Re: Gapping Pot Core

On Thu, 24 Jan 2008 19:28:37 GMT, qrk <SpamTrap@xxxxxxxx> wrote:

On Wed, 23 Jan 2008 23:43:06 -0800 (PST), WJLServo
<WJLServo@xxxxxxxxxxx> wrote:

On Jan 23, 9:25 pm, Winfield <winfieldh...@xxxxxxxxx> wrote:
W Letendre, WJLServo wrote:
Working on eval design for power inverter; will use a pot core for
transformer. Have picked a core size, based on allowable hysteresis
loss. Also have a pretty good idea what magnetization inductance
needed. Requires an AL value a good bit smaller than that of largest
gap core in vendor's catalog...

Thought of buying a few core sets with largest available gap, then
modifying them to test for proper indutance, and get vendor to make
a special order core, same gap, for production. But, damned if I can
figure out how to remove material from core center posts. Could just
use gap shims, I guess, but that would open up gap at periphery as
well as center of core, and I would not then be able to put core in
a shield cover without incurring eddy current losses in metal cover.

An E core would be easier, I think: just come in from the side with a
grinding wheel and shave down the center post, using a bit of flood
coolant to make a nice, smooth ground surface. Getting to the center
post of a pot core would almost seem to require a jig grinder, which
I don't have. Any suggestions?

My advice: During the prototype phase, add the spacer
and don't worry about the flux leakage and eddy-current
losses with the shield -- they will be small. Go ahead
and measure the inverter's performance. Take heart in
the thought it might do better with a proper ground core.
Second, add the shield to get a handle on magnetic-field
leakage, etc., which at any rate will be less with the
production pot core. What's the problem? If you're
happy with the prototype design, tested this way, the
production one will be a bit better! So... what's the
big problem?

Oh, more advice: just forget grinding your ferrite core.


Heh! Think your advice may be best! Only reservation is wondering how
easily the gap generated with spacer shim can be translated into a
production value. Had thought that a modified core with center post
ground to match desired AL could then be measured using dial indicator
to give an exact gap spec for vendor to use for production cores. At
the end of the day, though, may just use spacer, measure AL rather
than gap, and ask vendor to take AL value as their production spec...

W Letendre

Win's method is what I have used in the past. Beware that the junk
spitting out from the core can get into other electronics and cause
problems if your gap is large enough. I've used business cards as
shims. Seems to be the best use for them. For proto purposes, you can
use black electrical tape to hold the core halves together. Since you
are gapping your cores, the offical hardware isn't necessary for your
experimental stage.

You need diamond tools to gap your own ferrite. Best to use the shim
method.

Once you have a gap you like, simply calculate the AL of your core set
by measuring the inductance of a coil with a known number of turns. Be
sure your bobbin is nearly full as the AL value is more sensitive to
winding height with lower AL vaules (a 0.2 relative winding height
will give an AL value 6% lower at and AL of 160, 2% lower at an AL of
600 for many pot cores). When ordering custom gapped cores, all you
need to give is the AL value you want and they do the rest. However,
see if you can use a standard gap as that makes life much easier and
cheaper.

---
Mark

You can buy a set of non-conductive shim stock from McMaster for about
$30 which has color-coded thicknesses from half a thou to 30 thou.

For production, epoxy with glass spacers can be used to hold the core
halves together.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@xxxxxxxxxxxx Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
.

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