Transformers are everywhere in the electrical world. They raise voltage for long distance transmission and lower it so we can use electricity safely in homes, schools, and workplaces. At the center of every transformer is the core. Far from being a simple block of metal, the core is a carefully engineered path for magnetic flux that strongly influences efficiency, temperature rise, size, and cost. This article introduces how cores are built, why hysteresis and eddy currents cause losses, and what magnetizing current is and why it matters.
Core construction: limbs, yokes, and design
The transformer core provides the magnetic circuit that allows energy to transfer from one winding to another. Limbs, sometimes called legs, are the vertical portions where the windings are placed. When current flows in a winding, it produces magnetic flux that travels through the limb. In single phase units you typically see two limbs, while three phase units commonly use three limbs, one for each phase. Yokes are the horizontal sections that connect the limbs at the top and bottom, completing the magnetic circuit so the flux has a continuous path. If the limbs are the highways carrying flux, the yokes are the bridges that link them, and both are sized so flux flows without bottlenecks.
Cores are arranged in two broad styles. In core type transformers the windings are wrapped around exposed limbs and the flux travels up a limb, across a yoke, and down another limb. This approach is straightforward to build, cools well, and is easy to service, which is why it is common in distribution equipment. In shell type transformers the core surrounds the windings, splitting the flux so it flows around both sides of the coil. This layout supports interleaved, or sandwich, windings that reduce leakage reactance, a benefit in certain specialty applications such as furnace transformers.
Designers also choose among different limb arrangements to meet size and transport constraints. A single phase two limb core is simple and familiar. A three phase three limb core is compact and cost effective for many ratings. Very large units may use a five limb core to reduce height for shipping while providing additional magnetic return paths. Whatever the arrangement, the core is never a solid block. It is built from thin laminated sheets of grain oriented silicon steel, each one insulated from the next. Laminations limit circulating currents inside the steel and grain orientation steers flux along an easier path, both of which lower loss and heat. Practical tradeoffs enter every design decision. A single three phase core is generally cheaper than three separate single phase transformers, but using separate units can simplify replacement if one fails. Extremely large cores may be fabricated as sections and assembled on site to meet road or rail limits. Engineers sometimes increase the yoke cross section slightly to trim no load losses, accepting added weight to gain a small efficiency improvement.
Hysteresis: magnetic lag and heat
Core steel does not flip its internal magnetic alignment instantly when the applied field reverses. It lags, and that lag is called hysteresis. A useful way to picture it is to imagine a shopping cart with a sticky wheel: you put in extra effort to get it moving and again to stop it. Inside the steel, countless tiny magnetic domains behave similarly. Each reversal of the alternating field forces the domains to reorient, and the work required shows up as heat. If you plot flux density against magnetizing force you see a loop rather than a single line. The area inside that loop represents the energy lost each cycle. Narrower loops mean less loss, which is why grain oriented silicon steel, and premium grades such as Hi-B and laser scribed variants, are favored for transformer cores. Over time the heat from hysteresis contributes to warm running and audible humming, and although each cycle’s loss is small, it accumulates continuously whenever the transformer is energized.
Why It Causes Losses
Each reversal of the AC field works the domains back and forth. That work becomes heat in the core and contributes to warm running and audible hum.
Reducing Hysteresis Loss
- Grain oriented silicon steel (CRGO) aligns grains to ease domain motion.
- Hi B and laser scribed steels further narrow the loop and lower loss.
Key takeaway: hysteresis is magnetic lag. Less lag means less heat and better efficiency.
Eddy currents: circulating currents in the core
A changing magnetic field induces voltage not only in copper windings but also in the conductive steel core itself. That induced voltage drives small circulating paths of current within the metal called eddy currents. Like whirlpools in a river, these currents loop inside the material and do no useful work, but they do generate heat. If they grow large, they can create hot spots that stress insulation and shorten equipment life. Laminations are the primary defense. By slicing the core into thin, insulated sheets, designers interrupt the area available for current loops, keeping each loop small and weak. Thinner laminations reduce losses further by shrinking the possible loop size. The principle shows up in other fields as well. Some roller coasters and high speed trains use eddy current brakes that slow a moving metal fin as it passes near a magnet. In a transformer, the same physics would merely waste energy, so the lamination stack is engineered to suppress it.
Eddy Currents: Loops of Wasted Energy
A changing magnetic field induces voltage not only in copper windings but also in the steel core. That voltage drives small circular currents inside the metal called eddy currents.
Why They Are a Problem
- They create heat.
- They reduce efficiency.
- They can cause hot spots that damage insulation.
How Engineers Limit Them
- Use thin, insulated laminations.
- Thin sheets break up current loops and keep them small and weak.
- Each sheet’s coating prevents easy current flow between laminations.
Magnetizing current: the cost of keeping flux alive
Even when a transformer supplies no external load, it still draws a small input current known as magnetizing current. Energy is required to establish and maintain alternating flux in the core, to overcome the natural reluctance of the magnetic path, and to offset losses from hysteresis and eddy currents. In most power transformers the magnetizing current is a small fraction of the rated current, commonly on the order of a few tenths of a percent up to a couple of percent. Although small, this current tells a story about core condition and design. Because the relationship between flux density and magnetizing force is nonlinear, the excitation current is not a perfect sine wave. It contains harmonics, with the third often being most prominent in common core steels. On an oscilloscope the current trace looks peaky rather than smooth. Test technicians pay attention to this value during no load or excitation testing. A magnetizing current that is significantly higher than expected can point to problems such as partial saturation from excessive flux density, shorted turns, or unintended air gaps. Designers control the level by choosing adequate core cross section, using step lap joints to minimize gaps at mitered corners, and selecting high grade steels that reduce both hysteresis and eddy losses.
What Magnetizing Current Does
- Sets up the magnetic field in the core.
- Overcomes the core’s reluctance.
- Offsets hysteresis and eddy current losses.
Why these details matter
Core design separates efficient, reliable transformers from units that run hot and waste energy. Losses from hysteresis, eddy currents, and magnetizing current may seem minor in isolation, but a transformer operates around the clock for decades, so small improvements in materials and geometry compound into meaningful savings and longer life. Choices such as lamination thickness, grain orientation, joint geometry, limb arrangement, and yoke sizing are made not only for electrical performance but also for manufacturability and transportation. For someone entering the electrical field, understanding these fundamentals builds intuition. When you look at a nameplate value for no load losses, consider that it is tied to the hysteresis loop of the steel. When you see a stack of thin laminations, recognize that they are there to choke off eddy currents. When you measure excitation current during a test, remember you are gauging the energy needed to keep the magnetic circuit alive.
Final thoughts
Transformers are more than boxes that change voltage. Inside, the core guides magnetic flux and sets the baseline for how efficiently the machine will operate. Cores guide the magnetic path, hysteresis is magnetic lag that turns into heat, eddy currents are circulating currents inside the steel that also become heat, and magnetizing current is the small but essential current that sustains the magnetic field even with no load attached. Mastering these concepts early makes advanced topics such as leakage reactance, regulation, and loss evaluation far easier to grasp later on.
