Dave,
Also I forgot to mention, yes, I know a resistor will waste some power as
heat. However, my comment was based on stuff I read from Richard Hull:
"We found the best control of the input energy was secured by mixing both
resistive and inductive ballast in series with the power transorfmers
primary winding. The resistance is often well under 1 ohm and the
inductance under 5 milliHenries. These values are typical and will vary
from system to system.
Without power conditioning, the main transformer will saturate and power
will be wasted. If pure inductance is used, a see-saw action of the
transformer and ballast can result in some frightening current exchanges
between the two and uneven operation will result. Using only resistive
ballasting, a large amount of energy is lost in heat. With a balance of the
two, energy is saved, smooth operation is secured, and very long run times
are possible at high powers."
Source: http://qsl.net/f3wm/tesla/magnifier.html
So if we calculate power (heat) based on a 5KVA system (240v at 20a,
assuming 100% PFC) and use .333 ohm* resistance, using this formula:
P = (I^2) * R
P = (20*20) * .5 = 133 watts
*I think David Rieben is using three 1ohm resistors in parallel (in
addition to an inductive ballast) on his Green Monster Coil:
https://youtu.be/t23mGSW2k6c
From my other readings, regardless of (resistor) heat, maximum power
transfer should occur when the ballast and transformer are impedance
matched.
Thanks,
~Dan
On Tue, Jun 5, 2018 at 9:21 PM, David Speck <Dave@xxxxxxxxxxxxxxxx> wrote:
> Dan,
>
> A shorted welding transformer stores energy in an expanding magnetic field
> which is then returned to the power circuit as the sine wave of the input
> power waveform decays, kinda like compressing a spring. When you relieve
> the pressure on the spring, the spring pushes the energy back into your
> system.
>
> If you use a resistor, the resistor converts the energy to heat, which is
> lost, and not returned to the system. This is much less efficient. It
> would provide ballasting, at the expense of a lot of waste heat. Any
> resistance in the ballast components will generate waste heat, so you want
> to minimize that as much as possible.
>
> If you want an easily and infinitely adjustable ballast, you would use a
> saturable reactor. This is a special transformer which would perform the
> same function as a shorted welder, but in which the permitted current is
> controllable by a small low voltage DC current applied to a special set of
> control windings.
>
> Saturable reactors provide nice control for TC systems, but ones big
> enough to effectively control a pole pig can easily weigh half a ton, and
> are usually very expensive, unless you can find a surplus unit for cheap.
>
> Last spring, I bought an assembly nearly identical to the following one
> for $130 for the copper scrap value, but it cost me $580 in shipping fees
> to get it to my friend's factory shipping dock. "Some day" I hope to build
> a coil big enough to put it to effective use.
>
> https://www.ebay.com/itm/40-KVA-20-AMP-460-VOLT-HUNTERDON-SA
> TURABLE-CORE-REACTOR-TRANSFORMER-S-1207-1/292572873068
>
> The reactor that was in it looks just like the following one, but is mine
> "only" 52 KVA rated. I think there is a typo in the listing. The rating
> should be more like 60 KVA, not 600. I would speculate that a 600 KVA core
> would likely weigh several tons.
>
> https://www.ebay.com/itm/NEW-Hunterdon-Saturable-Core-Reacto
> r-Transformer-600KVA-60Hz-WE-1097-R-C98R2/282780054079
>
> (I have no connection to either seller.)
>
> You can buy a lot of welders to short out for $4,800 plus shipping.
>
> To understand saturable reactors, look up "magnetic amplifiers" in
> Wikipedia.
>
> Dave
>
>
> On 6/5/2018 11:22 AM, Daniel Kunkel wrote:
>
>> As already pointed out, for a current limiting ballast transformer, the
>> secondary coil will be shorted out, and the primary will be in series with
>> the load.
>>
>> However, I have been wondering what would happen if you put a (variable)
>> resistor on the secondary side of the ballast transformer (instead of a
>> dead short). I wonder if this would give you some degree of variable
>> current limiting? From all my reading, it seems like the best current
>> limiting uses BOTH inductance and resistance...so I wonder if resistance
>> located in this location would work.
>> ~Dan
>>
>
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