All electromagnetic devices that operate at power frequencies (50 or 60 Hz) use lamination steel sheet, which, for most devices (motors, small transformers, ballast transformers, etc.) is punched, annealed, and stacked by the manufacturer to form the magnetic core of the device. This manufacturing process has some major shortcomings. The most obvious is that a significant amount of scrap is generated in the lamination punching operation, sometimes exceeding 40 percent. Often the device manufacturer is driven to reduce scrap at the expense of optimizing the electrical efficiency. Also, the freedom to design the most efficient motors is limited by the two-dimensional constraints of stacking individual lamination sheets.
In addition to design and scrap considerations, one must look at market trends to see which devices are on the horizon and how the use of lamination steels will affect their economic and electrical efficiency. One area of growth is variable-speed permanent magnet motors. These devices use a permanent magnet rotor; that is, the rotor is solid rather than made from a stack of electrical steel laminations. The net effect is that now, since there is no use for the hole punched from the motor stator, the lamination scrap rate is dramatically increased.This has the effect of significantly increasing the cost of making the motor stator.
Finally, variable-speed motors place different demands on the lamination steel. The speed of permanent magnet motors is controlled by changing the frequency in the motor stator windings; frequencies as high as 1200 Hz are currently being used.
As the frequency increases, the core loss of the steel increases dramatically. This can be understood by examining the nature of the core loss.
The core loss Pc of an electrical sheet product can be considered to be composed of two components: the hysteresis loss Ph and the eddy current loss Pe (see Eq. [2.20]).
whereas the hysteresis loss increases linearly with frequency, the eddy current loss increases with the square of the magnetizing frequency (see Eq. [2.21]).
where t = sheet thickness
B = magnetic induction f = frequency p = resistivity of the steel
Thus, the higher the magnetizing frequency, the more important is the eddy current component of the total core loss of the steel. (Note that the physical properties of the steel that affect the eddy current loss are the sheet thickness and the steel resistivity.)
The resultant high core losses not only reduce motor efficiency but limit its operating range. These high core losses will ultimately have to be addressed by the motor designer as well as the steel manufacturer. On the steel side, as is obvious from Eq. (2.21), thinner gauges and higher alloy contents are needed to meet the growing demand for high-speed motors. Both the thinner gauge and the higher-alloy trends will increase the cost required to produce these steel grades. In addition, silicon added to the steel to increase the resistivity also decreases the magnetic saturation of the steel. This decrease in magnetic saturation negatively impacts many motor designs.
The Accucore concept (Bularzik, Krause, and Kokal, 1998, pp. 38-42) is directed toward addressing the aforementioned problems—a reduction in scrap, a greater electrical efficiency of the finished electromagnetic device, and almost limitless design freedom. An Accucore pressed magnetic component doesn't have the same design constraints that are inherent in a stack of laminations. A powder metal part is scrapless and is, therefore, free of design constraints based upon simply reducing scrap. Furthermore, three degrees of freedom are available to the designer rather than the two degrees imposed by simply stacking steel laminations on top of one another. The zero scrap and increased degree of design freedom will allow for optimization of the magnetic circuit around an improved electrical efficiency for the device.
Another advantage of a powder metallurgically pressed component is related to the frequency dependence of the core loss. It is possible to make a magnetic component that exhibits an almost linear dependence of core loss with magnetizing frequency, as opposed to the almost squared dependence of core loss on frequency of most lamination steel grades. Since variable-speed permanent magnet motors operate at frequencies up to 1200 Hz, higher efficiency in these devices is possible.