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Eddy Current

Eddy currents are loops of electrical current produced in a conductor due to a changing magnetic field or relative motion of the conductor inside a magnetic field. They flow in a plane perpendicular to the direction of the magnetic field. Because they resemble swirling eddies in turbulent water, they are called eddy currents.

Due to Faraday’s law of electromagnetic induction, Eddy currents produce their magnetic field. According to Lenz’s law, the current direction is such that it will oppose the magnetic field causing it. In the image below, an inducting coil produces a magnetic field. A conductor is brought near this magnetic field, resulting in swirling electrical currents on the conductor’s surface. Since the coil’s magnetic field and the eddy current’s magnetic field are opposite, it results in a magnetic drag.

Eddy Current

Properties

The magnitude of the eddy current is:

  • Directly proportional to the applied magnetic field’s magnitude
  • Directly proportional to the area of the loop
  • Directly proportional to the rate of change of magnetic flux
  • Inversely proportional to the conductor’s resistance

Applications

  • It is used in eddy current braking of trains and roller coasters. The brakes expose the metal wheels to a magnetic field, resulting in an eddy current. Eventually, the interaction between the applied field and eddy currents slows down the wheels. The faster the wheels spin, the more pronounced the effect is. It means that eddy currents can smoothly slow down a fast-moving vehicle without causing any discomfort.
  • It is helpful to detect flaws in metals. Eddy current testing applies non-destructive testing (NDT) method in which the test metal is not sacrificed. Any defect in the metal, like near-surface cracking, breaks the eddy current, which, in turn, reduces the magnetic field. This change in the magnetic field can be detected easily.
  • It is used in a speedometer. A speedometer consists of a magnet that rotates according to the vehicle’s speed. Eddy currents are produced in the drum, which rotates in the direction of the magnet. A pointer attached to the drum indicates the speed.
  • Since current produces Joule heating, eddy current is suitable in induction furnaces and cooktops. An induction furnace separates metals, while an induction cooktop is used for cooking food.
  • It can separate metals from nonmetals. It is helpful in waste management. For example, aluminum can be separated from trash.
  • It is used in magnetic levitation, providing modern high-speed Maglev trains with frictionless movement. Changing magnetic flux in the train produces eddy currents on the fixed conducting sheet over which the train levitates. The eddy current interacts with the applied magnetic field to produce levitation.

Drawback

Eddy current generates heat in a conductor creating energy loss. Inductors (other than the ones used for heating), transformers, generators, and electric motors suffer losses due to heating. To prevent it from happening, the metal core of these devices is laminated. Then, the core splits into several layers of insulated metal sheets.

Article was last reviewed on Thursday, November 24, 2022

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