EDDY CURRENT LOSS                     

EDDY CURRENT LOSS

When a.c. voltage is applied to a coil wound on a magnetic core, the current, not only induces voltage within the coil, but it induces a voltage in the magnetic core also which results in what is known as circulating currents or eddy currents. These eddy currents result in a loss in magnetic materials known as eddy current loss. The circulation of these currents

(a) shows the flow of eddy current in a solid core whereas

(b) shows in a laminated core. It has been observed that eddy currents flow crosswise in the core.

The magnitude of eddy current can be reduced substantially by increasing the cross-sectional resistance of the core. The cores are made of sheets called lamination. The thickness of laminations varies from 0.5 to 5 mm in electrical machines and from 0.01 to 0.5 mm in devices used in electronic circuits which operate at frequencies. Each lamination is insulated from the others by a coating of oxides or varnish or both so that it offers high resistance to the flow of eddy currents. The eddy current loss can also be reduced by using a high resistivity core material. The addition of a small percent of silicon says 3% to the iron will increase the resistivity significantly. The eddy current loss is given by 

where t is the thickness of laminations and Ke is a constant which depends upon the core material.

 

a. eddy current loss in core {solid core}

b. eddy current loss in core{laminated core}

The sum of the hysteresis and eddy current loss is known as core loss or iron loss. Since for a machine f and Em are constant, the core loss is also known as a constant loss of the machine. Eddy currents can be used to advantage to electric braking purposes e.g. in moving the iron and moving coil measuring instruments eddy currents are utilized to damp out the final settling of the instrument pointer. Also, there are some specially designed machines that utilize eddy current braking.