TECHNICAL NOTES

 
POWER TOROID TECHNICAL INFORMATION
The first toroid patent dates back to 1884. However, it's only during the past thirty years, as AMVECO and its predecessors developed practical manufacturing techniques, that large numbers of toroids have been applied in wide varieties of electrical and electronic products. The usage of toroids has grown rapidly because of the many features that permit new and innovative product designs. The following is a discussion of various aspects of toroidal transformer technology that will be useful to design engineers contemplating their application in new or existing equipment.

CUSTOM DESIGNS FOR OEM APPLICATIONS
Unlike their E-I counterparts, toroids lend themselves to custom applications because most designs don't need special costly tools, such as stamping dies (required for special lamination forms).

A strip of high-quality grain-oriented silicon steel is tightly wound in the shape of a clock spring, thus forming the core of the AMVECO toroid. The coils constituting the primary and secondary windings are individually wound, one at a time.

SMALL SIZE THROUGH REDUCED CORE AND COPPER LOSSES
From Faraday's equation for induced voltage in a transformer winding, we derive the following practical equation:

Erms = 4.44 x f x N x B x A x 10-8
Erms = induced voltage in winding
f = Frequency (Hz)
N = Winding turns
B = Flux density
A = Core cross section (cm2)

Increasing the working flux density will permit a lower number of turns and/or a smaller cross sectional core area. Experience has shown that working flux densities of 16k to 18k Gauss can be used in toroids.

Working flux densities of 12k to 14k Gauss are the practical limits for typical laminated cores with airgaps, Thus, toroid geometry may directly reduce core material (weight) or winding turns (copper losses).

FLEXIBLE DIMENSIONS
As long as the cross sectional area of the toroidal core is held constant, the height and diameter may be varied to meet the designer's requirement. The functional optimum diameter-to-height ratio is 2:1. For modern equipment design emphasizing a low profile, a 3:1 ratio, wider diameter and lower height may be more suitable. In cases where a smaller "footprint" is desired, a 1.5:1 ratio should be considered (narrower diameter, higher profile).

The only restrictions are those of the practical limitations of insulation and winding machinery, A minimum center hole must be maintained in order to permit the insertion of the winding machine shuttle into the center hole of the core.

REDUCED SIZE REDUCTION THROUGH DUTY CYCLE
A significant reduction in transformer size and weight may be realized in many cases where the transformer is loaded intermittently. In such cases, the load is energized for a small portion of the period. The period is much shorter than the overall thermal time constant of the transformer. The following equation applies: