Switchable Magnets Explained

tech_center-200x993Electro Magnets

A simple electro magnet shown (fig.1) incorporates a coil of N turns wrapped around a ferromagnetic steel horseshoe.

Direct current is applied to the coil which creates a magnetic force perpendicular to the coil.

Magnetic flux passes through the poles of the horseshoe which will then influence any ferromagnetic material that comes within the field area.

Electro magnets can be very powerful, more turns on the coil and or more current generates more force:  MMF = I x T.

The magnetic field is removed from the poles the moment the current is removed from the coil.

Permanent Magnets

In the case of a simple permanent magnet design, the horseshoe is interrupted with a permanent magnet.

The permanent magnet material would likely exhibit high coercivity (difficult to demagnetize), Rare-Earth, for example.

As its name suggests, magnetic flux is always flowing from north to south.  The horseshoe allows the flux to concentrate over the pole areas and influence any ferromagnetic material that comes within the field area.

With permanent magnets, MMF is increased as the magnetic length of the raw material is increased.

In this illustration the magnetic field is always present and cannot be removed*.
A high-coercive permanent magnet is very difficult to demagnetize, therefore, if we want to make it switchable, we need some method of redirecting the flux away from the pole surfaces.

In Fig. 3 we have introduced a second permanent magnet of equal proportions into the circuit.  This has the benefit of providing twice the amount of flux which increases the density of the field, creating an even greater attractive force when in the ON condition.

More important, the second magnet is free to rotate which changes the overall state of polarity.

As can be seen in Fig. 4, when the second magnet has been rotated, the flux emanating from both raw magnets has a better route to travel from one pole to the other, and is now diverted away from the pole surfaces placing the unit in an OFF condition.

Switchable permanent magnets can be very powerful and have the benefit over electro-magnets of better safety since there are less external factors that can affect performance*.  However, there comes a point when the physical friction of moving a magnet becomes impractical and expensive to solve.
Permanent-electro magnets combine the benefits of both electro and permanent magnet technology.

Permanent-Electro Magnets

In Fig. 5 we have replaced the moving magnet with a static low coercive one.

By placing a coil around this magnet and using a short electro magnetic field, this magnet can be energized fully.

In the ON condition, as with the previous permanent magnet, flux from both magnets are combined and directed through to the poles surfaces.
In the OFF Condition, the coil is re-energized in the opposite direction, thus re-magnetizing the magnet to opposite polarity.

Magnetic flux from both magnets is now re-routed internally leaving the pole surfaces free of magnetism.

Permanent-electro magnets, require electricity only for the switching process (<0.5 secs). They can be very powerful, they don’t generate heat and have no moving parts.

 

* High Coercive magnets can be influenced externally, either by high temperatures and/or by influence of high electro-magnetic fields.