Railway Traction Transformer Laminations: The Hidden Story of Mechanical Robustness

If you stand close to an electric train and listen, you’ll hear a low hum under the noise of doors and announcements. Behind that hum is a traction transformer working very, very hard – and at the heart of that transformer are stacks of thin steel sheets called laminations.

In railway traction duty, those laminations live a rough life: constant vibration, shocks from track joints, violent short-circuit forces, thermal cycling from heavy load patterns, and sometimes brutal climates. Traction transformer brochures talk a lot about efficiency and cooling, but the mechanical robustness of the lamination pack is just as critical for long-term reliability.

• In this article, we’ll walk through:
  • Why traction environments are so punishing for laminations
  • How lamination packs actually fail in the real world
  • The design levers that make a core mechanically tough (not just efficient)
  • How standards and tests tie into lamination robustness
  • What specifiers should actually ask transformer suppliers about laminations

1. Life on board: what traction transformers go through

Railway traction transformers aren’t sitting on a concrete pad in a calm substation. They’re either bolted to a bogie or chassis (on-board types) or sitting near the track (fixed installations) with frequent short circuits and current shocks.

On-board traction transformers must survive:

• Continuous mechanical vibration and random shocks per EN 61373
• Load cycles that swing hard with acceleration/braking
• Temperature changes from sub-zero depots to hot tunnels
• Contamination, moisture, and sometimes salt or dust

Fixed traction transformers under EN 50329 see fewer shocks, but they face frequent short-circuits and current shocks on the catenary or feeder lines.

• Major mechanical stress sources for laminations include:
  • Short-circuit forces causing high radial and axial stresses in windings and transmitted through the core and clamps
  • Magnetostriction, where the steel physically strains with changing flux and shakes the core
  • Vibration from wheel/rail interaction, transmitted structurally to the transformer frame and core
  • Thermal cycles that expand and contract steel and clamping structures
  • Handling, shock and transport loads before the transformer even sees a rail network

The big takeaway: in traction service, the lamination pack is constantly being “shaken, squeezed and heated.” If its mechanical design is weak, problems don’t show up in year one – they show up when the fleet is in service and outages hurt.

2. The lamination pack: more than just stacked steel

A transformer core is a carefully stacked 3D puzzle of grain-oriented electrical steel sheets, typically 0.23–0.35 mm thick, insulated and built into a frame. Properly designed, the lamination pack does three things at once:

1. Provide a low-loss magnetic path (the textbook reason for laminations).
2. Break up eddy currents, reducing core loss and heating.
3. Behave as a mechanically unified structure that can withstand vibration and fault forces without loosening or cracking.

That third role is the one that often gets under-explained. In traction service, you are essentially asking a stack of thin sheets, separated by insulation, to behave like one robust, damped mechanical body for 30–40 years.

• Laminations contribute to mechanical robustness by:
  • Creating many friction interfaces that help damp vibration
  • Allowing controlled flexibility so the core can “breathe” under magnetostriction without cracking frames
  • Working with clamping and yoke structures to share short-circuit loads
  • Providing a stable, flat support for windings and structural members when properly machined and stacked

If the lamination system is badly designed or assembled, the transformer may still pass type tests – but humming grows, bolts loosen, and in worst cases you start to see insulation wear and internal damage years later.

3. How lamination packs actually fail

Mechanical failure of laminations rarely looks like a dramatic fracture. Instead, it’s usually a slow, noisy story of loosening, rubbing and shifting under repeated stress.

Over time, small changes add up: varnish cracks, burrs bite, clamp pressure relaxes. What started as a perfectly tight lamination stack becomes a slightly rattling one, and every vibration cycle and short-circuit event makes it worse.

• Typical failure and degradation modes:
  • Loosening of clamps and yokes → core starts to “buzz” louder, vibration amplitudes rise
  • Fretting and wear at lamination edges, especially where burrs or misalignment concentrate stress
  • Delamination or flaking of insulation coating, reducing inter-laminar friction and altering eddy current paths
  • Buckling or local deformation of laminations near corners, joints or under tie-bars after major faults
  • Corrosion in damp environments, especially at lamination edges and bolt holes, making packs looser and noisier over time
  • Noise and vibration growth, often the first field-visible symptom of deeper mechanical issues

By the time you see serious performance issues, the core has typically gone through thousands or millions of micro-slips between laminations.

4. Design levers for mechanically robust laminations

The good news: lamination robustness isn’t magic. It’s the cumulative effect of a dozen design and manufacturing decisions that can be controlled, measured and specified.

From the perspective of mechanical robustness in traction duty, you can think of the lamination system as a tuned mechanical component, not just a magnetic one. That mindset shift alone makes engineers ask better questions about material, geometry and clamping.

• Key design choices that strongly influence mechanical robustness:
  • Steel grade and thickness
    • Thinner GO steel (e.g. 0.23 mm) can reduce magnetostriction-driven vibration; thicker sheets are stiffer but may be noisier.
  • Insulation coating and surface finish
    • Controls friction between sheets and helps damp vibration; good coating resists cracking and corrosion in railway climates.
  • Lamination geometry and joints
    • Step-lap or mitered joints can spread flux and force more evenly, reducing hot-spots of magnetostriction and mechanical stress.
  • Stacking accuracy and burr control
    • Poorly controlled burrs and misalignments act like miniature chisels under vibration, promoting fretting and noise.
  • Clamping system design (frames, tie-rods, yoke bolts)
    • Needs enough pre-stress to keep packs tight under short-circuit loads – but not so much that insulation is crushed or steel is overstressed.
  • Bonding and impregnation strategy
    • Varnish or resin impregnation can create a more unified and damped structure, especially for dry-type or cast-resin traction transformers.

The art is in balancing all of this with electrical performance, weight, and cost — especially on board where space and mass limits are ruthless.

5. On-board vs fixed: different worlds for the same steel

A lamination pack in a transformer hung under a high-speed EMU has a very different day-to-day experience than one in a concrete-housed trackside transformer. They’re governed by overlapping but not identical standards (EN 60310 for on-board traction transformers, EN 61373 for shock & vibration, EN 50329 for fixed traction transformers, plus IEC 60076-5 for short-circuit withstand).

Understanding those differences helps you target the right lamination design choices.

The core idea: same physics, different emphases. If your fleet is mostly on-board traction units, you almost want to think like an NVH (noise, vibration, harshness) engineer in the automotive world – just at much higher power levels.

6. Standards, tests and how they touch laminations

Standards rarely say “do your laminations like this,” but they describe stresses and test regimes that laminations must survive as part of the whole transformer.

EN 60310 sets out performance, safety and test methods for traction transformers installed on trains, including requirements that indirectly force robust mechanical design (thermal cycling, overloads, dielectric performance under vibration, etc).

EN 61373 defines shock and vibration test profiles for railway equipment – which on-board transformers must pass as complete assemblies. EN 50329 (fixed traction transformers) explicitly notes that these units are subject to frequent short circuits and current shocks, linking back to IEC 60076-5 for short-circuit withstand.

• For lamination robustness, the most relevant tests and methods are:
  • Short-circuit withstand tests (IEC 60076-5) – real-world verification that the core, windings and clamps survive forces equivalent to system faults.
  • Shock and vibration tests (EN 61373) – show that no mechanical damage or functional degradation occurs after imposed vibration/shock profiles.
  • Noise and vibration measurements – used increasingly to verify that mechanical design (including laminations) keeps emissions below project limits and to track condition over life.
  • SFRA (Sweep Frequency Response Analysis) – detects mechanical changes in windings and core by changes in frequency response, often used to compare “fingerprints” over time.

A transformer that barely passes a short-circuit test or vibration test is not the same as one that comfortably clears the bar with mechanical margins. Robust lamination design is part of building in that margin.

7. Condition monitoring: listening to the laminations

In traction, downtime is expensive and access to equipment can be awkward. That’s why there’s growing interest in using vibration and acoustic signatures to catch problems early, particularly in traction transformers.

Core vibration is now recognized as the major contributor to transformer noise, especially for power and traction units. Magnetostriction of grain-oriented steel drives much of this vibration, and changes in lamination tightness, clamping pressure or material condition show up as distinct changes in the vibration spectrum.

• Signs that your lamination pack is mechanically “unhappy”:
  • Noticeable increase in audible hum without a matching change in loading
  • New higher-frequency buzz components or tonal peaks in vibration measurements
  • Changes in SFRA curves suggesting internal mechanical shifts
  • Core hot-spots or abnormal temperature gradients on thermal imaging
  • Evidence of corrosion or loosened clamp hardware during inspection

Newer research even explores using dual-attention neural networks on vibration data to identify inter-turn faults in traction transformers at an early stage – the same data streams can help detect lamination-related issues as well.

The practical point: if you’re already collecting vibration data for condition monitoring, use it to track lamination health as well. It’s one of the earliest windows into mechanical degradation.

8. Turning this into a spec: questions to ask your supplier

If you’re a rolling stock OEM, infrastructure owner or procurement engineer, you rarely get to dig into lamination design details – but you can ask smarter questions that push suppliers toward more mechanically robust solutions.

Think of it as taking lamination robustness from “implicit” to “explicit” in your technical specifications and design reviews.

• Practical questions and requirements you can include:
  • Material and thickness
    • “Which grades of GO steel and thicknesses are used, and how were they selected with respect to vibration and magnetostriction?”
  • Lamination edge quality and burr limits
    • “What are the maximum burr heights and how are they controlled and inspected?”
  • Clamping philosophy
    • “How is clamping pre-stress calculated for short-circuit withstand, and how is relaxation over life accounted for?”
  • Vibration analysis
    • “Have FEM-based vibration analyses been performed on the core and tank; what natural frequencies were identified relative to traction harmonics?”
  • Environmental and corrosion measures
    • “What coating systems and sealing measures are used to protect lamination edges and clamps in the specified climate and pollution class?”
  • Test evidence
    • “Can you provide recent short-circuit test and vibration test reports for comparable designs, and how do they relate to our rating?”

Suppliers who have really thought through lamination robustness can answer these questions clearly and consistently. If the answers are vague or purely marketing-oriented, that’s a red flag.

9. Looking ahead: smarter laminations for smarter railways

Rail networks are getting more heavily loaded, with higher speeds, more acceleration, and more power electronics in the loop. That means more harmonic content, more dynamic loads – and more stress on traction transformers and their cores.

Research on transformer magnetostriction, vibration, and noise is moving fast, including improved grain-oriented steels, hybrid core structures and advanced simulation methods for predicting vibration from the microstructure level up to the full transformer.

We’re likely to see:

• Lamination steels optimized not only for loss but also for low magnetostriction and better vibration behavior
• More widespread use of resin-bonded or partially bonded core structures in traction duty
• Standardization of vibration performance metrics for traction transformers alongside noise and efficiency
• Deeper integration of machine-learning-based vibration diagnostics into fleet monitoring systems

For now, though, one simple principle holds:

If you treat lamination robustness as a first-class design goal – not an afterthought – your traction transformers will hum quietly, ride smoothly, and stay in service longer than the timetable expects.

And somewhere on a platform, a passenger will still just hear a quiet hum and assume everything “just works” – because you did the hard work on the hidden steel sheets that make it so.