RE: I slotted my 1256B variac core, now...

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Terry Oxandale" <toxandale-at-cei-dot-net> 

I was able to do it with one SawzAll blade, but I had to let the saw cool
down a couple of times in the process. To hold the core together, I removed
one of the 4 posts (those that position the core between the aluminum
framing) from the side of the core that was opposite of the side I was make
the cut on, and re-installed it in an empty hole in the frame next to the
cutting location and the other side of the cut already had a post positioned
there). This then provided support on both sides of the cut to keep the core
round (I cut the core at a location centered between the ends of the
winding). As I completed the cut, it became obvious the core laminations
were beginning to spread out in the immediate vicinity of the cut (about an
inch on either side), but the posts did a good job of restricting the
expansion of the laminations. To complete the process, I used a good sized
C-clamp (with a 3/4" thick piece of nylon block on the inside and outside,
of the core's new gap, as  arbors) to squeeze the core's laminations back
together snuggly. At this point the gap spread out to a slightly larger
width of about .100" from the original .060". I then filled this gap with a
high temp epoxy. Once the epoxy hardened, I removed the clamp, and the coil
retained the shape without any of the laminations popping out. I may
fabricate an extra post so that I can leave the displaced post in place just
to make sure the core will hold its shape later on as the variac is moved
and bumped around.

(un)Terry

  -----Original Message-----
From: 	Tesla list [mailto:tesla-at-pupman-dot-com]
Sent:	Friday, November 05, 2004 6:48 AM
To:	tesla-at-pupman-dot-com
Subject:	Re: I slotted my 1256B variac core, now...

Original poster: "Mark Broker" <mbroker-at-thegeekgroup-dot-org>

How are you clamping the laminations together?  A couple nuts and bolts?

Adding even a very narrow air gap should virtually eliminate the chance of
saturation.

A friend went through several SawzAll blades before taking it to a machine
shop, who I guess also went through a good bandsaw blade.  I have since
mused about mounting a 3-stack on a "good-sized" vertical end mill and
using a 1/2" cutter to make a gap.  But then we got a dedicated reactive
ballast that limits the 240V line to 80A (so that was a 19kVA Jacobs Ladder
for those who attended our event last weekend!), and we lost the desire to
slice variacs.

Mark Broker
Chief Engineer, The Geek Group


On Wed, 03 Nov 2004 20:47:58 -0700, Tesla list <tesla-at-pupman-dot-com> wrote:

 >Original poster: "Terry Oxandale" <toxandale-at-cei-dot-net>
 >
 >
 >Today I undertook the painful venture of slotting my variac core to reduce
 >the tendency to saturate when used to vary inductive ballasting. After
 >searching the archives I was unable to find any reference to a recommended
 >width of the gap that now exists in the core. Is the width of this gap
 >critically important, or just the fact that one has been created? Currently
 >the gap is about .060" (the width of the blade used to cut it) and as has
 >been mentioned many times, it is a coiled band of strap that is straining
at
 >the clamps wanting to bust open. Should I widen this gap some, or will this
 >be sufficient.
 >
 >Thanks,
 >
 >Un-Terry
 >
 >
 >