Eddy Currents – Benefits and Drawbacks in the Sensor Industry

Eddy currents – a phenomenon named after their circular/whirlpool-like character of flowing current (not after some scientist named Eddy). They are both beneficial and, in some cases, undesired physical phenomena involved in conductive materials.

Eddy currents represent loops of electrical current induced in conductive materials exposed to the changing/alter­nating magnetic field. This phenomenon was first observed by physicist Léon Foucault more than 150 years ago. Since then, eddy currents have found significant applications in everyday life.

The Physics Behind It – How Do Eddy Currents Work?

According to Faraday's Law, an electromotive force is generated if a conductive material is inserted into a changing magnetic field region.

Changing magnetic field creates circulating currents, perpendicular to the magnetic field, in the conductive material (steel), as depicted in Figure 1. These electrical loops also create Joule heat and additional localised magnetic fields, which is utilised in sensorics.

What if the eddy currents represent an undesired effect in the application? According to their nature, eddy currents flow in loops. Therefore, their presence can be suppressed by prohibiting loop creation e.g. reducing the thickness of the material (thin electro-steel sheets in transformers). This helps to reduce another characteristic feature of eddy currents – heat production, which presents energy loss. However, excess heating can be also beneficial – induction stoves, where using a thick pot takes advantage of huge eddy current loops producing Joule heat. Eddy currents can produce very high temperatures to even melt metals. In RVmagnetics, we employ eddy current heating during the preparation of our sensors

Generation of localised magnetic fields represents the main feature for the utilisation in sensorics. One of the biggest impacts lies in the non-destructive testing of flaws in conductive materials. The probe generates the eddy currents, however, when a crack is present in the material the continuity of current loops is disrupted, resulting in a smaller loop generating a lower magnetic field, which reveals the defect.

Not only defects, but metals itself. Creation, but most importantly, detection of the eddy currents, can be applied to sense metals in non-conductive environments, such as rebars in concrete. After the precise calibration of the probe, the displacement altogether with the position and thickness of conductive layers/objects can be measured. The same principle can be found in metal detectors which also offer an option to estimate the composition of the metal – gold/copper/i­ron/aluminium.

Advantages and Disadvantages

The advantage of such sensing represents the already mentioned non-destructive character, which cooperates with contactless measurement. A simple movement of the probe toward the conductive element can detect the current loops from a distance of a few millimetres. Eddy current sensors are notable for their high sensitivity, wide frequency range (100 Hz – 2 MHz), and robustness which makes them suitable for harsh thermal environments.

On the other hand, as with every other sensor, eddy current sensors have their limitations. Being limited only to conductive material with a penetration depth in order of millimetres. The measurement is strongly affected by the temperature of conductive material, as the temperature variations affect the resistivity of the material. In some cases, the Joule heat generation can be a disadvantage, since it will increase the temperature of the measured object. Finally, the performance of the eddy current sensors is dependent on the material properties, therefore, it must be carefully calibrated for the tested material under strict conditions with correct compositions and geometrical properties to obtain the quantitative results.

Eddy Currents and Sensing Industry

In the modern age of sensing industry, eddy current sensors enable precise contactless sensing possibilities for various applications dealing with conductive materials. Unlike the eddy current sensors, the MicroWire – magnetic sensor, can be influenced by the additional magnetic field created by eddy currents, which is interacting with the excitation field. However, Eddy current induced in the magnetic material is smaller than the current flowing through the coil. Magnetic field is proportional to the current, therefore magnetic field generated by eddy currents is usually much weaker than the coil’s magnetic field. Thanks to that property, with the correct setting of the electronics, magnetic field from eddy currents can be neglected.

What about the use-cases dealing with non-conductive materials? RVmagnetics’ Microwire does work for conductive materials, but also for non-conductive materials, where eddy current sensors are ineffective. Thanks to its micro-dimension, Microwire can detect structural integrity of both – conductive and non-conductive materials. Not only flaws, but also temperature, mechanical stress, magnetic field, and various interesting physical properties, which can improve your sensing possibilities.

Are you interested in how it works? Do not hesitate to contact our experts!