Gap spacers carry magnetic compressive force between reactor packets. [1] Use the actual loaded separation, not unloaded catalogue thickness, when relating spacer change to inductance. Separate initial seating, recoverable compression and permanent change under the actual load/temperature history.
Separate initial seating, recoverable compression and permanent change. Measure the same gap positions under a defined preload and temperature. An unloaded measurement after dismantling cannot directly reproduce the gap used by the operating magnetic circuit.
A paired record across three states
| State | Dimensional record | Electrical record |
|---|---|---|
| Initial assembly | Gap lengths under specified preload | Inductance at stated current/frequency |
| After thermal/load exposure | Same loaded geometry and temperature basis | Same inductance definition and excitation |
| After unloading/reloading | Recovery and persistent thickness change | Repeat after restoring original assembly load |
Predict the direction with a restricted model
For a linear, gap-dominated circuit with unchanged effective area and turns, L ∝ 1/gΣ. A small change gives ΔL/L ≈ −ΔgΣ/gΣ. Closing the total gap tends to increase inductance. This relation neglects saturation, fringing and core reluctance, so it is a consistency check rather than a calibration law.
If inductance changes in the opposite direction, first compare temperature, excitation and the measurement method. A change in core permeability or a nonlinear operating point can dominate the simple gap effect. Preserve these alternatives instead of attributing every drift to creep.
Propagate uncertainty through the paired gap ratio, including correlation if the measurements share a gauge or reference. A measured shift is meaningful only on matched loaded and thermal states. [2]
Translate spacer compression into inductance drift
Assumptions: a linear gap-dominated circuit retains turns and area; matched loaded gaps at 20 °C change from 20.0 to 19.0 mm, each with independent 0.10 mm standard uncertainty. L₂/L₁ = 20/19 = 1.05263, a 5.26% rise. First-order uncertainty in the ratio is 0.007642, or 0.764 percentage points. Its k = 2 lower drift bound is 3.73%, still above an assumed +3.0% allowable drift. Hold the loaded-gap condition and verify matched-excitation inductance; the calculation identifies geometric consistency, not proof that a particular spacer material crept. [1] [2]
References
- H. B. Duc, T. P. Minh, T. P. Anh and V. D. Quoc, A Novel Approach for the Modeling of Electromagnetic Forces in Air-Gap Shunt Reactors, Engineering, Technology & Applied Science Research 12, 8223–8227, 2022. DOI: 10.48084/etasr.4692.
- B. N. Taylor and C. E. Kuyatt, NIST Technical Note 1297, Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results, 1994, sections 5 and 6.