A method of grading molybdenum permalloy toroidal cores to predetermine required turns for a given inductance

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Virginia Polytechnic Institute


Molybdenum-permalloy powder cores are manufactured to fall within required permeability limits. However, these limits are sufficiently broad to cause sizeable labor loss in adjusting the number of turns for required inductance values; for example, an Arnold Engineering Company A-930157-2 core with a 1000-turn winding may range from 135 to 179 millihenries. It is therefore highly desirable that large-scale users of these cores grade them into groups by some simple inductance test. This permits cores to be wound with a predetermined number of turns, thereby essentially eliminating the labor of adjusting the number of turns of the winding to meet required inductance values.

The following work has been accomplished:

(1) The design and construction of a multi-turn split jig suitable for rapid testing of cores to permit economical grading.

(2) The construction of a modified Owen alternating-current bridge with component capacitor and resistor values such that the inductance of the unwound cores and the finished coils may be read directly from one decade resistance box in microhenries and millihenries, respectively.

(3) The preparation of a family of curves each representing a group of graded cores showing the number of turns required to yield a given inductance.

Sketches and photographs of a core, finished coil, multi-turn split jig, and the test circuit are included.