RE: oil dielectric

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "David Thomson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <dave-at-volantis-dot-org>


Hi Dave,

	Having done some thermal design & studied transformer
	design, I'd find that surprising.  Oil, in a conventional
	transformer functions in two ways:
		Electrical Insulation
		Thermal
	The thermal effect is by being heated by hot spots
	(core eddy currents, coil resistance losses), then flowing,
	by natural convection, or, less commonly, pumped
	circulation, to a cooler spot where it can dump heat.
	Sometimes the cool spot is aided by heat sinks ('fins').
  	cf any text on transformer design.

Does this also apply to air core transformers with no ferromagnetic
material?  The coil resistance losses are minimal in a resonant coil.  From
a practical standpoint, I've run my secondary for long periods of time and
there is no noticeable heating in any part of the circuit.

	I say nervous because a sealed container, can, if heated,
	even by a fault, 'rapidly disassemble':  Explode.
	Unlikely, probably, here, but possible.  (Power
	transformers can and have exploded, when not vented.)

You're talking the difference between an iron core transformer and an air
core transformer in resonance.  Further, the current flowing through the
wires of an air core transformer is a milliamp or less.  You can't heat the
wires (hence the oil) without appreciable current.

> I like this coil so much I'm going to wind another one.  This time
>I'll use finer wire if I can get some.

	In general, fine wire increases losses, limiting power,
	and increases heating...

In general, this is correct.  With this type of coil, it doesn't appear to
be the case.  The better the insulation of the coil, the higher the
frequency I can reach.  The higher the frequency, the higher the voltage (in
general.)  The higher the voltage, the lesser the current.  The lesser the
current, the smaller the wire can be.  The smaller the wire, the higher the
inductance and voltage (for the same length of windings.)  The higher the
voltage, the lesser the current, etc.

And of course, the coil will be operated at resonance, so the impedance will
be minimum.  I don't see how covering a coil with oil can be a problem.  If
anything, it is a great help, and this is attested to by Tesla in several
references. (His book mentions the importance of oil covered secondaries.)

Dave