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Re: MOTs...exploring the low end of TC voltage
Hi, Malcom.
Comments interspersed.
> Original Poster: "Malcolm Watts" <malcolm.watts-at-wnp.ac.nz>
>
> > The core will tend to saturate very easily because even though the
> > voltage differential between turns is the same as a first stage Mot,
> > (i.e. the gradations between 2kv and 4kv are the same as those
> > between 0 and 2kv), the level of power being handled is higher.
>
> Is the voltage across the winding the same though? Is the current through
> the winding the same as the first stage transformer?
I'm not exactly sure. My impression is that bottom feeding the second
stage tranny with 2kv is equivalent to running the primary at overvoltage.
Perhaps this is because the current fed into the secondary by the first
stage transformer contributes towards the number of flux lines through the
core.
> In simple 1-1 arrays, the second stage
> > would get warm, and the third stage, if there was one, would fail
> > within 1-2 minutes. So it is not simply a matter of core-coil
> > breakdown. With the solution I have now, the second 2 Mot stage is
> > ice cold, and it is the first stage which gets warm.
> Interesting results. If the secondary windings are connected in
> series then the currents through them must be identical and if the
> windings have the same number of turns and the cores are identical
> then identical Ampere-turns must be applied and those windings at
> least must be generating the same flux density in each core. What
> appears to be getting hottest - the secondary winding/s, primary or
> core?
The primary and the core get hottest. The only problems I have noted in
the secondaries result from core-coil breakdown, or complete
saturation--when both the primary and the secondary burn up.
I guess I would compare the 2 stage, 2 Mot case to the case of the
variac, which has to be derated to allow the entire 120V across the
winding. Dragging 2 kv into the secondary of a second Mot is like dragging
120v across the entire variac winding--it tends to lead to saturation unless
the core can be doubled in size (the 3 Mot case) or gapped. The reason the
3 Mot case doesn't saturate is that each of the 2 second stage Mots only
boost the voltage 1kv. From a design standpoint, taken together they ought
to look like a single Mot designed for 220 operation. In fact, in
comparison tests with an actual 220 Mot, they perform identically. They
handle about half the current at 110, (since the primary inductance is
double?), and about twice the voltage.
--Mike