Magnetic Core Losses Resulting from a Rotating Flux

Abstract

Most work in the past for determining magnetic core losses has been based on the assumption of an alternating flux. However,in parts of many machines such as generators, the flux rotates rather than alternates. Because of this, it would be advantageous when calculating the losses in a machine if data on losses due to a rotating flux were available. At the 1957 Conference on Magnetism,a paper was delivered in which the magnetic core loss comparator(also known as the magnetic probe) was described. Since that time, a procedure has been devised by means of which the magnetic core losses caused by a rotating flux can be determined using this instrument. One obtains a rotating flux by inserting the legs of a cross-shaped lamination into two sets of magnetizing coils, each set at right angles to the other. If the phases of the magnetizing currents going into each set of coils are adjusted so that the resulting fluxes are 90° out of phase (electrically) with one another, a rotating flux results in the lamination. Core loss readings are made every 30° (geometrically) about a point near the center of the lamination using the magnetic probe. The actual loss of the material can then be determined by adding the losses in any two mutually perpendicular directions. Core loss measurements were made under varying flux conditions ranging from a pure alternating flux (flux in only one direction) to a pure rotating flux (flux in two mutually perpendicular directions of equal magnitude). Intermediate to these two extremes, measurements were made with the flux densities in the two directions of unequal magnitude. This latter condition is a close approximation to that found in an actual generator. Utilizing the above technique, the magnetic core losses caused by a rotating and an alternating flux were determined for an oriented and a nonoriented silicon iron.With both materials, the rotating losses were substantially higher than the alternating losses with the relative increase being greater for the oriented iron. However, the oriented iron was still the lower loss material under all flux conditions.