When a Small Component Causes a Big Failure

Some failures announce themselves loudly. Others hide in plain sight, quietly developing until one day a machine that looked perfectly healthy suddenly isn’t. When a 2.3 MW wind turbine generator failed at a UK offshore wind farm operated by a leading renewable energy operator, it fell firmly into the second category. 

There were no warning alarms. No rising temperatures. No unusual vibration trends or electrical anomalies in the SCADA data. The generator had been operating normally for years, right up until the moment it failed. For a renewable energy operator, the immediate question was simple: replace it, or understand it? They chose the harder option—and asked Rubix to carry out a full root cause analysis.

What followed was several days of intense investigation at the Rubix Aberdeen facility, with the energy operators' engineers embedded in the process from start to finish. It was full-on and technically demanding. But it was exactly what the situation required.

Opening the machine – and opening minds

Once the generator was dismantled and the rotor carefully removed, the nature of the failure began to reveal itself. Deep within the stator, at a single slot position, a magnetic wedge had failed. Around 100 mm of that wedge was missing—later recovered from the opposite end of the casing, having been struck by the rotor and carried through the air gap. 

That missing wedge had been doing a critical job. Magnetic wedges lock the stator coils tightly into the core slots, preventing movement under electromagnetic forces. With the wedge gone, the coil beneath was no longer fully restrained. Over time, it was free to flex, vibrate and rub against the slot walls. Eventually, the insulation system could no longer cope. The coil flashed to earth, resulting in a localised but catastrophic stator winding failure.

What was striking was how isolated the damage was. The remainder of the stator windings, wedges and end-winding bracing were all in good condition. There were no signs of widespread ageing, thermal degradation or contamination. This was not a machine at the end of its life—it was a machine taken out by a very specific mechanical failure.

The detail that made the difference

As the investigation deepened, attention turned to the failed wedge itself. A closer inspection revealed a felt packer between the top of the coil and the magnetic wedge, installed during manufacture to compensate for a dimensional gap. In theory, the vacuum pressure impregnation (VPI) process should flood this area with resin, locking everything together into a solid structure. 

In practice, it hadn’t. The felt had distorted, creating voids beneath the wedge. Resin had been unable to fully penetrate and cure in these pockets, leaving localised weaknesses in what should have been a rigid insulation system. Over the years of operation, vibration and thermal cycling gradually exploited those voids. The wedge loosened, lifted into the air gap, and was eventually struck by the rotor.

At this point, the failure stopped being a mystery. Rubix engineers recognised the mechanism immediately—and crucially, recognised it as a known issue. 

The findings aligned closely with a technical note issued in 2020 by the OEM, detailing an upgraded insulation system and a slot geometry re-design introduced specifically to address limitations in earlier generator builds. The failed generator pre-dated that redesign.

Before Rubix had even opened the machine, we had already identified the failure mode—and independently proposed the same technical solution.

Repairing smarter, not just replacing 

With the root cause established, the conversation shifted from diagnosis to decision-making. A brand-new replacement generator would cost close to £200,000. Rubix proposed an alternative: rewind and upgrade the existing stator, removing the legacy wedge configuration and rebuilding the machine to a higher insulation class, Class H1.

The difference is more than just a letter on a spec sheet: a Class H winding system uses advanced insulating materials and resins designed to tolerate higher thermal stress, giving better heat endurance, greater resilience against electrical and magnetic stresses, and significantly longer effective service life compared with lower classes of insulation. 

Class H validates a thermal endurance of around 180 °C under continuous operation, significantly broadening operating margins compared to traditional systems and helping protect against degradation over time.

That upgrade also eliminated the legacy wedge design that led to the original failure and improved heat dissipation — meaning the rewound generator is not just as good as new, but designed to outperform a like-for-like replacement over its service life. And this was not a one-off decision. The wind farm operates 26 similar turbines. One failure, properly understood, had suddenly become a roadmap for preventing many more. 

Finding value beyond the failure

The stator failure may have triggered the investigation, but it didn’t define its limits. While the generator was dismantled, Rubix engineers identified several additional findings—none of which caused the failure, but all of which affected long-term reliability.

Excess grease had migrated into the air cooler, reducing thermal efficiency. Grease contamination had compromised the shaft earthing system, increasing the risk of bearing damage. Lubrication practices were found to be inconsistent with the actual requirements of the bearings, something Rubix was able to quantify precisely using grease and oil analysis data.

Each issue was fed back to the energy operator as an opportunity: small changes, backed by evidence, that could improve reliability and extend service intervals across the wind farm. 

Circularity, in the real world

As the generator now undergoes its upgraded rewind at the Rubix Chesterfield facility, the wider significance of the project becomes clear. This was never just a repair job—and it was certainly not a quick fix. Instead, it stands as a practical example of circularity done properly: engineering-led, evidence-based, and focused on long-term value rather than short-term replacement.

A key reason this approach was possible is Rubix’s unique UK-based capability to manufacture its own coils and perform full stator rewinds in-house. Very few workshops in the UK still retain the specialist skills, facilities and experience required to rewind and re-insulate large electrical machines to advanced insulation classes. It is a capability built over decades of hands-on engineering and allows Rubix to control every aspect of the process—from material selection and winding geometry to impregnation methods—rather than relying on off-the-shelf solutions.

That level of control makes a tangible difference. By repairing and upgrading the existing generator rather than replacing it, the renewable energy operator avoided unnecessary material consumption, transport emissions, and the environmental impact associated with manufacturing a new unit. More importantly, by extending the life of a critical asset—and applying those lessons across an entire fleet—the project reduced both cost and carbon without compromising performance or reliability. 

In an industry under increasing pressure to deliver more energy with fewer resources, this kind of engineering-led circularity truly matters. It demonstrates that sustainability is not only about deploying new technology, but about using existing assets more intelligently, more efficiently and for longer.

For the renewable energy operator, the outcome was confidence, clarity and a robust strategy for the future. For Rubix, it reinforced a simple truth: real value doesn’t come from replacing what’s broken, but from understanding why it broke—and making sure it never does again.

For more information on Power Transmission or if you’d like to find out how you could benefit from the same standardisation, contact your local Service Centre, who will be happy to discuss your options.
 

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