Environmental Engineering Reference
In-Depth Information
breakthrough in the understanding of losses in electrical machinery and it continues
to form the foundation of understanding M/G core loss today.
During the early 1920s, an insightful view of magnetic hysteresis was expoun-
ded by Professor Heinrich Barkhausen during his tenure at the Technische Hoch-
schule Dresden. According to Professor Barkhausen, hysteresis was a consequence
of magnetic domains, and this formed the basis of his theory. The Barkhausen
theory of magnetic domains was derived from experimental evidence that magnetic
induction was not a continuous function of magnetizing intensity in a ferromagnetic
medium, but rather a stepwise phenomenon resulting from domain wall pinning and
then alignment with the applied field. Over the intervening years other investigators
have shown that magnetic domain structure also has a pronounced effect on eddy
current losses. A magnetic steel sheet of thickness,
d
(e.g. a lamination sheet), con-
sisting of magnetic domains having an average width, 2
L
, in which the adjacent
domains are magnetized anti-parallel, can be shown to have a domain-model eddy
current loss,
P
ed
, relative to the classical-model eddy current loss,
P
ec
, that differ
noticeably at higher fields. Figure 8.1 illustrates this theory for the lamination steel
cases shown.
5
4
B
/
B
s
=1
B
/
B
s
<< 1
3
H
2
H
1
0
d
0
0.5
1.0
1.5
2.0
2 L
Domain size
2 L
Sheet thickness
d
Figure 8.1 Magnetic domain-model of eddy current losses
The lower curve in Figure 8.1 represents low values of applied magnetizing
intensity,
H
, so that the resultant induction in the lamination,
B
, is small compared
to its saturation value,
B
s
. When the field is increased the losses are higher because
the domain walls widen to the point that the steel is saturated once each half cycle.
At low frequency the domain walls remain flat and the average wall velocity is low.
Referring to Figure 8.1, the domain-model of eddy current converges to the clas-
sical model as the domain size approaches zero, because then the permeability
instead of being discontinuous as a result of domain size becomes homogeneous
on a microscopic scale. At the opposite extreme, when the domain size equals
the lamination thickness, the eddy current loss is double its classical value, as
shown in Figure 8.1.
