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Electropermanent magnet [Encyclopedia Magnetica]

Encyclopedia Magnetica

Encyclopedia of magnetics and electromagnetics.

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electropermanent_magnet

Electropermanent magnet

Stan Zurek, Electropermanent magnet, Encyclopedia-Magnetica.com, {accessed 2019-06-26}

Electropermanent magnet (also electro-permanent magnet) - a type of electromagnetic device which uses a combination of magnetically soft (e.g. iron), switchable semi-hard (e.g. Alnico) and hard (e.g. Nd-Fe-B).

FEM simulation of electropermanent magnet operation:
  • left (drawing): red - switchable magnet, blue - permanent magnet, orange - magnetising coils, light grey - yokes, dark grey - part being lifted
  • centre (FEM): magnet configured for lifting (light blue = low flux density, purple = high flux density)
  • right (FEM): magnet inactive

by S. Zurek, E. Magnetica, CC-BY-3.0

Operating principle

Mechanically switchable magnet technology was invented in 1960's1)2), but electro-permanent magnets were patented much later.3)4)

In some aspects the operation in similar to such devices as magnetic chucks. If the magnetic flux is guided in a closed internal magnetic circuit then there is no magnetic force produced outside of the device. The difference is that the switching action is achieved not mechanically, but electromagnetically.

Changing the path of magnetic flux and forcing it outside of the electropermanent causes any magnetic material in the vicinity to be included in the resulting magnetic circuit because the magnetic flux to close through the surrounding volume. This leads to magnetisation of the material and hence attraction, so that holding or lifting is possible.

Electric current is used to change the level of magnetisation of the semi-hard part. Once the magnetisation is changed the state is stable and no electric energy is required to sustain such state.5)6)

If the two magnets (permanent and switchable) can have similar value of remanence (e.g. around 1.25T) so if their volumes are equal the can cancel each other completely (no external force).

As an example, the coercivity of the permanent magnet (e.g. grade N40 NFeB magnet) can be 1000 kA/m and for the switchable magnet (e.g. sintered Alnico 5) can be 48 kA/m.7) The coil must be capable of generating an impulse of magnetic field strength significantly exceeding the coercivity of the switchable magnet, but significantly below the coercivity of the permanent magnet. Such impulse would ensure that the polarity of the switchable magnet is reversed without affecting the magnetisation of permanent magnet.

For instance, for the values given above, for a magnet 3.2 mm long, the suitable impulse could be 5.3 A into a coil of 80 turns, which would result with H exceeding 130 kA/m. This value is more than 275% of the coercivity of the switchable magnet, which is sufficient to ensure the reversal of polarity. On the other hand, it is only 13% of the coercivity of the permanent magnet, so that its magnetisation is not affected.

Practical applications

Because relatively low average power is required for the operation of the device (only pulse switching) such actuators can be used as miniature actuators in small-scale robotics (a few millimetres in size).8)9)

On the other hand, the fact that no electrical energy is required during passive operation the devices has some inherent safety advantages.10)

Lifting very heavy load (up to 40 tons) without danger of dropping the load when the supply of electricity is cut off (unlike ordinary lifting electromagnets) is possible.11)

In large devices during normal operation the electrical energy used is less than 5-10% as compared to conventional electromagnets.12)13)

See also

References

electropermanent_magnet.txt ยท Last modified: 2019/06/03 18:29 (external edit)

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