Comparison of Graphic Results With Theory

The following equations are repeated from the March, 1942, article:

If they are combined by eliminating p., the following equation results:

TrOJnl \ I

This equation yields a figure for the value of inductance at which the top of the humped curve should occur for the particular measuring frequency employed. This is compared in the following table with the value obtained graphically from each plot.

Figure 10. A coil wound with 972 turns of No. 20 enamel, measured at 400 cycles.

In the same way the following equation _ 1 SpiSAoc . ___

gives the maximum height which should be reached by the humped Q curve. This likewise is compared with the graphically obtained value in the table.

It will be noted that the agreement between theory and measurement is fairly good when the vagaries of ferromagnetic materials are borne in mind.

Air-core Point

The fact that the point representing measurements on an air-core coil falls on the curve as closely as other points may at first thought seem strange. However, this point represents the extreme limit of low permeability where copper loss is the sole factor in determining Q. For this condition the flux cutting the copper is identical with that for an interleaved core.

In order to demonstrate that this point belongs on the curve, extra points to define the curve were taken for Figure 9. The two points to the right of point A represent air gaps of 1 M inches and 1inches, respectively. The third point corresponds to a f^-inch gap, the longest normally measured.

Eddy Currents in Copper

It will be noted that a number of points in the middle of the Q curves (that is, for moderate-length air gaps) of Figures 10 and 11 do not fall on the curve. This is attributable to increased eddy-current losses in the copper.

Figure 11. The same coil measured at 1 kilocycle.

lam. mat. 746 SS 746 A

calc. act. 2.76 2.95 2.59 2.43

calc. act. 28.0 30 26.3 25

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