Manfred Mornhinweg wrote:
> 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.
>
If I recall correctly, (and its a very long time since I did my EE
degree), it can be shown that a transformer has maximum efficiency when
the copper loss is equal to the iron loss. Hence if either one is higher
than the other, the efficiency suffers. As you say, if the load is not
constant. things will get more complex - at zero load, the efficiency is
zero of course.
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