Electrical Apparatus magazine, July 2006 Dealing with stator core damage

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.

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Featured Technical Article
July 2006

On this page is a summary of the Electrical Apparatus July 2006 featured technical article, by Richard L. Nailen, P.E. , "Dealing with stator core damage" Behebung von Ständerkernschäden ... Gestion des dommages de noyau de stator ... Para Tratar el Daño del Núcleo del Estator To order backissues which contain the foreign language summaries and the full article, call 312-321-9440 or visit our online webstore. See also links to additional technical summaries, below.

 

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