Eddy Currents

Author: E.E. Kimberly

Fig. 7-10. Eddy Currents and Their Paths

A rod of solid metal, when subjected to an alternating magnetic field, has electromotive forces set up in it. The rod may be considered as being made up of concentric laminating rings, as in Fig. 7-10 (a). The alternating field cuts the rings and generates alternating electromotive forces in them. These electromotive forces cause alternating currents, called eddy currents, to flow through the resistance of the rings. The magnitude of an eddy current is designated as Ie, A power loss and a rise of temperature result from eddy currents.

Fig. 7-11. Laminated Magnetic Paths

If the rod of Fig. 7-10 (a) be divided into many slender rods insulated from one another, as in (b), the eddy-current paths will be broken up and the power loss caused by these currents will be very greatly reduced. Such a subdivision of an iron path is effective in mitigating eddy currents but is so poor mechanically that it is unsatisfactory for most practical purposes.

In electrical machinery, eddy-current losses are mitigated by making the magnetic paths of sheet-steel laminations. A sheet thickness of about 0.014 in. is commonly used. Fig. 7-11 shows the laminated structure of a transformer and one of a toy a-c motor. In alternating-current machines, the eddy-current loss is usually much less than the hysteresis loss. Lamination increases the resistance per volt induced in the eddy-current paths, and so greatly mitigates the power loss caused by such current.

Inasmuch as eddy currents are caused by voltages generated in the iron, they are proportional to the rate of change of flux. In the iron core of a coil, therefore, they are proportional to the maximum flux density and to the frequency. In the armature of a d-c motor or generator they are proportional to the flux density and to the speed. Therefore, the losses caused by them are proportional to the square of the maximum flux density and to the square of the frequency or speed.