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On 7/15/06 at 12:01 AM Tom W8JI wrote:
>> The Heathkit 220 and a few other models had around 15%
>> regulation which is right on the edge of
>> saturation (they ran a high flux density to save on weight
>> and cost).
>
>The SB220 transformer was designed for a kilowatt INPUT DC,
>which was the legal input when it was designed. This was
>about 450 mA at about 2200 volts. It was a good conservative
>design for that power level. At rated power on a good power
>line it sags less than 10%.
Why does Heathkit show in their manuals the open circuit voltage, and the full
load voltage that
calculates to 15% sag (OCV / FLV )? They show this in several of their models.
The maximum flux density can be calculated easily by the core size which I've
done. Even for using 3.5%
silicon steel lams, Bmax comes up to around 16 kilogauss. After 15 kilogauss,
the magnetizing current
greatly increases and it's recommened to be ran at 15 kilogauss or under though
it supposed to be able
to run 17 kilogauss max. 4% silicon steel should be ran at about 14 kilogauss
or under. I'm not
saying the transformer is designed wrong. It was designed to save on weight and
money. However,
the end user will feel the difference in his wallet when they receive their
power bill, and the regulation
could be better. Without knowing what the actual iron is they used in the core,
a conclusion can only be
guessed at as to the suitability of the design.
>
>The problem is most people seem to think it was designed for
>1000-1300 watts CW output, and they expect a 1000 watt INPUT
>power design to be stellar at two or three times the rated
>CW dc input.
>
>>They don't saturate, but under full
>> load they're cutting it pretty close.
>
>A regular power transformer certainly does not work the way
>that statement might lead us to believe.
I should have worded this different. No they don't saturate under load.
However, the
amount of iron is what controls the power output in watts. Not enough iron and
you have
a high flux density, a higher magnetizing current, and more sag. You also have
increased heat
both in the iron and the windings. It's best to run them at a lower flux
density to keep
everything cool and provide a longer life for the transformer plus help with
the sag. The sag is
controlled by the transformer design and other components.
>
>Maximum flux density, which means closest operation to
>saturation, occurs with NO load. As load is increased flux
>levels do not increase. Increased current, because of
>resistive losses in the primary circuit, actually causes the
>transformer's flux level to decrease. When a transformer is
>designed the highest primary voltage under no load is used
>to set flux density at a safe level.
correct
>
>The actual mechanism inside a transformer is the secondary
>develops a counter-MMF. This opposing flux would reduce flux
>density, but primary current increases in order to try and
>maintain the **same** flux density. When mains and primary
>resistances carry more current from increased load, the
>primary voltage drops slightly. This REDUCES flux density,
>moving the transformer further from saturation. Not closer
>to it.
correct
>
>If the transformer had a separate secondary winding with
>constant current you would actually see the voltage on it
>decrease with any increase in load on the secondary with
>variable load. This voltage decrease is because flux density
>decreased.
>
>73 Tom
>
>
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Best,
Will
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