Hysteresis & Eddy Current Losses Explained: Causes, Impact, and Minimization

Every design engineer grapples with the critical challenge of maximizing motor and transformer efficiency while keeping a close eye on production costs and thermal management. If you’ve ever found yourself weighing the trade-offs between different core materials or wondering why a perfectly designed device is still getting hot and wasting power, you’re in the right place. The culprits are often two invisible energy thieves: hysteresis and eddy current losses.

These phenomena, collectively known as “core losses” or “iron losses,” are fundamental hurdles in electromagnetic design. They are the unavoidable price of manipulating magnetic fields to generate motion and transfer power. Understanding them isn’t just an academic exercise; it’s essential for creating efficient, reliable, and cost-effective electrical machines. This guide will break down these complex topics into clear, actionable engineering principles to empower you to make more informed design decisions.

In This Article

• Understanding Core Losses: The big picture of energy loss in magnetic systems.
• Hysteresis Loss: A deep dive into the magnetic “memory” effect.
• Eddy Current Losses: Uncovering the unwanted electrical “whirlpools” in your core.
• Key Differences and Similarities: A head-to-head comparison.
• The Real-World Impact: Why these losses matter for performance and your bottom line.
• Minimization Techniques: A guide to selecting the right materials and design strategies.
• Practical Applications: Where these losses have the biggest impact.
• Your Engineering Takeaway: Key points to remember for your next project.

Understanding Core Losses in Electrical Systems

Before we dissect the two main types of loss, let’s zoom out. Whenever you use an alternating current (AC) to create a fluctuating magnetic field in a material—the very principle behind transformers, motors, and inductors—you will inevitably lose some energy. This energy doesn’t contribute to the useful work of the device. Instead, it converts directly into heat within the magnetic core.

This wasted energy is what we call core loss. It’s a critical factor because it directly dictates the efficiency, operating temperature, and ultimately, the lifespan of your component. Core loss is primarily the sum of two distinct phenomena: hysteresis loss and eddy current loss. While they both stem from the changing magnetic field, their physical origins are quite different. Getting a handle on both is the first step toward controlling them.

Hysteresis Loss: The Magnetic “Memory” Effect

What is Hysteresis Loss?

Imagine trying to rapidly bend a metal paperclip back and forth. You’ll notice it gets warm at the bend. Why? You’re expending energy to overcome the internal friction and restructure the metal’s grain. Hysteresis loss is conceptually similar; it’s a form of “magnetic friction.”

It is the energy consumed to repeatedly reorient the magnetic domains within a core material as the magnetic field rapidly switches direction. This energy is lost as heat in every single AC cycle.

The Physical Mechanism: A World of Tiny Magnets

To understand hysteresis, you need to think of a magnetic material like iron as being composed of countless microscopic regions called magnetic domains. You can picture each domain as a tiny, powerful bar magnet with its own north and south pole.

Visualizing Hysteresis with the B-H Curve

Engineers visualize this process using a B-H curve, or a hysteresis loop.

• H (Magnetic Field Strength): This is the “effort” you apply with your electrical coil.
• B (Flux Density): This is the “result”—how strongly the material becomes magnetized.

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Two key terms from the B-H loop are retentivity (the residual magnetism left after the external field is removed) and coercivity (the amount of reverse field needed to bring the magnetism back to zero). For low hysteresis loss, you want a material with low coercivity.

Factors Affecting Hysteresis Loss

Hysteresis loss isn’t constant. It’s influenced by:

• Material Properties: The intrinsic coercivity of the material is the single most important factor.
• Peak Flux Density (Bmax): The stronger you magnetize the material in each cycle, the higher the loss. The loss is roughly proportional to Bmax raised to a power (typically 1.6 to 2), a relationship defined by the Steinmetz equation.
• Frequency (f): Since the loss occurs in every cycle, the more cycles per second, the greater the total power loss. Hysteresis loss is directly proportional to frequency.

Eddy Current Losses: Induced Currents Fighting the Flux

What are Eddy Current Losses?

Now let’s switch gears from magnetic friction to an electrical phenomenon. If hysteresis loss is like a paperclip getting warm from bending, eddy current loss is like the heat generated in an induction cooktop.

Eddy current loss is the energy lost due to circulating currents induced within the body of a conductive magnetic core. These currents are a direct consequence of the changing magnetic field and serve no useful purpose, only generating heat through a process called Joule heating.

The Physical Mechanism: Faraday’s Law at Work

The principle here was famously described by Michael Faraday. Faraday’s Law of Electromagnetic Induction states that any change in the magnetic field through a conductive loop will induce a voltage (electromotive force or EMF) in that loop.

Factors Affecting Eddy Current Loss

Eddy current loss is highly sensitive to a different set of parameters than hysteresis loss:

• Frequency (f): This is the biggest driver. Because the induced voltage is proportional to the rate of change of the magnetic field, doubling the frequency doubles the voltage. The resulting power loss (proportional to voltage squared) quadruples. Eddy current loss is proportional to f². This makes it a massive problem in high-frequency power electronics.
• Peak Flux Density (Bmax): A stronger magnetic field also induces more voltage. Eddy current loss is proportional to Bmax².
• Conductivity (σ) / Resistivity (ρ): A material with high conductivity (low resistivity) will allow large eddy currents to flow for a given induced voltage. Therefore, you want materials with high electrical resistivity to minimize these losses.
• Thickness (t): This is the most critical geometric factor. A thicker conductor provides a wide, easy path for large eddy currents to circulate. The loss is proportional to the square of the thickness (t²).

Key Differences and Similarities: Hysteresis vs. Eddy Currents

Similarities:

• Both are components of total core loss.
• Both manifest as unwanted heat generation.
• Both reduce the efficiency of electromagnetic devices.
• Both are unavoidable in AC magnetic circuits but can be minimized through design.

Impact of Hysteresis and Eddy Current Losses

These losses are far from trivial. Their combined effect has significant engineering and economic consequences.

• Energy Inefficiency: Globally, core losses in transformers and motors account for a substantial portion of all electricity generated. For a distribution transformer that is energized 24/7, even a small percentage of loss adds up to a massive amount of wasted energy and money over its lifetime.
• Thermal Management Challenges: All of this lost energy becomes heat. This heat raises the operating temperature of the device, which can damage winding insulation, reduce component lifespan, and decrease performance. An old rule of thumb states that for every 10°C increase in operating temperature, the life of electrical insulation is halved. This often necessitates complex and expensive cooling systems (fans, oil, heat sinks).
• Performance Degradation: The energy used to overcome core losses is energy that isn’t available at the output. This results in lower power output for motors and voltage drops in transformers.
• Economic and Environmental Costs: Wasted energy means higher electricity bills for consumers and utilities. On a larger scale, it means more fuel must be burned at power plants, contributing to a larger carbon footprint.

Minimization Techniques: Reducing Core Losses

Fortunately, engineers have developed effective strategies to combat both types of core loss. The approach is a two-pronged attack focused on material selection and geometric design.

Reducing Hysteresis Loss: The Material Solution

Since hysteresis is an intrinsic property of a material, the primary solution is to choose the right one. The goal is to select a soft magnetic material with a very low coercivity—a “skinny” B-H loop.

• Silicon Steel: This is the workhorse of the industry. Adding a small amount of silicon (around 3%) to iron significantly reduces coercivity and increases permeability.
• Grain-Oriented Electrical Steel (GOES): The grains are aligned in a specific direction during manufacturing. It offers excellent properties but only in that one direction. It is ideal for the predictable magnetic flux paths in a transformer lamination core.
• Non-Oriented Electrical Steel (NOES): The grains are random, providing good magnetic properties in all directions. This makes it perfect for applications with rotating magnetic fields, like motors and generators.
• Amorphous Metals: These materials have a non-crystalline, glass-like structure. This eliminates the grain boundaries that pin domain walls, resulting in extremely low hysteresis losses—sometimes 70-80% lower than traditional silicon steel.
• Nanocrystalline Alloys: These advanced materials offer a combination of the high saturation flux density of silicon steel and the low losses of amorphous metals.
• Ferrites: These are ceramic magnetic materials. While they can’t handle the high flux densities of steel, they are excellent insulators, which makes them indispensable for high-frequency applications.

Reducing Eddy Current Loss: The Geometric Solution

The fight against eddy currents is primarily won through clever physical design. The goal is to obstruct the paths these currents want to take.

• Laminations: This is the single most effective technique. Instead of using a solid block of metal for the core, we construct it from a stack of very thin sheets, or laminations. Each sheet is coated with a thin insulating layer (like varnish or an oxide). This construction breaks up the large, continuous paths for eddy currents into thousands of tiny, high-resistance paths, drastically reducing the overall loss. Using thinner electrical steel laminations can reduce eddy current losses by over 95% compared to a solid core.
• High Resistivity Materials: This strategy attacks the problem from a material properties angle. As mentioned, adding silicon to iron to create silicon steel laminations not only helps with hysteresis but also significantly increases the material’s electrical resistivity. This higher resistance directly impedes the flow of eddy currents, converting less energy into heat.
• Using Ferrite Cores: For high-frequency applications (like in switch-mode power supplies), eddy currents are the dominant loss factor. Ferrites, being ceramic insulators with extremely high resistivity, are the go-to choice because they practically eliminate eddy currents altogether.

Practical Applications and Significance

Understanding and managing core losses is crucial across a wide range of electrical equipment:

• Transformers: In distribution transformers, which are always on, no-load losses (which are almost entirely core losses) are a continuous drain on the power grid. Using high-grade materials and optimized designs is essential for grid efficiency.
• Electric Motors and Generators: Core losses in the stationary and rotating components are a major source of inefficiency and heat. The performance of both the stator and rotor depends heavily on managing these losses, especially in high-performance or continuously operating motors.
• Inductors and Chokes: These components, vital in power electronics, often operate at very high frequencies where eddy current losses can become extreme if not properly managed with the right core material (like ferrites) and design.
• Renewable Energy Systems: The efficiency of wind turbine generators and solar power inverters relies on minimizing every watt of loss. Advanced magnetic materials play a key role in making these systems more effective and economically viable.

Your Engineering Takeaway

Hysteresis and eddy current losses are two sides of the same coin—the coin of core loss in AC magnetic devices. While they are often discussed together, it’s critical to remember their distinct origins and, therefore, their distinct solutions.

Here’s the summary to carry into your next design meeting:

• Core Loss is Wasted Energy: It’s the sum of hysteresis and eddy current losses, and it shows up as heat, reducing efficiency and lifespan.
• Hysteresis is Magnetic Friction: It’s caused by the energy needed to flip magnetic domains back and forth. The solution is material-based: choose soft magnetic materials like silicon steel or amorphous alloys with low coercivity (a “skinny” B-H loop).
• Eddy Currents are Electrical Whirlpools: They are unwanted currents induced in the core by the changing magnetic field. The solution is structural and material-based: use thin, insulated laminations to break up current paths and select materials with high electrical resistivity.
• Frequency is a Key Differentiator: Hysteresis loss increases linearly with frequency ($f$), but eddy current loss increases with the square of the frequency ($f^2$), making it the dominant concern in high-frequency designs.

These losses are an ever-present challenge, but they are not insurmountable. Through careful material selection, thoughtful geometric design, and a solid understanding of the underlying physics, you can effectively manage their impact. This empowers you to build more efficient, cooler-running, and longer-lasting electrical systems. If you’re tackling a specific design challenge, consulting with specialists can help you navigate the trade-offs and select the optimal core solution for your application’s unique demands.