| If I read you correctly, you want to use a second secondary (lower power
| rating) which is tapped and put in series with the main secondary. Now once
| you do this, you have in effect a single secondary with taps just as in a
| conventional tapped secondary. Sure the "tapped section" is lower power-
| because it is a lower voltage but it still has to handle the same current.
| Nothing is gained.
| The problem in tap changing is not "power" but the current being switched.
No, that is not what I tried to explain. I'll try again:
The main transformer would have 2 secondaries. These 2 secondaries are NOT
wired in series with each other. The smaller of these secondaries will have
taps. The tapped smaller secondary feeds another smaller transformer. The
larger secondary of the main transformer, and the only secondary of the smaller
auxiliary transformer, would be wired in series. So the taps are only dealing
with the current of the lower power "tapping section". The smaller secondary
of the main transformer, and the primary of the auxiliary transformer, can be
wired for whatever voltage/current works out best.
| In either case the voltage driving short circuit current on tap changing is
| that between taps
| Delta V =A(delta n) Delta Z =B(delta n)^2. where delta n is the change in
| turns between taps. The short circuit current on such a change will be
| proportional to 1/(delta n).
| If you want fine control, then you could go to sliding carbon brush as in a
| variac. The first idea of a separate transformer feeding a variac will not
| solve the "too low" voltage problem of the variac because you are still
| dealing with an autotransformer.
In that first scheme, adjusting the variac to the lowest voltage would be
reducing the voltage contributed by the boost transformer. There is still
the original supply voltage going around the variac, "plus" (actually minus)
the buck voltage (to select the range I want). Since the variac is an
autotransformer itself, it merely feeds the primary of the boost transformer.
Note that in this case the "boost" transformer is wired as an isolation
transformer. I should have mentioned that. If needed, I guess I could draw
some ASCII diagrams or try to get something made graphically (all the tools
I have to do that suck, except for Visio which needs Windows to run and I
don't have a spare machine to do that at the moment).
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