The following is a paper by H. Aspden published by the Institution of
Electrical Engineers as Monograph No. 164M (January 1956) and later in Proc.
I.E.E. vol. 103C at pp. 279-285 (1956).
THE EDDY-CURRENT ANOMALY IN ELECTRICAL SHEET STEEL
A theory which accounts for the well-known discrepancy between eddy-current
losses in electrical sheet steels and the experimentally observed values is
presented. The anomaly is shown to be due partly to the magnetic inhomogeneity
arising from ferromagnetic domain structure and partly to a time-lag effect
caused by the finite speed of domain boundary movements. A new experimental
approach to the study of the eddy-current anomaly is described. This involves
the use of a method of measuring the anomaly factor as it applies
instantaneously at a point in the magnetization cycle.
The advance reported in this paper was the step of measuring the eddy-current
loss anomaly factor, not as an averaged effect taken over a full cycle of
magnetization, but rather as an incremental effect over selected portions of the
B-H loop. It was found that the main anomaly effect was occurring over the low
flux density range. Indeed, over such a range it could be far in excess of the
mean value normally attributed to the phenomenon.
In an extreme case one
can show that magnetic domain inhomogeneities could account for a loss anomaly
factor as high as 3 in thin sheet steel, but the research reported here revealed
loss factors appreciably higher than 3. It followed that a time-lag effect
enhancing the hysteresis loss could not be ruled out.
these notes being written some 43 years on from the date of the subject paper,
the author now admits that the research suffered from a rather grave omission in
that no account had been taken of the effect of the heat generated by
eddy-current and hysteresis loss. This heat would flow from the magnetized core
in the plane of the laminations and so in a direction at right angles to the
magnetic polarization within the magnetic domains.
By the Nernst Effect
this is a recipe for the induction of mutually orthogonal electric fields
powered by tapping that heat and it is now realized that this would, in fact,
add to the induced EMF driving currents around the eddy-current loop, enhancing
eddy-current flow and so escalating into an anomalous loss effect.
only excuse which the author can offer for this oversight is that there seemed
at the time no reason to even think about thermoelectric effects affecting
electric current flow in a single metal, steel, and even had that been
contemplated the limited temperature range involved would hardly have suggested
scope for efficient conversion of heat into electricity on the scale needed to
explain the anomaly. Now, also seeing all this in retrospect, there is good
reason for questioning the validity of what one had been taught concerning the
second law of thermodynamics, or at least the applicability of this law to
specific conditions prevalent inside a metal conductor where absolute
temperature has no significance and only the temperature differential
contributes to the effect considered.
This aspect of the eddy-current
anomaly can be far more important than the direct implications of the actual
power loss. Indeed, one can begin to see scope for a new method of electrical
power generation drawing on the ambient heat of our environment. This prospect
gains strength from the discovery that eddy-current anomaly factors much higher
than 3 were observed, particularly over the low flux density range where the
maximum action attributable to the Nernst Effect will occur. The reason for this
is that the domain magnetization is then equally shared by the two polarization
directions, so that the augmenting EMFs induced will see flow paths of least
See also the related papers: [1956a]