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Re: Re : resistive V inductive ballast
Hi Scot,
At 08:35 PM 09/10/1999 -0500, you wrote:
>
>
>Tesla List wrote:
>
snip...
>
>as far as fully "resistive" components to control current input to a pig....
>I spent a small fortune on electric hot water heater elements with the
>intent to
>run them in paralell (but in series as a set to the pig) with switches to turn
>on and off each element. What i found was that yes it does control the current
>going into the pig BUT ... it also reduces the voltage ( the more elements
>turned on = more amps to the pig). It seems kind of defeating to go about it
>this way .... having great current control but to only see the supply
voltage
>going to the pig drop to 150 or so volts ( with a 40 or so amp current draw
>thru
>the elements) Output from my TC was at best very minimal , an occasional
single
>streamer of 24" would pop out. Previously I had my welding machine hooked
up in
>series with the pig... I got great results ... 6-8 foot long streamers or
>multiple 4-5 foot long bolts of miniature lightning ;) ... but alas the
>insulation on the primary windings of the welder got a bit toasty, needless to
>say i had to retire the welder from coil applications and return it to a
>life of
>welding only.
>
>
>Now here is my question.........
>
>why am i seeing such a drastic drop in voltage by going to a resistive ballast
>compared to the inductive ballast i was once using????
>
>
>Scot D
>
>
The key thing you should notice is that the resistors get hot while the
inductor stays fairly cool. The resistors work by dissipating extra power
in the form of heat. Real energy is being lost there.
The inductor limits the energy into the coil by changing the "phase" of the
voltage and current going into a transformer. If you have say 100 volts at
10 amps and the phase is the same between voltage and current, the power to
the coil is 1000 watts. However, if the phase is 90 degrees apart from
voltage and current, the delivered power is zero since the peak of the
voltage coincides with a zero current point. A book on AC electronics will
explain this better than I can but there is an equation that governs this:
Power = V x I x COS(b)
P = Power is the delivered power in watts
V = Voltage is the AC RMS voltage level
I = Current is the AC RMS current level
COS(b) = the cosine of the angle between the voltage and current waveforms.
At 90 degrees COS(90)=0 so no power goes through.
Now "theoretically", your resistive ballast and inductive ballast should be
able to give equal performance. However, inductors store energy just like
capacitors do where resistors do not. They also have saturation and
significant dynamic effects that resistors do not. This is why resistor
ballast systems seem to run so smoothly where inductive ballast systems can
get uneven with bumping and kicking. The interaction of inductors and
resistors in the primary of a pig system are fairly complex and they are
definitely not equal. We now have some nice computer tools to study such
things perhaps it is time to look into this further...
Cheers,
Terry
References: