[Prev][Next][Index][Thread]
Re: Tunable Primary coil, update
At 02:25 PM 1/19/97 -0700, you wrote:
>Subscriber: nikki-at-fastlane-dot-net Sun Jan 19 14:05:29 1997
>Date: Sun, 19 Jan 1997 10:49:02 -0600 (CST)
>From: Bert Pool <nikki-at-fastlane-dot-net>
>To: tesla-at-pupman-dot-com
>Subject: Tunable Primary coil
>
>Just thought I'd give a quick update on the tunable primary I've been
>building for my large magnifier.
>
>It ended up being 29 inches in diameter. Magnifiers typically use a
>cylinderical primary with a tightly coupled secondary inside, and this uses
>this same design. Insulation between the two is usually very heavy poly
>steeting.
>
>All my primaries in the past have been roughly tuned by tapping the turns
>for approximate tune, then half or quarter turns for final tune - sound
>familiar? In about 1976 I came up with a design for a primary that allows
>you to change the spacing between the turns to do precision tuning, even as
>the coil is running. I built such a primary for a very large coil (primary
>was 9 feet in diameter) and the tuning worked beautifully. This 29 inch
>primary is the only other primary that I've built using this technique.
>Because you have at least six vertical pipes which pvot in the center, and
>the fact that each place where the wire connects to an upright has to have a
>pivot as well amounts to drilling a lot of holes and making a lot of moving
>parts.
>
>I started out building an open frame out of 2 inch and 1.5 inch pvc pipe to
>form a cage-like structure upon which to wind the primary. I found 12
>verticals to be too many, operation was stiff and mechanical resistance of
>the large coax wound on the form was excessive. I reduced the number of
>uprights to six, and changed the pivot points on the 1.5 inch pvc from
>threaded pipe fittings to a simple plastic pin arrangement and now it works
>like it is supposed to.
>
>A friend and I will be measuring the amount of inductance shift the tuning
>arrangement provides later this Sunday. We also are abount to insulate the
>secondary and hope to fire up this magnifier for the first time!
>
>I'll post the results of the measurements and whether we set any smoke free
>later.
>
>
>Bert Pool
>nikki-at-fastlane-dot-net
>
>
Wild Bill and I fired up the new magnifier this weekend. We tested the
tunable primary, and it allows tuning of equivalent of about one turn. Here
are the measurements:
-at- 9 turns we could adjust from 76 to 86 uh
-at- 8 turns 64 to 74 uh
-at- 7 turns 53 to 61 uh
-at- 6 turns 42 to 46 uh
-at- 5 turns 33 to 36 uh
-at- 4 turns 24 to 26 uh
-at- 3 turns 17 to 17.5 uh
-at- 2 turns 10 to 10.25 uh
-at- 1 turn 5.5 uh, no change
As you can see, the amount of change drops as you reduce the number of
turns, which is to be expected. Most obvious change is when you tune near
max turns.
In operation, we were able to tune output from a few spindly sparks to much
more robust and longer sparks simply by operating the mechanism while the
coil was running.
On the down side, coupling appears to be lower than we'd like. This due to
the extra 4 inches in total primary diameter required by the extra pvc
fittings. We are going to do a lot more work on testing effects of coupling
in this system.
Our secondary, which is wound on a 16 inch diameter by 33 inch form, uses 10
gauge pvc insulated wire. It is very heavily insulated, and we've had no
sign of failures yet. We wrapped the entire secondary with three layers of
30 mil poly. Then we wrapped the bottom third of the coil with 20 to 25
POUNDS of 5 mil mylar insulation. We covered that with a couple layers of
90 mil poly, then covered the entire secondary with a final double layer of
90 mil poly. Insulation thickness of the poly and mylar over the coil at
the bottom is 1 inch!
One stupid thing that I did without even giving it a second thought is when
I set the plywood sheet which has the primary, secondary, caps and gaps on
it, on top of my hydraulic lift table (we had to use a lift because the
secondary alone weighs 107 pounds) and I didn't even think about the fact
that the secondary and primary were sitting right on top of a 150 pound
steel table. You can imagine how much of my primary's magnetic field was
going directly into the steel table and never making it to the secondary
(duh!) We'll be putting the coils on an insulating stand before the next
set of test runs.
We were running an 8 inch by 62 inch diameter toroid. Resonator coil was 4
inch by 12 inch, 30 gauge kynar wire. Initial firings were only 40 inch
sparks, as a 62 inch toroid does not leave much side room inside a two car
garage. After doing a lot more tuning and coupling testing, we'll move
outside and crank her on up.
All in all, I was quite pleased with the first firing. We didn't smoke any
coils, the caps didn't blow up, all the insulation held, and our variac and
transformer are still good. This was much better than our attempts at
building an oil filled driver design!
More to follow next week.
Bert Pool
nikki-at-fastlane-dot-net