How fast can you turn a motor before the magnet powderizes?

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Discussion Overview

The discussion revolves around the structural integrity of magnets in motors, specifically focusing on how fast a magnet can rotate before failing. It explores the factors influencing this failure, including material properties and mechanical constraints, and considers different modes of failure under high rotational speeds.

Discussion Character

  • Technical explanation, Debate/contested

Main Points Raised

  • One participant assumes that the stator and windings will not burn out and questions the maximum rotational speed a magnet can achieve before structural failure occurs.
  • Another participant notes that the failure depends on the material of the magnet and the tolerance of the bearings that support it.
  • A different viewpoint discusses the compressive and tensile strengths of NIB magnets, raising questions about the failure mode at maximum RPM, specifically whether it would break apart or powderize, and the roles of tension, compression, and shear in this process.
  • Another participant describes a typical failure mode due to over-speed, suggesting that it usually results in tensile failure of the armature components, which can lead to catastrophic mechanical failure, including components being ejected from the motor.

Areas of Agreement / Disagreement

Participants express differing views on the specific failure mechanisms and conditions, indicating that multiple competing perspectives remain without consensus on how magnets fail at high speeds.

Contextual Notes

The discussion does not resolve the assumptions regarding material properties, the specific conditions under which failure occurs, or the definitions of failure modes.

Schtoogie
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Assuming the stator and windings won't burn out, how fast can a magnet rotate before it structurally fails?
 
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Depends on the material the magnet is made from, and also the tolerance of bearings that constrain the motion of the magnet.
 
So, NIB magnets have a way higher compressive strength (950 MPa) than tensile strength (75 MPa). As you spin it the outer edge is under greater compressive force, while the inner edge is in greater tensile force. So how does it fail at max RPM, by breaking apart or by powderizing itself? Tension or compression? And what about shear?
 
The mode of failure due to over-speed is usually a tensile failure of the armature components with the result that the armature components hit the stator poles and the motor locks.

If you allow a winch with a DC motor and gearbox to free-run under load without power applied, the rated RPM of the motor can be exceeded. What I have seen happen is the strips of copper commutator being thrown out of the armature taking the attached winding leads and brushes with them. It makes a mess.
 

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