Re: oil dielectric

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

Original poster: "davep by way of Terry Fritz <twftesla-at-qwest-dot-net>" <davep-at-quik-dot-com>

> 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?


	No core heating.  The rest applies.

> The coil resistance losses are minimal in a resonant coil.


	I can't see why.  Resonance implies high currents.  Arcs
	demonstrate high currents.  Resistance losses are
	utterly insensitive to frequency.

> From a practical standpoint, I've run my secondary for long

>periods of time


	Air is an effective cooler.  Or is this the oil filled one?

> and there is no noticeable heating in any part of the circuit.

	What power levels?
	Typical _Tesla_ transformers have large exposed areas
	(as compared to a conventional transformer), so cool
	themselves well.)  And the power in the secondary is
	a deal lower than that in the primary.  cf the
	'efficiency' discussions.


> 	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.


	I'm talking an assembly described by the builder as sealed.

> Further, the current flowing through the wires of an air core

>transformer is a milliamp or less.


	perhaps THIS air core transformer.

> You can't heat the wires (hence the oil) without appreciable

>current.


	Tesla coil systems involve high voltages and arcs/sparks.
	arcs and sparks ignite things.

 
>>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...


	.and losses... limiting voltages...
> 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.


	Frequency is generally set by inductance.  Finer, longer
	wire tends to more inductance, lower frequency.

> The higher the frequency, the higher the voltage


	I can't think why.

> (in general.)  The higher the voltage, the lesser the current.


	Until the arcs/streamers start.

> The lesser the current, the smaller the wire can be.


	The larger the losses, the lower the efficiency.

> The smaller the wire, the higher the inductance


	the lower the frequency.

> and voltage (for the same length of windings.)


	??
	More wire, means more inductance, or a smaller winding.
	Smaller winding means lower voltages.

> 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.


	Minimum impedance maximizes current...

> I don't see how covering a coil with oil can be a problem.


	Oil is ignitable, notably by electric arc/spark.

	The description was of oil 'in the center',
	rather than 'covering'.

> If anything, it is a great help, and this is attested to by

>Tesla in several references.


	Indeed.  Tho he did not use it in the largest coils.

> (His book mentions the importance of oil covered secondaries.)


	Which.
	He wrote several.

	best
	dwp