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Re: Sam Barros' 5000Watts Car Ignition coil update: tests, and
At 03:36 AM 2/22/99 -0700, you wrote:
>Original Poster: Sam Barros <sambarros-at-yahoo-dot-com>
> I am trying to calculate how much power, in theory, an ignition coil
>would be able to handle. I suppose I would have to look into how much
>current the wire can take, and than see if it is more, or less than
>what it takes to saturate the core... Any ideas there? The wire on the
>primary is 0.50mm diameter, about 240meters long, and oil-immersed.
>The resistance of the windings is 3Ohms.
I would consider the total resistance of the wire, core loss and thermal
effects. I measured these for my car coils (search the archive for my data &
comments) by measuring the 3db tuning bandwidth, from which I derived the
coil's Q, and from measuring reactance of the coil at the frequency in
question, using signal generator and scope, I found the effective resistance
(both AC and DC resistance) of which the AC was significantly greater,
probably due to the skin and magnetic parallel effects.
After you know the resistance, you figure the average power dissapation. I
then tried a crude thermal analysis. Using values of thermal conductivity &
dissapation for paper, oil & epoxy I estimated the power to create around a
200 degree C thermal gradient in the center of my car coil immersed in
flowing oil (driven by a CPU cooling fan). The coil has a laminated paper or
epoxy - copper structure, so I could just SWAG or guestimate.
I was dissapointed to calculate my coil couldn't handle over a few hundred
watts dissapation without burning. That means if I couple the coil to a
constant load, I could let the coil dissapate 200 Watts in heat while the
load gets 600 watts. I could probably push the power to 1000 watts for 5 -
10 second runs, then let it cool down for a while.
I figured it would be good to somehow measure the resistance of the copper
coil, using it as a thermistor, to let me know when I'd pushed it as far as
I could go. I planned to run without a core, letting the car coil feed a
capacitor, resonating around 8KHz. Search the archive for the rest. The
core, being a solenoid, will saturate gradualy, and eat half of you power.
>
> Is this correct for the maximum theoretical output of an ignition
>coil? And, if so, is di/dt given in Hz?
>
> V = L di/dt
>Where:
> V is the output voltage
> L is the inductance in Henrys
> di/dt is the rate of change of current flow as the field collapses in
>the coil.
Yes, but remember capacitance, which is very significant, will reduce that
value greatly.
> How about this for power output?
>
>P = (L di/dt)^2 / R
>where P is the power in watts and R is the total resistance of the
>coil secondary, the plug wire and the ionized spark gap.
P=V^2/R is for direct current/time. It will work if you integrate (average)
your pulse.
P=I^2 * R too.
You need to know Q (AC losses) core losses (which will depend on your power
levels)
Also, how
>important is the inductance of such a coil in coildriver applications?
>Did you all get it and didn't think it
>was worth a comment or was there a problem while I tried to send the
>file?
You must be using the new file naming convention, it left blanks in the file
name which irritated me when I had to use Microsh*t's Windowz Explorer to
rename & view.
My 2 cents is the best coil will have the best thermal dissapation
character. My TC efforts have stalled presently, when I was conteplating my
own induction coil based on an Allied Metglass core, with teflon insulated
windings, with forced convection oil immersed cooling. Being SCR pumped, Fo
around 4KHz, and 100KV + output voltage.
How practical capacitors and spark gaps will be at 100KV + potentials I hope
to find out.