Steel Core Inductor Ei Core Design

B Vodvarka 11/05/2009

Steel Core gapless Inductor design Using EI cores or C cores These calculations are going to get you in ballpark when some of the varialbes are unknown, like steel type core gaps...etc

Lets start with determining what inductance we will need, lets say we have a pole xfmr and we need 240Vac at 60Hz RMS input to drive it, and we want 35 amps of current: We need to determine the max voltage so we need to convert RMS to peak voltage: 240Vac x 1.414 = 339.36 Vac Peak. Now we have the peak voltage we need to know what inductive reactance we need to place our current at 35 amps: 339.36Vac Peak / 35 Amps = 9.696 ohms of reactance is needed. And now we know the reactance needed, we can now find the inductance required: 9.696 ohms / (2pi x 60hz) = .0257 henries or 25.7 mH. Now we turn to the core and determine the turn count: mpl := 20 N :=

a := 6.875

mpl⋅ L⋅ 10 3.19⋅ a⋅ μ

μ := 2000

L := .0257

8

N = 34.232

Where: mpl= Magnetic path length a = Core area (center leg width x stack depth) L = Inductance μ = Permeability of the steel N = Turns Using a permeability of 2000 for steel cores will get reasonably close for all practical purposes If the steel type is known the μ can be taken from the steel Specification sheet if more accuracy is desired The mpl can be reasonably estimated by using half the width of the center leg and measuring all the way around the window, so for example if the window measured 1" wide and 3" High the the mpl can be estimated by adding .5" to the width and .5" to the height in this case it would be (1.5 x 2)+(3.5 x 2) = 10" This calculation assumes no gaps in the EI core with the EI butted together tightly, of course there always gaps but for all practical purposes this calculation will be within a few turns of the target. You can add a few turns to compensate for the gaps that do exist and stacking factors...etc Calculating gapped reactors is done a little differently as the gap needs to be accounted for.