Understanding Eddy Current Loss: How to minimize it? Tutorial
Eddy Current Loss Definition:
In this post we will discuss about various methods of reducing the Eddy current loss in the electrical equipment.
Before that we will get refresh about what is eddy current loss? and How the eddy current is produced?
- The magnetic field surrounding a coil which is carrying AC current varies with time.
- This varying magnetic field induces voltages in nearby conductive material like metal equipment cabinets, transformer cores and so on. The resulting current is known as Eddy current.
- The currents flow in a circular manner like eddies in the brook, so they are called as eddy current.
- They create unwanted power loss which is known as eddy current loss.
- Additional power is required from the supply to make up this loss
Eddy Current Concept:
As shown in the figure, consider an iron-cored solenoid which connected to a supply via an on/off switch.
- When the switch is closed, the current flows through a coil will increase rapidly. The coil current will reach to some steady value which will depend upon the coil resistance.
- A magnetic field will be generated due to the flow of coil current.
- The magnetic flux is proportional to the coil current and it will increase from zero to some steady value. This changing flux(dΦ/dt) expands outwards from the centre of the iron core.
- As the flux linking with the core is changing, an emf will be induced in the core.
- Since the iron core is a conductor, the induced emf will produce a current to be circulated around it. This is known as an eddy current ( it circulates similar to the manner created by an eddy of water).
- The direction of the induced emf and eddy current will be determined by applying Fleming's right hand rule.
As the eddy current circulates in the core, it will produce a heating effect. This is normally an undesirable effect and is referred as eddy current loss. This loss is negligible in dc circuits as it will occur during circuit turn on and turn off duration only. But in AC circuits the eddy current will be flowing continuously, in alternate directions.
How to minimize Eddy Current losses?
- In order to reduce the eddy current loss, the resistance of the core should be increased. In other words, low reluctance should be retained.
- In devices like transformers, the core is made up of laminations of iron. ie,the core is made up of thin sheets of steel, each lamination being insulated from others.
- As the laminations are thin, they will have relatively high resistance.
- Each lamination sheet will have an eddy current circulates within it.
- The sum of individual eddy current of the laminations are very less compared to that of using single solid iron core.
- The eddy current loss is proportional to f2. So at higher frequencies, the eddy current loss is very high.
- Under such conditions, the use of lamination sheets are not enough.
- For this type of application, ferrite cores or iron dust cores are used. Using these materials, the eddy currents are limited to individual grains, so the eddy current loss is reduced considerably.
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