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Autotransformers – Part 2


I have had it pointed out to me that my SPICE simulation of an autotransformer doesn’t account for a less-than-unity coefficient of coupling across the magnetic path. The idealized result of that can be seen as follows:

Figure 1 Autotransformer step-up with unity coupling coefficient.

Figure 2 Autotransformer step-down with unity coupling coefficient.

I guess it’s time for me to say “mea culpa.” I must be getting careless in my old age. However, the two models above can be embellished to account for their non-unity coupling coefficients along the following lines:

Figure 3 Autotransformer step-up with non-unity coupling coefficient.

Figure 4 Autotransformer step-down with non-unity coupling coefficient.

The two additional inductances are by definition not coupled to each other while the L1 and L2 are “mutual inductances” in their respective illustrations. Mutual inductance as a topic unto itself can be examined at many sources. For purposes of this autotransformer model, this is one:

https://en.wikipedia.org/wiki/File:Mutual_inductance_equivalent_circuit.svg

Numerical values in the SPICE model can be adjusted as needed for more precise coupling coefficient modeling.

John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).

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