Factors Influencing Core Loss

The hysteresis loss, as has been shown previously, is proportional to the area of the hysteresis loop, which in turn is affected by the heat treatment applied to the sheet steel. The hysteresis loss is thus kept low by proper annealing methods. The hysteresis losses are also related to grain sizes. The larger the grain size, the lower the hysteresis loss. The eddy-current losses vary as the square of the "lamination" thickness, and in order to keep the eddy-current losses low, the thickness of laminations must be kept as low as is economically feasible. The eddy-current losses also vary as the square of the frequency. A common lamination thickness for 60-cycle operation is 0.014 in. (29 gage), although thicker laminations are not unusual at that frequency. Laminations of 0.001 to 0.003 in. and cores of powdered iron or powdered nickel-iron alloys (known as permalloy) are used for radio-frequency applications.

Improved characteristics, not only in the sense of reduced core losses but also in increased permeability, are achieved in grain-oriented steel. This is accomplished in the manufacture of the steel sheets or steel strips by cold rolling in reducing the material to the desired thickness. The grain-oriented materials are characterized by the property that the core loss is much lower and the permeability much higher when the flux path is parallel to the direction of rolling than when it is at right angles to it. This is taken into account in the manufacture of the most efficient power and distribution transformers.

Generator laminations made of this material are segmented, being of such design that the major flux path does not deviate more than about 15° from the rolling direction.

Eddy current-losses determined by test on silicon steel have losses about 50 percent in excess of those computed from Eq. 3-106 or 3-107. The reason this excess is so great in the case of silicon steel is that the grain size for this type of iron is especially large. Generally the larger the grain size the greater the eddy-current loss for a given resistivity. Other factors tending to increase the eddy-current losses over and above the calculated values are low values of interlaminar resistance, bolts or rivets through the laminations to hold them together, etc. If the frequency is too high or the laminations are too thick, the flux will not completely penetrate the laminations and the eddy-current losses will actually be greater than normal.

Nevertheless, Eqs. 3-106 and 3-107 show the relative effects of the various factors that influence the eddy current losses and are therefore quite useful for estimating purposes.