Misconception About the Rotor Strength of Eddy Current Separator

5th of 5 Eddy Current Separator Misconceptions Focusing on Rotor Strength

This is the 5th and final installment in a short series of blogs discussing misconceptions about Eddy Current Separation.  are used extensively throughout the recycling industry to separate non-ferrous metal (e.g. aluminium beverage cans, shredded aluminium and copper etc) from non-metallic materials.

How Important is Magnetic Rotor Strength?

Stronger is better, or is it?  Also, what does ‘stronger’ actually mean?

The principle of eddy current separation is based around a rotating magnetic field with changing polarity.  In accordance with Faraday’s Law of induction, electric currents are induced with conductors entering the rotating field (eg non-ferrous metals such as aluminium).  By Lenz’s Law, the induced eddy currents create a magnetic field that opposes the magnetic field that created it, thus resulting in the conductor being repelled away from the magnetic source.

Therefore, if the rotating magnetic field is stronger, then the repulsive effect must be greater.  Unfortunately, in operation, separation is not as simple as that.

In theory, stronger magnetic rotors producing better levels of separation could be true, but in practice the manufacturing and design process means that there are limits to any magnetic rotor.  All Eddy Current Magnetic Rotors are constructed from permanent magnets attached to a carrier pulley.  The size of the permanent magnet (both in length around the rotor and thickness) dictates the throw of magnetic field.  Longer and thicker magnets produce deeper magnetic fields than shorter and thinner magnets.

Irrespective of whether the magnet is long or short, the maximum magnetic intensity is on the pole (surface) of the magnet.  Therefore, in practice, there has already been a loss of magnetic intensity by the time the field reaches the surface of the feed conveyor belt as it has to pass through:

  • A non-metallic shell;
  • An air gap between the shell and the magnetic rotor;
  • The belt;

If very short magnets are used a very shallow magnetic field is produced and this may only just reach the surface of the belt.  In contrast, longer magnets throw a deeper magnetic field.

This means that you could have a magnetic rotor with short exceptionally strong Rare Earth Magnets that produce a weaker magnetic field at the point of separation (ie on the surface of the conveyor belt) than one constructed from longer standard strength Ferrite Magnets.  This also makes it very difficult to classify any magnetic rotor as being ‘strong’ or ‘weak’ as this definition is totally dependent on where the measurement to support that decription is taken.  For example:

Rotor design On the surface of the belt 10mm above the surface of the belt
Rotor 1 Short, exceptionally strong Rare Earth Magnets with a shallow field Strongest Weakest
Rotor 2 Long, standard strength Ferrite Magnets with a deep field Weakest Strongest

The key to magnetic rotor selection need to be based on what needs to be separated.  If the non-ferrous particle is large (eg an aluminium can) and you want the magnetic field projected into the centre of that particle for maximum separation effect, then a deep magnetic field with longer magnets is required.  For small non-ferrous metal particles (eg as found in plastics) then a shallow magnetic field with shorter magnets will produce the best separation.

Understanding these principles is vitally important when considering which design and type of Eddy Current Separators to purchase.  Simply being advised that the actual strength of the magnetic field is the ‘strongest on the market’ will not determine if that Eddy Current Separator design is right for a particular application.

Other blogs in this series on Misconceptions about Eddy Current Separation include:

1 of 5 Rotor Speed

2 of 5 Belt Speed

3 of 5 Belt Length

4 of 5 Ferrous Metal Removal

For further details on the Bunting range of Eddy Current Separators, Magnetic Separators and Metal Detectors please contact Carlton Hicks (carltonhicks@buntingeurope.com) or our technical sales team on:

Bunting Metal Separation System Recapture Plastics-4

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