How to put Air-Gap in EI cores?

You don't interleave the laminations and keep the I's and E's on their own sides - slide a piece of paper or plastic of the required thickness between the sides and tighten the frame or housing.

One can't just do the center this way, for that you need to lap the center down with abrasives, or buy iron that is already stamped.

Hand lapping is somewhat variable in results. Place I worked at we used a 1/2 inch thick piece of plate glass, epoxied and screwed the E's together and a strip of emery cloth. - that was only done for very specialized transformers where we wanted part of the core to saturate so we weren't cutting away the whole center leg. That was very rare (once in two years). The paper in the gap is pretty common . . .

BTW cutting and grinding operations on stacked or wrapped laminations frequently results in laminations connecting to their neighbors - that raises eddy currents. The cut end may be etched with acid to remove the connecting burrs.

Its a solid ferrite EI core, not laminated core. How to create air gap in the center leg (while not creating air gap in the side legs) ?

Or is this not possible ?

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for ferrite cores you have to specify the gap when you order them. rw

The gap, if any, in a ferrite core is usually specified when you buy it.

What is the purpose of the gap? Seems logical if the goal is to prevent or delay saturation, a gap across all three legs should work. This assumes all windings are on the center leg and the center leg is twice the area of the outside legs and the gap is supposed to be the same across the entire face of the center leg.

I don't know how to calculate it, but slipping a piece of paper in doesn't permanently wreck the core so you could experiment, or if you know what the expected impedance of a winding should be, you could check that.

The only downside I can see is the radiated field may be greater.

or try grinding it with an abrasive . . . ferrite is tough stuff but it can be ground down. Give it a try and see how long it will take. A fine strip of emery cloth on a flat hard surface, run the center leg on the abrasive and let the outer legs ride on the surface plate.

I did derive an equation, using basic equations of magnetic circuits:

l_new = l_cen * 2 * A_side / (2 * A_side + A_cen)

where:

l_new is the new air gap across all 3 legs l_cen was the old specified air gap in the center leg A_side is the cross sectional area of a side leg A_cen is the cross sectional area of the center leg Assumption: both side legs have symmetrical geometry

Hence, putting the l_new air gap across all 3 legs would produce similar magnetic resistance as putting l_cen air gap in the center leg alone.

Mike

ferrite can be ground back with the right abrasives, do you intend to do it by hand, or do you have access to a milling machine of the apropriate scale?

it may be possible to buy pre-gapped E's

ISTM that halving it would do that, the lines of flus would pass throgh one 0.2mm gap into the I and another 0.2 back into the E for a round trip gap of 0.4

doing that will increase electromagnetic radiation

Bye. Jasen

If it is that easy . . . Magnetic flux decreases with the cube of the distance, and leakage flux may have other consequences. Build it and see what happens.