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“Dealing
with stator core damage" With
trilingual summary
Heat loss caused by magnetic hysteresis and eddy
currents in stator core laminations contributes both to increased
winding temperature and reduced efficiency. Although the hysteresis
component is governed by the metallurgy of the lamination material,
eddy current loss depends upon the degree of electrical contact
between adjacent lamination surfaces, which are coated with
insulation to inhibit such contact.
A severe electrical fault, excessive core heating
during removal of a failed winding, or a mechanical breakdown
allowing the rotor to rub the stator – these are the usual ways in
which the electrical separation between laminations can be
compromised. Such damage is diagnosed by magnetizing the core while
its temperature rise is monitored. Excessive eddy current loss
results in unacceptably rapid heating.
Repairs can be made by disassembling and restacking
the core using either new laminations or existing laminations that
have been reinsulated. Proper reassembly involves careful control of
the clamping pressure as the core stack is built, and holding the
finished stack together in a way that minimizes electrical resistance
between laminations. Too loose a stack risks movement and eventual
breakage of core teeth under the electromagnetic vibratory forces
present during machine operation. Too much pressure will needlessly
increase core loss.
If only the inner diameter of the core is damaged,
etching that surface with a mild acid solution may break the contact
between lamination edges resulting from the grinding action of the
rotor contact. Not the subject of any standard, this method requires
an experienced operator, and is not always successful.
Another repair method can reach more deeply into the
core to deal with more extensive damage. That involves dipping or
spraying the core with an acid solution that chemically converts
lamination surfaces into a corrosion resistant phosphate layer. In
experiments on both large and small stators, this method has reduced
core loss as much as 25 percent and core temperature by one-third.
From “Dealing
with stator core damage," Electrical Apparatus
July 2006
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