The Evolution of Transformer Cores: From Simple Laminations to Advanced Boltless Designs
If you've ever looked at a power transformer and thought, "It's just a big metal box with coils inside," you're not alone. But here's the truth: the transformer core—the heart of every transformer—has undergone one of the most remarkable engineering evolutions you've never heard about.
And understanding this evolution isn't just academic trivia. It's the kind of practical industry knowledge that separates engineers who thrive in the power utility world from those who struggle to find their footing.
The Early Days: Simple Laminations
In the early days of transformer design, cores were built using hot-rolled, non-oriented steel. The steel had no preferential magnetic direction—meaning the magnetic flux could travel in any direction with roughly equal ease. Manufacturers stacked laminations and clamped them together using steel bolts passing through both the limbs and yokes.
It worked. But it wasn't efficient.
The problem? Hot-rolled steel had relatively high core losses. Every time the transformer was energized—which is essentially 24/7/365—energy was being wasted as heat in the core. For utilities operating thousands of transformers, those losses added up to significant operational costs.
The Grain-Oriented Revolution
Everything changed in the mid-20th century. In 1958, Allegheny Technologies developed cold-rolled grain-oriented silicon steel (CRGO).
Unlike conventional steel, grain-oriented electrical steel is specially processed to align its crystalline structure in a single direction. This alignment gives the material superior magnetic properties when flux flows along the grain direction—but it comes with a catch.
The catch? Any factor that requires the flux to deviate from the grain direction will increase core loss. This meant engineers had to completely rethink how they designed and built transformer cores.
The Challenge of Grain-Oriented Steel
Grain-oriented steel presented two immediate challenges:
First, bolt holes became a problem. Previously, manufacturers simply drilled holes through the core plates for clamping bolts. But with grain-oriented steel, these holes distorted the flux path and created unnecessary losses. The loss of effective cross-section also meant designers had to increase the diameter of the core limb unnecessarily.
Second, corners became critical. At the intersection of limbs and yokes, the flux needed to turn—which meant it had to cut across the grain direction. This turning created additional losses.
The solution? Engineers began cutting the corners of laminations on a 45° mitre. These mitred corners allowed the flux to transfer to the adjacent face rather than cross an air gap directly in its path. Core plates at these mitred corners had to be overlapped so the flux could transfer smoothly.
The Shift to Banded Limbs
As grain-oriented steel became standard, manufacturers moved away from core bolts on the limbs, replacing them with bands of either steel (with an insulated break) or glass fibre. The insulated break in the steel band prevented current flow in the band itself, and the band was insulated from the core to prevent shorting out individual laminations at their edges.
Top and bottom yokes continued to be bolted, however, since the main structural strength of the transformer is provided by the yokes together with their heavy steel yoke frames.
The Boltless Revolution
By the late 1970s, increasing economic pressures to reduce losses—particularly core loss, since it's present whenever the transformer is energised—pushed designers toward totally boltless cores.
Why go boltless? Three reasons:
Punching holes through core plates increases loss in the material by creating stress and distortion.
Bolt holes force the flux to deviate from the grain direction.
Eliminating bolt holes allows marginal reduction in core weight—every kilogram counts when you're dealing with transformers weighing hundreds of tonnes.
On large cores, achieving boltless construction requires a high degree of design sophistication to ensure structural strength isn't sacrificed. The limbs have little rigidity without clamping bolts until the windings are fitted. In some designs, the hard synthetic resin-bonded paper tube onto which the inner winding is wound provides the clamping for the leg laminations.
Temporary steel bands clamp the limbs during assembly, and these are stripped away progressively as the winding is lowered onto the leg. The top yoke—which was built in place initially to ensure precise fitting—is removed, the windings are installed, and then the top yoke is replaced, suitably interlaced into the projecting ends of the leg laminations.
Step-Lapped Joints: Pursuing the Last Percent
Much of the loss in modern transformer cores arises from the yoke-to-limb joints. Engineers have given considerable thought to the best method of making these joints.
One particularly clever innovation is the step-lapped joint. Instead of a simple overlap with only two plate configurations, a step-lapped joint might use as many as five different plate lengths. This allows the flux transfer to be gradual through the joint, ensuring a smoother transfer and thus lower corner loss.
Why isn't this used everywhere? Because more lengths of plate must be cut (increasing costs), and replacing the top yoke after winding installation becomes a more complex process requiring greater care and labour.
Interestingly, step-lapped joints have found wider application in distribution transformers. Why? Because the corner joints represent a larger proportion of the total core in a small distribution transformer than in a larger power transformer core, making such improvements more worthwhile.
The Next Frontier: Amorphous Metal Cores
Just when you thought transformer core technology had reached its peak, along came amorphous metals.
Amorphous alloy cores offer significant reductions in core losses compared to conventional grain-oriented steel cores. In fact, dry-type amorphous alloy transformer cores can reduce no-load losses by 75-80% and no-load current by 80% compared to conventional silicon steel transformers.
The regulatory momentum behind this technology is accelerating. The U.S. Department of Energy has proposed rules that would require distribution transformers to use amorphous steel cores to meet new energy efficiency standards. These standards, taking effect in 2029, will reshape the transformer market.
The global amorphous metal cores market is predicted to grow from US$980 million in 2025 to US$1,441 million by 2031. China is the world's largest market for amorphous core distribution transformers, holding about 30% share, followed by North America at 24% and Europe at 15%.
Why This Matters for Your Career
Here's the thing about all this technical evolution:Â it's not taught in university courses.
When Mike graduated from engineering school and entered the power utility industry, he discovered that the theoretical knowledge from his degree provided a foundation—but wasn't adequate for even the most basic engineering job functions. He couldn't even communicate properly with coworkers because of industry-specific lingo and practices that take years to learn on the job.
The same is true for transformer core technology. You can study electrical machines in a textbook, but you won't learn:
Why a tall, slim core has lower losses than a short, squat core of the same weight
Why building factor—the relationship between core loss of an assembled core and the product of core weight multiplied by specific loss—is typically about 1.15 for a well-designed core of grain-oriented steel
How manufacturers standardise on ranges of plate widths, usually varying in 10 mm steps, and why this affects core design
Why core earthing is critical and how inadequate bonding can create arcing that breaks down insulation and oil
Most importantly, you won't learn these things by searching the internet. Knowledge of the "how-to" isn't commonly found online, and individual teams within companies often keep knowledge to themselves.
The Opportunity
The power utility industry is experiencing unprecedented growth. The global power transformer market is projected to grow from USD 30.38 billion in 2025 to USD 41.62 billion by 2030. The transformer market overall is estimated to grow from USD 64.64 billion in 2025 to USD 88.48 billion by 2030.
This growth is driven by rising electricity consumption, large-scale investments in grid infrastructure modernisation, and the accelerated integration of renewable energy sources.
The industry needs professionals who understand not just theory, but practical, real-world engineering. Professionals who can walk into a utility company and immediately contribute—not spend years figuring out the basics.
What Mike Can Teach You
Mike has spent years working in various roles within the power utility industry. He's taught engineering concepts to the public, fellow engineers, and power line professionals. Now, he's taking everything he's learned and making it accessible.
These courses teach real-life skills that are applicable to the industry—skills that will help you land your dream job. They don't waste your valuable time with irrelevant theory. Instead, they give you the foundations that will launch your career in the power utility industry.
Mike believes that knowledge and skills in this industry should be affordable and open to all. There are no other courses out there as comprehensive and as well explained, catering specifically to the power utility industry.
Ready to transform your career? [Link to course page]
