Carl,
With much lower core loss and less than half the primary power and half
the secondary power consumed the regulation should be much better than
the 15% Ameritron rates it at in the 811H.
No. Unless you managed to change the laws of physics.
The voltage sag in a well made transformer (which I hope this is, but I
don't know) comes mostly from the resistance of the windings, and only
to a much lesser degree from the leakage inductances. The resistance of
the windings is the same regardless of operation voltage, and the
leakage inductance is also pretty constant, except when the core
saturates, which is not the case here.
So, the _absolute_ voltage sag in this transformer depends on the
_current_ it is delivering, and not on the power. It will suffer
essentially the same absolute voltage drop at a given load current,
regardless of the voltage level at which it is operating. So, the
_relative_ (or percentual) voltage drop at a given load current is twice
as bad when the transformer is running at half the voltage.
In intermittent duty at half voltage you should be able to pull a bit
more current at that 15% regulation but temperature rise should be
measured first.
It's true that from the thermal point of view you can pull _slightly_
more current when running at half the voltage, as long as the voltage
sag is acceptable for the application. Since the core loss will be much
lower at half the voltage, you can afford some more loss in the
windings. But only some more, not much more, because transformer
windings, specially in larger transformers, are usually limited by the
thermal conductivity from the winding center to its surfaces. Working at
half voltage, the core will stay much cooler, so the windings can
dissipate some heat through the core, but you will soon run against the
limit given by thermal conductivity between winding layers.
But there is more to it: The core loss of a given transformer depends on
applied voltage and frequency, so it stays essentially constant
regardless of load current, except for the minor effect of voltage drop
in the primary winding slightly _reducing_ flux density and thus core
loss, when operating at higher current. Instead the loss in the windings
depends on the square of the load current.
Transformers designed to stay idle for much of the time, then deliver
full current for short times only, are optimally designed by accepting a
peak loss in the windings that is much higher than the constant core
loss. A typical HV transformer for a linear amp might have a core loss
of 20 watts or so, almost constant as long as the amplifier is on, while
the loss in the windings will vary from almost zero while the amp is
idling (only a tiny load due to the bleeders), to as much as 200 or 300
watts during peak load! So, during relatively high duty cycle operation
the transformer will get hot, while during low duty cycle use it will
stay pretty lukewarm, and during long time idling it will be only
slightly cooler than that. The alternative would be designing a
transformer for higher core loss, which allows somewhat lower copper
loss. That would be done with a transformer intended to run at full
power whenever it is on, but definitely not for a ham linear amplifier!
So, with a transformer designed for ham amps, saving most of the core
loss by running at half voltage will allow only a _slightly_ higher
power loss in the windings, and due to the quadratic dependance of
wiring loss on current, that will allow only a _very slightly_ higher
output current, before overheating the windings and making the
insulation fail.
And of course, higher load current makes the voltage sag problem larger too.
The bottom line: When using a given transformer at half its design
voltage, it can be used only very slightly above half its power rating,
and the percentual voltage sag will be twice as bad. On the positive
side of things, its loss during idle time will be extremely low. And the
exact amounts of these effects depend largely on the tradeoff between
core loss and wire loss chosen by the designer of the transformer.
Manfred
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