Again a crystal clear explanation. Thanks again Manfred!
So I guess the only remaining option will be to remove the heating
winding which is located between the primary and the secondary and
replace it with about 50 turns of enameled wire of the same size as the
primary and wire it in series with the primary. Steve Thompson also
offered this solution on this list. Thanks Steve!
Then of course I will probably have about 500 V less at the secondary.
This would be too much. I need 3000 V for my GS-35B. But it would be
just fine for a pair of 4CX250Bs.
Maybe an intermediate solution with only a few turns?
Back to the workbench. Time to experiment again!
Thanks to all who responded.
Angel Vilaseca HB9SLV
Manfred Mornhinweg a écrit :
> Hi Angel,
>
>> I guess that microwave oven transformers designers also try to use as
>> little copper as possible, because of its high price.
>
> Yes, of course they do! This approach generally leads to rather
> longish transformers (laminastion stack much taller than the with of
> the center leg). Also, such transformers are more suited to low copper
> loss and high iron loss - exactly what is needed for a transformer
> that will work at full load whenever it's plugged in. On this topic,
> it's interesting to note that a transformer with a rather square
> center leg tends to be better suited for intermittent or low load,
> while one with a tall stack (highly rectangular center leg) is more
> suited for full load.
>
>> When I measured the curent across the primary of the microwave oven
>> transformers, I increased the voltage gradually.
>> From 0 to about 110 V, the curve voltage / current was straight
>> (linear). But from 110 to 220V, the curve is not linear any more, it
>> is definitely parabolic. Exactly as predicted:
>>
>>> At the same time, iron loss will be higher, also roughly to the
>>> square of flux density.
>
> This "parabolic" curve has nothing to do with the losses! It's simply
> a copy of the magnetization curve of the iron. This curve tends to be
> roughly linear up to about 0.8 Tesla, then it starts bending to a
> flatter slope as its permeability decreases due to the increased
> magnetization. The curve will eventually get essentially horizontal at
> something above 2 Tesla, a level at which you are left with the
> permeability of air, and thus an immense primary current!
>
> The fact that you see the current curve starting to bend at about 110V
> means that at this voltage the transformer is working at about 0.8
> Tesla. So, at 220V it's working at around 1.6 Tesla - a pretty usual
> figure for such transformers that are run at full load for a minute
> and then switched off.
>
> Note that the exact values, which I gave as 0.8, 1.6 and 2 Tesla,
> change according to the exact steel alloy used, but these values are
> quite typical.
>
> That the losses also change roughly to the square of the flux density
> and thus to the voltage, is merely incidental. To really measure the
> losses, you would have to measure the real part of the primary
> current, either by using a real power meter, or by measuring the
> magnitude and phase angle of the current against the voltage on a
> scope, and calculating the real part in the current. This can be hard
> and imprecise, because the real part is usually much smaller than the
> reactive part.
>
>> If we had only de-watted current here, I guess that the current
>> increase would be linear:
>
> No. Even with lossless iron, the curve still would be nonlinear,
> rising fast as the iron approaches saturation.
>
>> voltage across an impedance means a current is circulating. It's the
>> Ohm's law. So it should be linear.
>
> Ohm's law is linear, but the impedance in this case is not constant.
> As you increase the voltage, and the iron gets closer to saturation,
> the impedance goes down. That's why the current increases more than
> proportionally to the voltage.
>
>> I did not leave the transformer on long enough, so I didn't notice
>> any heating.
>
> In a transformer that size, thermal stabilization will probably occur
> only after two hours or so.
>
>> So knowing all this, what I am going to try is this: two microwave
>> ovens with their primaries in series. This will decrease magnetic
>> flux. With both transformers about the same size, voltage should
>> divide evenly between the two primaries.
>> The secondaries will also be wired in series, so the total secondary
>> voltage should remain around 2200 V.
>
> That's correct. Or if you need half the voltage, you can connect the
> secondaries in parallel, which would force a better sharing between
> the transformers.
>
> > These ovens are about 800 W units,
>> so using 2 transformers would be just right for maximum legal power.
>
> Big mistake! By using the transformers in this way, you will have
> twice the resistance, so the voltage drop with load current will be
> twice as much as that of a single transformer. On the modulation peaks
> you should be able to overload the transformers by perhaps 30% or so,
> but running at half voltage, this still means just about 550W from
> each, or slightly over 1 kW from the pair - enough for a 700W output
> amp, but not for legal limit!
>
> The problem is that running these transformers at half their voltage
> is going too far! Running them at about 170V instead of 220V should be
> about optimal for amp use. But who has that line voltage...?
>
> If they were mine, I would probably rip them apart and rewind them for
> the exact specs I need! On the other hand, that isn't much economy,
> because the wire can't really be re-used, and the iron is cheap. The
> only sense is in actually getting a usable core, because it can be
> hard to locate a source that sells transformer laminations in small
> quantities.
> I bought material for two 10kVA transformers a few weeks ago, and the
> wire was almost 70% of the total bill, with the iron being just 20%!
> The remaining 10% was for insulating material, impregnating varnish,
> and other odds and ends. Copper is becoming excruciatingly expensive!
>
> Manfred.
>
> ========================
> Visit my hobby homepage!
> http://ludens.cl
> ========================
>
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