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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