If anyone wants to see what is required to operate at high voltages in open air without significant corona loss or breakdown, take a look at some of the images of large Cockroft-Walton and Marx generators. The conductor junctions consist of huge spheres. The conductors themselves are frequently cylinders.
Steve White
Cedar Rapids, Iowa
----- Original Message -----
From: "jimlux" <jimlux@xxxxxxxxxxxxx>
To: tesla@xxxxxxxxxx
Sent: Friday, October 12, 2018 10:02:25 AM
Subject: Re: [TCML] Primary voltage considerations
On 10/11/18 8:46 PM, Steve White wrote:
> Hello Greg,
>
> I haven't worked with anything over 14.4 KV AC (20 KV peak DC) so I can't comment directly on your question. I do see two significant problems here though. The first problem will be corona loss from the 30 KV. It will be quite difficult to minimize this in an open-air system (no oil). The second problem is that the desire to charge your capacitor to 30 KV will make it even more difficult for you to find suitable capacitors. Using the 3x rule of thumb for oil-filled pulse capacitors, you would need a capacitor rated for a peak voltage of 90 KV ($$$$). This also will possibly make it more difficult to achieve the other specs that you want. It is all a careful balancing act.
>
> With regard to spark gap loss, it may not be as much as you think. I found a paper about a year ago that measured the gap losses in several different gases and several different pressures. As I recall, the peak current levels under test were quite high (1000s of amps). The gap loss was less than 100 watts as I recall. 100 watts on a 15 KVA tesla coil is quite small. Granted, the test apparatus was not a tesla coil spark gap and some things were different but it was close enough to yield some important information for us. I don't have the link to the paper anymore but if you search the archives during the last year you should be able to find my posting where I did include the link. You should probably search under "spark gap loss" or something like that.
>
> I would stick to your original peak voltage of 18 KV. Your life will be much easier.
Every few years, someone comes up with an inexpensive surplus X-ray
transformer (100kV kinds of voltages) - Life is really hard above around
30-40 kV - corona is a big issue, as is plain old breakdown.
Remember the rule of thumb is that you want 1" of clearance between
conductors per 10-15 kV - 30kV equipment tends be big and bulky.
Also consider the minimum conductor radius for no corona. Breakdown
field in air is about 70kV/inch. The field at the surface of a
conducting sphere V/radius, so a 1" diameter sphere at 35kV is right at
the breakdown - and a bit of surface roughness is all it takes.
Cylinders work pretty much like spheres -it's the smallest radius of
curvature that's important.
>
> Steve White
> Cedar Rapids, Iowa
>
> ----- Original Message -----
> From: "Gregory Peters" <greg.j.peters@xxxxxxxxx>
> To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
> Sent: Thursday, October 11, 2018 5:41:33 PM
> Subject: [TCML] Primary voltage considerations
>
> Hi All,
> I am building a 15kVA 3 phase DC coil and depending on transformer configuration I have the option to resonantly charge the primary cap to around 18kV or around 30kV.
> I would prefer 30kV as the gap losses and primary heating are significantly reduced. However, the most I have worked with in the past is 24kV. I am concerned unwanted arcing between components might become an issue - others have written about it getting hard to manage at these voltages. 24kv was no issue for me.
> Which would you choose? Any experience here?
> Regards,
> Greg
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