[Prev][Next][Index][Thread]

Re: Idiot's Guide



Hello Tom, all,
                  More info.....
> > <large snip>
> > > Eventually almost ANY insulation can be broken down. Some people wind
> the
> > > Secondary wires further apart. That lowers the Q of the circuit. 
> > 
> > Actually, space winding can reduce winding losses by reducing the 
> > proximity effects in the winding. I have a spacewound coil that 
> > clocks in with an unloaded Q around 300 and is not particularly small.
> > One thing you do lose is inductance for the same wire gauge. 
> >
> 
> Excellent observation. Tesla himself had some of his coils progressively
> space wound. If you look at the old photos of his Magnifier you will see
> that the windings on the secondaries are not spaced at regular intervals. I
> remember reading somewhere that this was done because the electrical
> stresses are not evenly distributed. Does that relate to what you are
> calling the proximity effects, Malcolm, or is that something totally
> different?

Proximity effect is an extension of the skin effect. By immersing 
adjacent wires in each other's magnetic fields, you get an uneven 
distribution of current around the periphery of the wire, in other 
words, current bunching. This raises the effective resistance of the 
wire. This is most pronounced in single layer windings. In bunched 
windings, only the outside turns suffer this badly.

> I also recall that Tesla made some coils in which he actually
> had several different diameters of wire used on the SAME secondary. I am
> not sure however whether this was an experiment to Find Out if there was
> any Value to doing that, or whether it was an Implementation based on his
> having already determined that such an arrangement was desireable. I see
> this technique applied on his Magnifier Coil design, but not on what we
> might call his Classic Tesla Coil design. Anyone out there have more
> information on any of this?

We had a discussion on this last year. It seems that it would be ideal
to have a wire graded in thickness from bottom to top to match the 
current distribution in the wire. Problem is that most of the 
effective inductance appears below the top of the resonator so 
robbing the coil of inductance at the bottom where it is most 
effective (subjected to the greatest loading). There may be a drawback
to a closewound design if this is done that has been noted by Mark 
Barton a la conical coils - the symmetry of the winding is 
effectively disrupted and the Q is greatly lowered (poorer 
selectivity). He noted that he could tap the primary virtually 
anywhere and still get a significant output.

> > I think the accepted engineering definition of Q is that it is the 
> > inverse of the dissipation factor of the circuit - in other words, it 
> > relates to circuit losses. 
> >
> 
> Malcolm is right. I was attempting to put things in very simple terms.
> Maybe I got TOO simple. Maybe someone out there can e-mail me a Good Simple
> explanation of Q that I can use instead of the one I gave. It needs to be
> simple enough that a Beginner can understand it, and Exact enough that it
> doesn't make Engineers Shake Their Heads in Dismay.

Two fundamental definitions spring to mind:
(1) The ratio of inductive reactance of a coil to the effective AC 
resistance of the coil.  wL/Rac
(2) The ability of a resonant system to retain energy while it is 
oscillating.  SQRT(L/C)/Rac   for an electrical case.

Both relate to losses as Rac being the bottom term shows.
 

On primary geometry: In my book a coil is a coil. Clearances are 
certainly a problem that spirals can overcome. A coil with bunched 
windings will exhibit the highest inductance for the length of wire 
used to make it but will have severe insulation problems. Personally, 
I prefer the flat spiral. I have found it is a routine matter to
get k's to about 0.18 with a flat coil (higher with a saucer). The 
coil need not have a greater diameter than the secondary either. I 
currently sit a large resonator on top of a flat primary whose 
inner diameter is considerably less than that of the resonator. An
acrylic slab separates the two. (FWIW)
  
> > > Ever notice that sometimes going near an operating Tesla Coil 
can cause > it
> > > to change the size and even the NATURE of the Sparks? That is because
> there
> > > are interesting ELECTROSTATIC Effects Also At Work. The Secondary coil
> has
> > > a Capacitance that must be taken into account. The problem is the
> > > Capacitance of the Secondary is affected by many physical parameters
> such
> > > as thickness of insulation, kind of insulation, number of turns,
> exposed
> > > surface area on the outside of the coil, height of the coil, width of
> the
> > > coil, closeness to other objects, and of course, ANYTHING YOU PUT ON
> THE
> > > TOP OF THE SECONDARY. PHEW! It's NO WONDER you can't just crank a lot
> of
> > > info into a computer program and expect it to spit out complete plans
> for a
> > > Coil that is guaranteed to work the First Time you fire it up!
> > 
> > I have to disagree with that. Well tested formulae that can predict 
> > raw Cself have been posted on the list a number of times. It is true 
> > that one assumes a reasonable degree of isolation from other objects
> > and also that toploads add to the static figure. Also that 
> > sparks/corona/ion clouds add to this figure when the coil is 
> > operating.
> > 
> > Malcolm
> 
> My point was not to disparage the work of those who have tried to gather
> the multitude of factors into mathematical formulae and try to produce
> computer programs that tell us EXTREMELY USEFUL INFORMATION. I APPLAUD all
> such efforts. We need them badly! Chalk it up to a bad attempt at humor on
> my part. I was trying to make the point that there are MANY factors that
> enter into the design of a Tesla Coil. Some of these Factors Change with
> the Weather, if you know what I mean!

Points made by you and Richard Hull noted and agreed. I would say 
however that you can get an initially sparking design by calculation
(useful if no test gear is around). It *will* need some primary tap 
adjustment for optimum output in most cases though.

Malcolm