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RE: [TCML] Re: Hot Primary
I believe Greg's twin coils only have a one-turn primary; therefore
heating of the inner turn would not be an issue, right?
An absolutely astute blond observation by Hank
-----Original Message-----
From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
Behalf Of Steve Ward
Sent: Tuesday, February 23, 2010 10:18 AM
To: Tesla Coil Mailing List
Subject: Re: [TCML] Re: Hot Primary
Hi Greg
On Mon, Feb 22, 2010 at 9:56 PM, Greg Leyh <lod@xxxxxxxxxxx> wrote:
> Dave Leddon wrote:
>
> As I push my DRSSTC into ever higher currents (presently limited to
3000
>>> amps) I am experiencing some significant heating in the primary
circuit. I
>>> can see that in the future, as we push the envelop with even higher
power
>>> silicon, we may have to resort to water cooling. Even with 3/4-inch
od type
>>> L soft copper water pipe (was that ever a bear to bend) the primary
becomes
>>> too hot to touch after running the coil for one minute. I even
changed the
>>> interconnecting wires from #4 to #2 welding cable to prevent
meltdown, but
>>> oddly the larger wire gets just as hot. I wonder if there isn't
some sort
>>> of rf heating effect unrelated to voltage drop which is generating
some of
>>> the heat. A related issue which has been bugging me for some time,
why are
>>> the active primary turns progressively warmer as you move inward
toward the
>>> secondary? One would think that all turns carry the same current.
>>>
>>
>
>
> I'm unfamiliar with the particulars of your primary drive circuit, but
it
> seems to me that there shouldn't be *any* noticeable heating of the
copper
> at all, otherwise the unloaded Q of the primary will be very low.
>
Hopefully we can increase your familiarity ;-). The DRSSTC has the
capacity
of running much longer drive periods, whereas transient excited systems
are
limited by losses and coupling coefficients. The capacity for running
many
cycles of RF per spark event, allows the operator to really drive up the
primary RMS current to huge levels because the duty cycle can be
significantly higher. But back to numbers... the Q for this system is
easily more than 10, as im betting his primary voltage is about 10X the
H-bridge output voltage when he hits 3kA, and with no secondary in place
im
guessing the primary envelope is a fairly straight ramp, meaning the Q
is
likely in the 30-60 range at least. Perhaps if Dave has a DSO he could
capture his primary current shape (with no secondary in place) ramping
up to
3kA. It would be important to capture the bus voltage as well (because
bus
voltage sag would make the primary current ramp taper off, artificially
looking like lower Q). From there we could determine the actual Rac of
the
primary, at least to some degree of certainty based on how good the data
is.
>
> When I first brought up the twin prototype, I noticed the unloaded Q
of the
> primary circuits was horribly low... about 10. Even then, the IGBT
heatsink
> was the only thing that got warm to the touch.
Can you give us some idea of what the RMS primary current was? I doubt
its
near the 100's of amps that Dave is likely operating at. DRSSTCs do pay
the
penalty for higher RMS currents, though this is often a design issue, or
the
operator just pushing the system very hard. I bet Dave could run at
half
the primary peak current and a lower duty cycle and still make really
nice
sparks at a higher efficiency, but sometimes its more fun to push things
:-).
> To raise the Q, I had to beef up the copper in key areas. I
discovered
> the weak points by passing 100A DC continuously through the entire
primary
> circuit, and surveying IR drops point-to-point with a DVM. After a
bit of
> copperwork I got the unloaded Q above 30 and stopped there, since the
loaded
> Q is well below 10. The primary is a single turn of 6" Cu strap, as
can be
> seen near the bottom of this pic:
>
I like the idea of this method, since it allows for convenient and safe
measurement of purely resistive loss. I might give that a try on some
of my
systems.
Steve
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