The Misconception of Eddy Current Rotor Speed

1st of 5 Misconceptions About Eddy Current Separation

Eddy Current Separators 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.  One of the first Eddy Current Separators was built by the Bird Group in the UK in 1981 and the separation system has now become common place in recycling operations across the world.  But there are still some misconceptions about eddy current separation.

The magnetic rotor needs to rotate as fast as possible.

An Eddy Current Separator consists of a Magnetic Rotor with alternating magnetic poles rotating inside a slower rotating non-metallic shell.  Eddy Currents are induced into a non-ferrous metal particle when exposed to the changing magnetic polarity.  The eddy currents cause an electric current to be generated in the non-ferrous metal particle that produces its own magnetic field.  This magnetic field reacts with the rotating magnetic field of the rotor, resulting in a repulsive effect that throws the non-ferrous metal away from the rotating magnetic field.

Bunting_Metal_Separation_Module_Recapture_Plastics-7454

It would be fair to assume that increasing the number of polarity changes in one second would have an effect on the separation.  In practice, the actual amount that a non-ferrous metal particle is thrown reaches a peak and as the rotor speed increases then separation actually deteriorates.

To inject the maximum amount of repulsive energy into a non-ferrous metal particle there needs to be a dwell time when the two opposing fields meet and if this is too short then the injected energy is less.  It is considered that very high rotation speeds do not allow for sufficient dwell time.  Nevertheless, manufacturers commonly claim that Eddy Currents with the highest rotating speed are the best designs.

In reality, the optimum separation speed will be unique to each design of Eddy Current Magnetic Rotor, but is commonly between 2,600 and 5,000 RPM.

Also, there are other considerations such as maintenance and wear.  As the rotor speed increases there is a higher wear factor on key components such as bearings.  Then the question has be asked that if one more item of metal is recovered by a Magnetic Rotor spinning at 5,000 RPM than one operating at 3,000 RPM but the resultant bearing wear is high, is it better to run at the lower speed.

Bunting will be exhibiting at Waste 16 on 9th June 2016 and RWM at the NEC in Birmingham (13th – 15th September 2016) and will be available to discuss the technology and physics behind eddy current separation.

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:

 

 

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