Power Donuts

Rewinding at the core of these toroid transformers

Two reasons have been cited to explain the efficiency of toroidal power transformers over conventional stacked lamination designs:

1. The windings are placed directly on the core, distributed over its entire circumference and minimiz-ing the amount of copper wire in the windings. By comparison, in a stacked core transformer the windings are placed on bobbins surrounding only the core legs, thus producing a larger diameter winding that requires a greater amount of copper wire. As a result, the winding resistance and losses are higher than in the toroid.
2. Cores used in toroidal line power transformers consist of grain-oriented silicon steel strip, wound into a flat tubular shape under controlled tension. The primary and secondary windings are placed uniformly around the core, with the magnetic flux parallel to the grain orientation in all parts of the core. As a result, there is a very low emitted (stray) field. The core may also operate safely at 15-16 kilogauss, as opposed to 12-14 kilogauss common to an E-I design. Additionally, the core ring draws a much lower excitation current than a core made of stacked laminations.

Besides efficiency issues, toroids are physically smaller and lighter than traditional transformers, making them suited to companies pursuing reduction in size of their products. So, how does one improve on the toroid transformer?

The engineers at Alpha-Core Inc, Bridgeport, CT, say they found an answer in the construction of the core. The circumference of a circle is about 12% less than that of a square with the same area, and 18% less than a rectangle with the same area and a 1:2 side ratio. The O-core transformer line begins with the silicon steel strip and slits it in such a fashion that when wound into a core, the resulting cross section is a perfect circle rather than a square or a rectangle. This further reduces the length of wire needed to circumscribe a given core cross section, resulting in further weight and loss reduction.

The O-core does not require epoxy coating, which adds 20-30 % to the cost of a bare toroid, to protect the copper wiring from being nicked or cut on the corners of a rectangular cross-section. Taping with 50% overlap is sufficient core insulation, in turn permitting the use of heavier wire at lower winding tension. The O-core design also eliminates the barrel shape of windings placed on a square core. Practically, the larger total cross section of wire in the core ID leads to a substantially higher VA rating based on core weight. This means a higher copper/iron ratio than ordinary toroidal transformers, in a smaller, lighter and more efficient package.

—RM