How it works : Eddy Currents

EDDY CURRENTS


Above: when a permanent magnet is moved across the surface of a metal-for example, aluminium-turbulent electrical currents called 'eddy currents' are produced in the metal. Although these currents are random in nature the' produce their own magnetic fields which create a dragging force between the plate and the magnet.

As the term suggests, eddy currents are a phenomenon of things that flow, and this includes fluids such as water and air or less tangible things such as an electric current. When liquid flows, such as water flowing from a tap, it may do so in an orderly manner, almost as if it were still flowing in a pipe. But if the tap lacks the right shape of nozzle, the water falls out of it in an irregular manner, with droplets separating from the main stream, whose general shape is constantly changing. Such flow is called turbulent. Smooth flow is called laminar hecause the fluid need not flow at the same rate throughout the cross-section of the stream, but can flow in thin layers (laminae) which can slide over each other smoothly.

The same two kinds of flow can refer to the passage of air over the wings of an aircraft, laminar flow being desirable and turbulent flow occurring under certain circumstances, for example when the aircraft stalls. High-speed motor boats and cars are streamlined in an attempt to create laminar flow, but even a streamlined motor boat leaves little whirlpools behind it in the water. It was these whirlpools which were first given the name 'eddy currents' or 'eddies'.

In modern technology the term is given different interpretations by different authors. Some would imply that an eddy current contains a rotating component, while others use the expression to denote irregular or turbulent flow in general.

The term is often used to describe electric currents flowing in a large sheet of conducting metal, as opposed to current flowing in an ordered manner in a wire, where insulating material is used to prevent the current from taking short cuts from one turn of a coil to the next.

In electrical terminology, eddy currents are almost entirely associated with electromagnetic induction, that strange phenomenon whereby a moving or changing magnetic field passing through a conducting material induces a current in it. Thus a magnet pulled across a sheet of copper, aluminium or brass will produce irregular current flow, including whirlpools very similar to those found in the wake of a ship. The phenomenon is used in household electricity METERS and in electromagnetic clutches. The current always tries to effectively oppose the relative motion between sheet and magnet. To put it another way, the magnet effectively tries to drag the sheet along with it.

The use of the phrase 'eddy current' is extended by some authors to the rotating part of an induction motor (see ELECTRIC MOTOR) on the grounds that the rotor consists not of insulated wires but of a copper or aluminium 'cage', the bars of which are sunk in slots in an iron core. The definition usually assumed is, however, that an eddy current infers a randomness which is not to be found in an induction motor, for the designers of such machines go to considerable trouble to ensure that the induced currents are highly organized.


Reproduced from HOW IT WORKS p833