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DEVELOPING AN AC CURRENT GENERATOR

by Ernesto Gradin & Aubrey Kagan

Start ý Features ý Magnetics ý Primary and Secondary Turns ý Primary and Secondary Winding ý Primary Inductance ý Hardware ý Firmware ý Analog to Digital Conversion ý User Interface ý Sources and PDF

PRIMARY WINDING

The length of wire required for the primary is found by the perimeter of the cross section multiplied by the number of turns. For our case, the perimeter is (2 ý 2.15) + (2 ý 1.60) = 7.5 cm. The length of wire is 190 ý 7.5 = 1425 cm @ 15 meters.

In order to choose the correct wire for the primary, you need to consider the inside circumference of the toroid. Figure 3 shows the basis of the calculation. The interior diameter of the core is 39.3 mm. The internal circumference is p D = 3.14 ý 39.3 @ 123 mm. The maximum diameter of a wire that will fit into this circumference is:

with one layer. More than one layer is permitted subject to practical considerations. Using wire tables, magnetic wire of AWG22 size is used (0.71 mm with insulation).

Figure 3ýA cross section of a toroid shows the distribution of wires for a single layer of windings. The diameter of the wire is chosen to fit inside the inner circumference of the toroid.

The resistance of the primary (R_P) is the product of the wire resistance (0.53 miliohms/cm for AWG22 copper) and the wire length R_P = 0.00053 ý 1500 @ 0.8 ohms. The reflected resistance to the secondary is:

SECONDARY WINDING

Based on wire tables, you need a wire gauge of at least AWG#12 for 10 A. It has a resistance of 0.052 milliohms/cm. Allowing for 100 cm of wire (only about 52 cm is needed for the actual loops), the wire resistance (R_S) is 5.2 milliohms. The total internal resistance (R_i) is the sum of R_P and R_S (i.e., 5.2 + 1 = 6.2 milliohms).

If R_i plus the sum of the additional resistance of the wire and contact resistance of the module under test exceed 25 milliohms, the 14 V_RMS used to drive the primary would be insufficient. Increasing the primary voltage would increase the flux density, and the core would saturate.

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