reverse magnetizing field is required.

• The energy expended in overcoming this opposition to the changing magnetization is the origin of the hysteresis loss.

• If you plot B against H over a cycle of magnetization and demagnetization, the resulting curve forms a closed loop called a hysteresis loop.

The Energy Loss: Each time the magnetic material goes through a complete cycle of magnetization and demagnetization (traces out the entire hysteresis loop), energy is lost in the form of heat. This energy is proportional to the area of the hysteresis loop. The larger the area of the loop, the greater the energy loss per cycle.

Desirable Material Properties: For transformer cores and other applications where minimizing losses is important, materials with high magnetic permeability is preferred to achieve strong magnetic fields with small currents, since $B_{ext}={\mu }_0\text{H}$ therefore higher the ${\mu }_0$ higher $B_{ext}$.

Eddy current loss

It's another form of energy loss that occurs in conductive materials when they are subjected to time-varying magnetic fields. These losses occur as heat${}^{[2]}$ within the material.

Induction of Eddy Currents: According to Faraday's law of electromagnetic induction, a relative motion between magnetic flux and a set of conductor(s) induces an electromotive force (EMF) the conductor(s). In a bulk conductive material (like the core of a transformer) subjected to a time-varying magnetic field, these induced EMFs create circulating currents within the material. These circulating currents are called eddy currents.

Direction of Eddy Currents and opposition: The direction of these eddy currents is governed by Lenz's law which states that the magnetic field produced by them opposes${}^{[3]}$ the change in the original magnetic flux that induced them.

Factors affecting Eddy Currents: The magnitude of the induced eddy currents, and hence the eddy current loss, is dependent on:

• The frequency of the time-varying magnetic field. Higher frequencies lead to larger induced EMFs and thus larger eddy currents.
• The conductivity ($\sigma$) of the material. Higher the conductivity, the larger eddy currents induced.
• The strength of the magnetic field.
• The geometry and size of the conductive material. A larger surface area allows for more eddy currents to form, increasing the overall loss.

Minimizing Eddy Current Loss: In devices like transformers and inductors, eddy current losses are undesirable as they reduce efficiency due to heating. Several techniques are used to minimize these losses:

• Using materials with high magnetic permeability and low electric conductivity, such as ferrites. A higher permeability core requires smaller magnetizing current to establish the same flux density, reducing energy wasted as heat. A lower conductivity results in higher resistance to the flow of eddy currents, thus reducing the loss.
• Laminating the magnetic core: Instead of using a solid core, the core is built from laminated thin sheets of the magnetic material coated with an insulating varnish or oxide layer. The lamination confines the eddy currents to smaller loops within each thin layer, thereby reducing the surface area and significantly reducing the energy loss. The insulation between the layers breaks the conductive paths, preventing eddy currents from flowing across them.

Footnotes

1. Ferromagnetic material refers to a material that exhibits strong magnetism in the presence of an external magnetic field and retains its magnetization even after the external field is removed. This phenomenon occurs due to the alignment of magnetic moments of atoms in the material in the same direction, creating a strong net magnetic effect.

2. Reason why eddy current loss occurs as heat : All conductive materials have some level of electrical resistance. When eddy currents flow through the electrical resistance of the material, they encounter opposition. This opposition causes energy to be dissipated from the kinetic energy of the moving electrons and transferred to the atoms of the conductor, increasing their vibrational energy. This increase in the vibrational energy of the atoms results in an increase in temperature of the material, which is what we perceive as heat energy.

3. Reason why Lenz's law opposes the change in the magnetic flux : Lenz's law states that the induced current will flow in a direction such that its own magnetic field opposes the change in the magnetic flux that produced it.

• The opposition is necessary to maintain the principle of conservation of energy. Consider what would happen if the induced magnetic field aided the change in the original magnetic flux:

  • A small change in flux would induce a current.
  • This current would create a magnetic field that reinforces the original change.
  • The increased magnetic field would then induce an even larger current.
  • This process would escalate, creating energy seemingly out of nowhere (a violation of conservation of energy).

• By opposing the change, the induced EMF and the resulting eddy currents act to maintain the existing magnetic state. They resist the increase if the magnetic flux is increasing and try to compensate for the decrease if the magnetic flux is decreasing. If the induced effects were to support the change, it would lead to a perpetual motion like scenario, generating energy without an external input, which is impossible.