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Cracking of sintered ceramic (ferrite) magnets in PM motors

Cracking of sintered ceramic (ferrite) magnets in PM motors

Cracking of sintered ceramic (ferrite) magnets in PM motors

(OP)
Have others encountered difficulty with cracking of sintered ceramic magnets in fractional HP permanent magnet motors?  Do these cracks grow with time, and can they produce loose chunks that will jam the armature?

The magnets in question are epoxy bonded to the inside of a heavy gage steel tube, roughly 3" in diameter.  The shell seems quite rigid and does not appear to be the cause of the problem, nor a potential solution.

Cracking of the magnet is generally detectable only by dye penetrant test during incoming inspection.  No field failures have been reported to-date.  It is uncertain as to whether or not these cracks extend through the entire cross section of the material. Circumferential cracking has also been found, perhaps indicating delamination. To-date, no loose pieces have been found.  In this particular application, jamming of the motor armature can pose a life-safety hazard.

Research indicates that sintered ceramic (ferrite) magnets commonly have porosity, cracks, chips, etc.  Manufacturers indicate that dye penetrant, x-ray and similar NDE techniques should not be used since a high rate of unnecessary rejections would result.  Our magnets do have small, inconsequential nicks and chips at corners.

The general consensus is that cracking occurs during or after baking the assembly at 350F to cure the epoxy.  We have also succeeded in producing similar cracks by hammering on the OD of the shell.  Loose magnets attributable to poor adhesive bonding have previously been a problem, however, the present epoxy material and process appears to provide a very strong, rigid bond.
 

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