RE: Interesting inductor observation

["Oxandale, Terry" <Toxandale-at-SPP-dot-org> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

My friend and I had this very same conversation last week. Here was a
copy of what I sent to him:



  Thanks for the comparative note on your ballasting. My intuition (not
technical by any means) is that a pure air core reactor would be the
ideal reactor. This is because the field lines are purely reacting with
adjacent conductor and are not "contaminated" with the core
magnetization. A big iron core with a few windings is basing the
reactance upon the core, where the "no core, and many winding" reactor
is basing it's reactance on the current countering itself directly. It's
almost like the core has a lot of electrical inertia and there by
comparing it as low friction-high inertia object, verses the air core
reactance being compared to high friction-low inertia. 
  When I was still testing relays (a long time ago) this was very
apparent. On air core reactors (about 10 feet in diameter, helix wound
with a 1" stranded conductor spaced 2" on center and about 8 feet tall)
we never had inrush current problems because the counter emf was
immediate and opposite to the exciting current. But on the iron cored
reactors, we had to have a separate relay to keep the reactor from
tripping itself off upon initial energization because the core took so
much energy to magnetize (inrush current). Once it was "on", the
overcurrent protection for the inrush current only, was no longer
needed, and the reactor did it's job normally. This was what was
happening to my tesla coil. With the RSG constantly "hitting" the
ballast, it was only seeing the inrush of current for that huge toroid
ring, instead of being used as counter emf. You can take two different
designs of reactor, one iron and one air core, with identical inductive
reactance, and they will have completely different operating
characteristics. I am going to look into making a pure air core reactor
here soon.

Not technical by any means, but imperical (did I use that right).

Terry


> ----------
> From: 	Tesla list[SMTP:tesla-at-pupman-dot-com]
> Sent: 	Thursday, July 20, 2000 9:51 PM
> To: 	tesla-at-pupman-dot-com
> Subject: 	Interesting inductor observation
> 
> Original poster: ghub005-at-xtra.co.nz 
> 
> Something that might be of interest...
> 
> I was talking to a transformer specialist today. 
> Although not an EE, he has been designing and 
> manufacturing transformers and inductors for the 
> last few years. He says that when designing an 
> inductor for a current limiting application, it 
> is not enough to simply specify the inductance 
> and the line voltage for the inductor.
> 
> Apparently, different inductors with the same 
> inductance (but different physical designs) will 
> often behave completely differently when placed 
> into a live circuit.
> 
> He said that an example is when you tune a TC's 
> input current with an variac-inductor, measure 
> the inductance of the variac, and build a fixed 
> inductor with the same inductance. It seems that 
> you will invariably get different performance 
> results from the coil when you switch between the 
> inductors.
> 
> This effect does not apear to be attributable to 
> core saturation in the inductors. Also, the 
> impedance of the two inductors is measured as 
> being (almost) the same.
> 
> Does anyone out there know why this effect 
> occurs? Presumably it is a magnetic effect - 
> possibly to do with the flux density or 
> magnetising currents? Or maybe it is just a 
> tuning phenomenon that is peculiar to the TC.
> 
> I would be interested to know what others think 
> about this, or if anyone else has experiences 
> this.
> 
> Safe coiling,
> 
> Gavin Hubbard
> 
> 
>