Variable Inductance

Quoting Ed Sonderman:

 > Richard,

 > I think perhaps I have discovered a method to drive my remaining
 > ground rods in.  I believe one of the local rental agencies rents a
 > 3/4" electric impact hammer that would do the job.  I rented an
 > electric jack hammer several years ago to chip rock to install a water
 > line.  The same tool should work with a hollow socket on the end to
 > hold the ground rod. 

Sound better than using your body to beat them into the ground! I am
sure there is an easier way. How about digging a shallow trench with 
a pickaxe out to the septic system, and forget the additional ground 
rods? Or how about digging one shallow hole and using the salt water 
ground? Just suggestions.

 > I reread Henry Transtroms book again last night.  It is a good 
 > basic electricity book although some of the terms are outdated.  
 > I understand the concept of making an inductor with a variable 
 > iron core but I haven't came up with a design to solve the mech-
 > anical problems. The core needs to slide easily in and out of the 
 > inductor with low friction and without wearing out the coil form 
 > (ie don't use cardboard).  For my use, the form could probably be 
 > a plastic pipe. A square form might be better although more diffi-
 > cult to wind.  I would need to come up with some mechanism to move 
 > the core in and out with some precision.  

I have thought about this at length, but have never gone past the 
rough design stage. I would be happy to bounce my incomplete ideas
off all of you... Maybe someone else has some input too?

In the four inch diameter tubing I believe there is ABS thin wall 
and PVC thin wall which telescope fairly neatly. I had thought about
getting a 30 pound spool of bailing wire and cutting the wire into
yard long sections. The cut wire could be layed out on some waxed
paper and coated with polyurethane or other sealer. Once dry the
lengths of cut wire could be tightly bundled, wrapped with a few 
spots with plastic friction tape to hold it, and fitted inside the 
smaller plastic tube. The cylinder core would then be potted with 
epoxy to prevent vibration. 

Around the larger plastic tube a coil four inches in diameter and
three feet long could be wound with some #6 wire. The coil would 
probably need to be wound with bared stranded or solid enamel 
covered copper, then coated with enough epoxy to pot the winding. 
The core would be drilled and tapped at one end for a mounting 
bolt, then fitted inside of the coil and mounted in a simple
wooden frame. A crank handle and draw screw would be fitted to the 
mounting bolt on the core, the coil would remain stationary. By
cranking the handle and drawing the core in and out of the coil,
the inductance would be varied.

 > The coil would need to be fixtured in some kind of frame and 
 > maybe use a lead screw attached to the core passing through
 > a fixed threaded piece and put a handle on the end?  I'm not at all
 > sure of the values yet or the size of the core needed or where I would
 > get the core.  I do have access to old transformers out of 36v 25A
 > battery chargers but I don't think the core pieces would be long
 > enough.  Do you know anyone who has built such a device?

Unfortunately I do not. However, since you do not need a lot of 
inductance (having resistance in parallel), the single winding over
the sliding core I described above would seem to be large enough.

Anybody else wish to comment? I have been thinking about a variable
inductance current limiter of this design for some time, any thoughts
would be appreciated.

It is a shame that I had no storage space with all the changes going
on here. My old neighbor and longtime friend is in the truck body 
business. They moved about two months ago and scrapped three old
Lincoln welders. The secondary windings could have been stripped off
and the rest of the unit could have been used "as is". I could have
had them for free, but had no place to put them, already have a 
welder of my own, and figured the weight would make shipping out
of state prohibitively expensive.

Richard Quick

... If all else fails... Throw another megavolt across it!