Environmental Engineering Reference
In-Depth Information
The underlying theory of domain-model eddy current loss is that the spatial
inhomogeneity of permeability due to large domain size leads to higher eddy cur-
rent losses than what would otherwise be calculated using the classical Steinmetz
model. In ferromagnetic material the eddy currents are localized to the moving
domain walls where it can reach high values. Because of this localized eddy current
flow, the losses are higher than if the currents were more evenly distributed in the
lamination. Therefore, the larger the domain size in the magnetic steel, the fewer
the domain walls, and the faster they must move in response to a given flux change
at a given frequency.
The components of core loss can be summarized as [1]:
Classical eddy current losses that result from circulating currents in the bulk
iron material produced by changing magnetic fields. For example, an M/G that
is designed for 5 V/turn will have 5 V/macrocirculating path in the iron lami-
nations. This induced voltage gives rise to circulating currents that dissipate
power as Joule heating in the iron. Bulk circulating currents can be minimized
by increasing the material resistivity (e.g. adding silicon, aluminium, phos-
phorous, manganese etc.) or by decreasing the lamination thickness. However,
lower thickness flat rolled steels can have issues with surface texture, surface
treatment and coatings.
●
Anomalous eddy current losses are generated by circulating currents as a result
of flux changes due to uniform domain wall motion. In typical magnetic steels
having grain sizes of 100
m
m, there may be two magnetic domains per grain.
The empirical relation for anomalous eddy current loss due to domain wall
motion is
●
q
H
wm
s
f
3
B
P
e
wm
¼
G
s
where
H
wm
is the hysteresis dependency on grain size,
G
s
. Small grain size
results in lower domain wall velocity and high hysteresis, whereas large grain
size results in low hysteresis component. In other words, there is an optimal
grain size for each specific frequency of operation. Decreasing the material
conductivity will lower the loss.
Hysteresis loss is caused by alternating currents induced by erratic domain wall
motion. Domain walls are pinned at precipitates, so minimization calls for low
silicon, carbon and nitrogen content. The presence of inclusions resulting from
insufficient time to float out slag, plus lattice defects due to re-crystallization,
and tendencies to relatively large grain size will all increase hysteresis loss.
Furthermore, magnetoelastic effects due to lattice strain and surface strain are
reduced through annealing and surface coating.
●
Net core losses in the hybrid propulsion traction motor can be substantial and
accountable for some integral percentage of overall fuel economy loss. The next
two sections cover the classical loss model and some extensions that are applicable
to calculating core losses in hybrid M/G.
