Instabilities of electromagnetically levitated bodies and its prevention


Instabilities of electromagnetically levitated bodies and its prevention

Priede, J.; Gerbeth, G.; Mikelsons, A.; Gelfgat, Y.

Electromagnetic levitation is a well-known technique for containerless processing of metals and alloys both in the solid and in the molten state. In experiments the levitated bodies often times show different types of instabilities resulting in a rotating and oscillating motion of the sample. In the paper we analyze the physical reasons for such spontaneous instabilities, and conclude on possible measures against it. The first type of instability considered is that due to possible coupling between the electric current passing through the magnetic system and the variation of position of the levitated body. It is shown that oscillatory motion of the body may be unstable if the electric power supply regime permits the electric current passing through the magnetic system to vary depending on its effective inductance. Another type of instabilities may occur because of the coupling between the motion of the body and the electric currents induced in the body itself. This effect may cause a spontaneous spin-up of the spherical body, occuring if the magnetic field frequency exceeds a certain critical threshold depending on the configuration of the field. It is shown that this type of linear instability may be completely suppressed by imposing a steady magnetic field of a strength comparable to that of the oscillating one. As a third type of instability we consider small vibrations of the spherical body occuring due to the finite diffusion time of the magnetic field into the body resulting in a delay of the induced currents with respect to the body position. Again, such vibrations may occur if the non-dimensional field frequency exceeds some specific threshold. This threshold and the
underlaying mechanism will be explained. The theoretical predictions are verified by model experiments using solid Al or Mg spheres. These instabilities can obviously be avoided by system parameters below the corresponding non-dimensional reshold. If this is not possible in reality, an active damping method using DC magnetic fields has been developed, too. The DC field can either be produced by permanent magnets or by an electromagnetic superposition to the levitation coils. Experiments will be shown demonstrating the stabilization effect due to the DC magnetic fields.

  • Lecture (Conference)
    The 3rd International Symposium on Electromagnetic Processing of Materials (EPM2000) April 3-6 / Nagoya, Japan, Published by: The Iron and Steel Institute of Japan, pp 352-357
  • Contribution to proceedings
    The 3rd International Symposium on Electromagnetic Processing of Materials (EPM2000) April 3-6 / Nagoya, Japan, Published by: The Iron and Steel Institute of Japan, pp 352-357

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Publ.-Id: 2558