[TCML] primary voltage
jimlux at earthlink.net
Thu Jan 7 17:38:57 MST 2010
>From: Brandon Hendershot <mrbrandman at cox.net>
>Sent: Jan 7, 2010 3:32 PM
>To: Tesla Coil Mailing List <tesla at pupman.com>
>Subject: Re: [TCML] primary voltage
>Reading your post was kind of disturbing! You said all that happens
>somewhere inbetween 30 and 100 kV?Is there a tighter more precise
>range you know of? Because my coils primary circuit voltage will be
>hovering around 36kV. Should I be too worried about insulating every
>little point and wire?
Why yes you should! Or at least worrying about making sure that they're large radius of curvature. They make little shiny balls (like split shot sinkers) that you can stick over solder joints, etc., to help. A bit of time browsing industrial HV supplier catalogs and websites will show you how professionals build HV test gear: http://www.rossengineeringcorp.com/ is a good source.. Glassman HV, etc. also.
Get a copy of the "North Report" (High Power Microwave Transmitters, by William North).. it's online in various places.
If it matters at all, I'm installing a Terry
>filter too. Those chunky resistors won't affect the issue much I assume?
>On Jan 7, 2010, at 1:13 PM, mddeming at aol.com wrote:
>> Hi Kevin,
>> While some people have experimented with primary voltages in to
>> 30-100 kV range, there are several distinct problems with primary
>> voltages much above 15 kV (rms).
>> 1) Costs: The number of caps in an MMC goes up as the square of the
>> voltage (twice V = 4 x number of caps). Above ~15 kV you are also
>> talking custom-made transformers: Cost and weight increase
>> exponentially with voltage.
Not necessarily. With higher Vpri, you don't need as much Cpri, to keep the same "energy per bang". Basically, capacitor cost is proportional to energy stored. 4 uF at 1 kV is the same energy as 1 uF at 2 kV. If you've got 1 uF, 1kV caps, you put 4 in parallel for the first case. You hook up 2x2 series parallel for the second case.
Now, for smaller Cpri, Lpri will need to increase to keep fRes the same. That means that Ipk is going to be lower, so less I^R loss and less Vgap loss (since Vgap is a fixed voltage plus a sqrt(I) term)
As far as cheap transformers, there are lots of 50-100kV X-ray transformers out there in the several kVA and up range. (100 mA @ 110kV is a pretty common ballpark)
>> 2) Corona problems: above about 20 kV, every point, twist, kink,
>> bend, or screw head in the primary wiring becomes a source of corona
>> leakage which is power lost.(but the blue glow looks "cool".to
>> some). These losses increase rapidly with voltage level.
This is going to be the biggest problem.
>> 3) Insulation breakdown: most HV wire tops out at 30-40 kV then you
>> start needing to get into X-ray equipment cables, or custom, or home-
>> made cables made from coax. Even wire run through plastic tubing
>> starts to have problems at higher voltages.
1/2" coax (RG-8, RG-213) works ok at 30kV, and even up to 50kV if you're careful. At least if the insulation fails, the shield is grounded, so the cable just starts smoking, but you don't have 50kV shorting to something else.
real HV cable is available for this, it has a semiconducting layer to make the effective diameter greater, and very careful internal design. I doubt you could make it at home. Plastic tubing breaks down pretty fast, unless it's much larger and basically you're using it to hold oil. You inevitably have an air gap between conductor and insulator with tubing, and you'll get corona there, then treeing, then total breakdown.
>> 4) Unintended Coupling: as voltages go up, there is an ever
>> increasing tendency of currents in the wire to couple capacitively
>> or inductively to nearby objects and power, telephone,etc. lines,
>> charging them to "unpleasant" levels and wasting spark energy doing
I don't know about that. At these frequencies the dominant coupling mechanism is probably going to be magnetic fields, which are proportional to current. Electrostatic charging by induction (or just with charge spraying off and accumulating elsewhere) *is* a big problem with HV DC, but with AC, it isn't as bad. The E field from your secondary is probably much larger than the Efield from your primary, in any case.
>> In short, it is much more cost, weight, and safety efficient to keep
>> primary voltages at the level of mass-produced transformers and
>> minimize the number of caps needed to still keep a good working
>> Hope this helps,
>> Matt D.
>> -----Original Message-----
>> From: makinglightning at comcast.net
>> To: Tesla Coil Mailing List <tesla at pupman.com>
>> Sent: Thu, Jan 7, 2010 12:27 pm
>> Subject: [TCML] primary voltage
>> What is the practical upper limit on the primary tank circuit?
>> What is the highest primary tank voltage that people have used?
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