Impedance and the tap range: the check most spreadsheets skip
Reactance is computed from the radial build. Move the taps and the build moves with them, which is why a design can pass at principal tap and fail at the extremes.
· 6 min read · Engineering
Reactance is geometry
Leakage reactance comes from the flux that links one winding and not the other, and that flux lives in the gap between the windings and in the windings' own radial thickness. So the percentage impedance is fixed by the mean turn diameter, the duct width, the radial builds and the electrical height, together with the ampere-turns.
That makes it a geometric quantity, which is good news for calculation and bad news for approximation. Anything that changes the build changes the impedance, including changes you made for another reason entirely, such as adding conductor area to hit a load-loss guarantee.
Taps change the geometry
On a tapped winding, moving from principal tap to an extreme tap changes the effective ampere-turns and, depending on where the tap section sits, the axial distribution of them. The impedance follows. A design that lands neatly in the middle of the guaranteed band at principal tap can sit outside it at maximum or minimum tap.
The arrangement of the tapping section matters as much as its size. Taps taken from the middle of the winding keep the ampere-turn balance far better than taps taken from one end, which is why the middle arrangement survives short circuit better as well.
Why the check gets skipped
In a spreadsheet, the impedance calculation is usually one formula fed by cells that describe the principal-tap geometry. Repeating it at each tap means duplicating the whole block, and once it is duplicated it stops being maintained. So the extreme-tap check quietly becomes something the designer does when they remember.
The consequence shows up in the test report, or worse, in a parallel-operation complaint after commissioning. It is a cheap check to automate and an expensive one to omit.
What good practice looks like
Compute the impedance at maximum, principal and minimum tap every time, show all three against the guaranteed band, and show the margin as a percentage rather than as a pass or fail. A design with two per cent of margin and a design with 0.1 per cent both read as pass, and they are not the same design.