Magnetics measures inductance in a Kelsall Permeameter Cup. Actual wound inductance outside a
Kelsall Cup is greater than the value calculated due to leakage flux and flux developed in the winding.
The difference depends on the core size, permeability, core finish thickness, wire size, and number of
turns, in addition to the way windings are put on the core. The difference is negligible for 125μ and
higher and turns greater than 500. The following table is a guide to the differences that one might
experience:
No. of Turns Measured Inductance compared to Expected
1000 0%
500 +0.5%
300 +1.0%
100 +3.0%
50 +5.0%
25 +8.5%
The following formula can be used to approximate the leakage flux to add the expected inductance.
This formula was developed from historical data of cores tested at MAGNETICS. Be aware that this will
only give an approximation based on evenly spaced windings. You might expect as much as ±50%
deviation from this result.
Leakage Inductance = 292*N*1.065*Ae/(Le*10^6)
where leakage inductance in (nH/Turn2).
N = number of turns
Ae = core cross-section (mm2)
Le = core magnetic path (mm)