Development of Ferromagnetic Metallic Glasses into Low Loss Power Transformer Cores

Citation

Floyd, Michael Cameron Dawley (2018) Development of Ferromagnetic Metallic Glasses into Low Loss Power Transformer Cores. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z9QF8R27. https://resolver.caltech.edu/CaltechTHESIS:10262017-091901955

Abstract

Currently, 3% of energy losses in the U.S. electrical grid occur at power transformers. With a transition to Metglas, transformer efficiency could be increased, but is Metglas the best replacement material for power transformers?

With this in mind we develop a Fe-based metallic glass for its glass forming ability and soft magnetic properties. During this development we identify a redox reaction of boron oxide by Si during melt fluxing of the Fe-based glass, which promotes an unexpected exchange of Si and B in the alloy. Taking this reaction into account, a unique optimization strategy is implemented, enabling oxide purification of the melt coupled with a significant but predictable shift in composition. This leads to an optimized Fe-based glass demonstrating a global peak in glass forming ability. Following boron oxide fluxing in the high temperature melt, alloy with composition Fe_57.5Co_20.2Si_10.2B_2.05P_10.05 transforms to Fe₅₇Co_19.2Si_6.8B_7.4P_9.6, and increases its critical rod diameter from 1 mm to 5 mm. The alloy also demonstrates excellent soft ferromagnetic performance characterized by a magnetic saturation of 1.53 T.

While developing the above alloy, we also analyzed the effect of varying thickness of a Fe₆₈Mo₄Ni₃Co₅Si₁P_11.5C₅B_2.5 transformer core as a function of frequency to discover if there was a minimum in the losses. We did not find a single minimum, but found that the optimal thickness exhibits a logarithmic dependency on frequency. This dependence suggests the optimal thickness of a core ranges from 100−400μm, instead of in the < 50μm range currently used. These larger optimal thicknesses are unexpected if anomalous losses are not considered, but the dominance of the anomalous losses at low frequencies, or for thin samples, validates the need for thicker power transformers. While other amorphous metals and casting techniques will yield varying results, the logarithmic dependence on frequency and the 100−400μm optimal thickness range should be broadly applicable.

Thesis Files