Inside a transformer's iron core

Paul Scherrer Institute (PSI) researchers have developed a method of looking inside iron core transformers, enabling them to see the tiny magnetic structures inside a transformer at work during the transforming process.

The researchers say this is a significant step towards understanding how transformers work and the development of more efficient transformers in the future.

“The transformer’s ring-shaped magnetic iron core is a fundamental element necessary for voltage increase or decrease,” explained Christian Grünzweig, who led the research. The tiny magnetic domains within the core play an essential role in this process. The magnetic orientation within each domain is uniform. Experts refer to the boundaries between these as domain walls. If the iron core is magnetised, this results, at a microscopic level, in all domains pointing the same way. In other words, the domain walls disappear.

“The decisive factor for an efficiently functioning transformer is domain-wall mobility,” said Benedikt Betz, the first author of both studies and doctoral student in Grünzweig’s team. This is because our power lines carry alternating current with a frequency of 50 Hertz. As a result, a transformer’s iron core is re-magnetised 100 times per second, being re-poled from north to south and vice versa in rapid succession. The domains are hence thrown backwards and forwards: the greater their flexibility, the better the transformer performs.

The methods available so far have only allowed indirect observation of domain-wall behaviour. The neutron grating interferometry developed by Christian Grünzweig at the PSI 10 years ago within the framework of his doctoral thesis now permits direct imaging of the domain walls.

“You can think of the domains as garden plots, separated from each other by fences,” said Grünzweig.

“Using neutron grating interferometry, we are now able to see these fences — meaning the domain walls, not the garden plots themselves.” In the scientist’s images obtained by neutron grating interferometry, the domain walls appear as black lines.

Grünzweig’s team has investigated what happens when a transformer is connected to a direct current, which is first increased and subsequently decreased again. As the voltage increased, the black lines disappeared, showing that the iron core was uniformly magnetised in one direction. Only in this state does the iron core transfer voltage efficiently. Once the voltage was subsequently reduced, the lines — and the domain walls they represent — reappeared. This first study provided the basis for further investigations.

In a second study, the researchers mimicked a more realistic scenario by applying an alternating current. When varying the voltage and the frequency of the alternating current, they found that there were certain thresholds of each of these parameters, beyond which domain walls either disappeared or appeared to freeze.

“These insights do not lead directly to better transformers,” Christian Grünzweig admitted. “What we are doing is offering science and industry a new examination method.”

The results of the two studies were published in the specialist journal Physical Review Applied.