Re: space winding?

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Paul Nicholson <paul@xxxxxxxxxxxxxxxxxxx>

Someone wrote:
> It reduces the inter-winding capacitance for higher output
> potential.

Someone else wrote:
> space windiing ... advantages??  reduces the capacitance of the
> secondary

It reduces the turn to turn internal capacitance alright, but this
has hardly any effect (1% or less) on the operation of the system.

There's something of the order of 100pF of capacitance between
adjacent turns, but there's not much voltage across it.  Consider
1000 pairs of turns, each providing 100pF charged to an average of
a hundred volts or so: the combined energy there is only 0.0005
Joules - a tiny fraction of the typical total stored energy of the
secondary.

A much bigger contribution is due to the capacitance between much
more remote turns.   Each pair contributes only a tiny capacitance
- a small fraction of a pF, but there are many more pairs to count,
plus there's a much higher potential difference across each.  Their
total effect far exceeds the tiny amount due to the C between
adjacent turns.

An even bigger contribution when h/d is greater than about 1.0 is
the C between each turn and the surroundings - external capacitance.

In a typical TC secondary, the C is proportioned something like

  adjacent turns: 1% or less
  longer range turns:  10 - 20% depending on h/d
  external C:  about 80-90%.

Internal C becomes more significant for overtones.  Turn to turn C
only becomes significant at very high overtones.

Gary Lau:
> The purpose of space winding a coil is to reduce proximity
> effect losses in the coil.  The proximity effect increases
> the effective resistance, hence losses, in a conductor,
> when it is close to the adjacent turn.

Right.  The magnetic field from close turns pinches the current
flow into a smaller part of the wire's cross-section - the
resistance rising in inverse proportion to the area used.
--
Paul Nicholson
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