An interesting explanation of how a MOT compares with other
transformers. Thanks Manfred!
I guess that microwave oven transformers designers also try to use as
little copper as possible, because of its high price.
Measuring the voltage produced by the heating secondary, I found that
the transformers are based on 1 Volt per turn.
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.
If we had only de-watted current here, I guess that the current increase
would be linear: voltage across an impedance means a current is
circulating. It's the Ohm's law. So it should be linear.
I did not leave the transformer on long enough, so I didn't notice any
heating.
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. These ovens are about 800 W units,
so using 2 transformers would be just right for maximum legal power.
As usually, the end of the secondary that is normally connected to the
core will be lifted, isolated and hipoted for safety. I will tie this
two ends together.
Vy 73 de Angel HB9SLV
Manfred Mornhinweg a écrit :
> Dear all,
>
> it seems that a little basic information about transformers could be
> good here.
>
>
>> I have also seen these microwave transformers draw a lot of primary
>> current with no load connected to the secondary, a lot more, in the
>> order of 3 to 5 amps..
>>
>
> Yes, microwave oven transformers will indeed typically take more primary
> current at no load, than some other transformers. The reason is that
> they are designed for very short duty cycles. Below is more insight.
>
> When designing a transformer, one of the basic decisions one has to make
> is how much flux density to put into the core. There are clear
> tradeoffs. A higher flux density is obtained with proportionally fewer
> turns. So, designing for higher flux density will make the copper loss
> squarely lower, because the wire is both shorter and thicker, assuming a
> given core size. At the same time, iron loss will be higher, also
> roughly to the square of flux density. So, by choosing flux density one
> can trade iron loss for copper loss. In addition, fewer turns result in
> much lower inductance, and thus much increased magnetizing current (the
> one that is 90 degrees off-phase and thus means no power consumption).
>
> Now the point is that iron loss is present all the time, regardless of
> load level. Copper loss instead is linked to the square of the load
> current. So, when a transformer is idling, iron loss is important and
> copper loss is not, and while a transformer is delivering a high
> current, both are important but the rapidly increasing copper loss can
> easily dominate.
>
> For this reason, a transformer designed to be be plugged in for long
> times but actually delivering current only for short times, such as that
> for an ICAS amplifier, should be designed with a relatively low flux
> density. Instead, a transformer that will deliver full load whenever it
> is energized, such as a microwave oven transformer, is better off using
> a high flux density. So, a designer, wishing to minimize the size of his
> microwave oven transformer, will push for a very high flux density,
> because the high iron loss and also the resulting high magnetization
> current are perfectly acceptable in view of the much lower copper loss.
> Instead, someone making a linear amplifier needs to care a lot for the
> no-load dissipation, because after all the amplifier will be idling for
> easily 90% of the total on time! So, for the amp it's acceptable to have
> high copper loss during the brief moments of full power operation, in
> exchange for very low losses during the long periods of standby, or
> lower levels in the modulation curve.
>
> Some microwave oven transformers might overheat even with zero load,
> when plugged in for hours. Others will not overheat from this, but in
> any case will get pretty hot. A transformer designed for a linear
> amplifier instead should get barely warm while idling. On the other
> hand, the voltage regulation of a microwave oven transformer will be
> better than that of a similarly sized transformer designed for an amp,
> due to the lower wire resistance.
>
> My advice: If you like to build amps and have a cheap source of
> microwave oven transformers, by all means try them, but whenever
> acceptable arrange the circuit to power down the transformer during RX!
>
> Manfred.
>
> ========================
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> http://ludens.cl
> ========================
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