Re: Overcoupling?

To: tesla-at-pupman-dot-com
Subject: Re: Overcoupling?
From: "Antonio Carlos M. de Queiroz" <acmq-at-compuland-dot-com.br> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
Date: Fri, 07 May 1999 23:50:52 -0600
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

Reinhard wrote:

> The primary coil´s magnetic field induces a current into the secondary,
> right? Changing the coupling factor also changes the amount of magnetic
> field that is effectivly "being seen" by the secondary. As you increase the
> coupling, you also increase the "amount of secondary" (couldn´t find any
> better words for this) being encased by the magnetic field. This increases
> the output voltage of the secondary coil.

Actually, this only increases faster the secondary voltage. The maximum
value is strictly restricted. by energy conservation, to:
V_secondary_max <= V_primary_max*sqrt(Cprimary/Csecondary).

> At one time or the other, you will
> hit the maximum voltage the secondary wire can take (meaning from one
> winding to the next). If you surpass this coupling factor and go even higher
> the wire´s insulation breaks down and you get racing sparks. You can
> compensate for this by increasing the secondary loading. This will then
> decrease (!) the secondary voltage, yet you will get longer sparks,
> because the topload stores more Joules (voltage is within "safe" values,
> but C is bigger, hence more Joules). This would also explain to me, why
> magnifiers allow a higher coupling coefficient. In a magnifier setup the
> secondary coil is much more loaded with Ctopload, Ctertiary (and
> topload) and Ltertiary. The current flow in such a setup is increased,
> but due to the loading, the voltage does not climb to destructive levels.

The secondary voltage in a magnifier is significantly lower than the
top voltage of the tertiaty coil. That is the reason why it is possible
to use tight coupling in the primary-secondary transformer there.

> The voltage levels allowed in the secondary are dependant on the total
> insulation. My testbed coils were all just wound with wire and not
> coated. I experienced the racing spark phenomenon many times. My
> 8" coil was coated (before winding) with several layers of PU varnish,
> wound, coated with 2 layers of resin (with sanding in between) and two
> final layers of PU varnish. In other words the coating is very thick. With
> this coil I can couple pretty high. The primary to secondary distance is
> around 1.9". I can raise the primary(!) approximately 4.5" above the
> lowest secondary turn and get absolutely no interturn arc overs. At
> ~5" above, I start getting the phenomenon of racing sparks.

This means that high voltage gradients appear in the small sections
of the secondary, and I guess that without corresponding high voltages
at the top of the secondary. Malcolm already pointed the two possible
causes that I imagine that can be at play: distorted voltage profile
due to the proximity of the primary, what eventually degenerates
to sparking between the coils, or excitation of high-order resonsnce
modes. Where does these racing sparks appear?

Antonio Carlos M. de Queiroz