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wrote in message
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In alt.engineering.electrical Don Kelly wrote:
| Yes -you are shorting a part of the winding but the switching is a bit
more
| complex than that so that short circuit currents are limited to
reasonable
| values. It is a multistep operation with reactor switching. On-load tap
| changers are expensive and are generally limited to applications where
this
| is absolutely needed (I have seen one where the tap changer was nearly
as
| large as the transformer).
I was thinking of what I might do to get some fine voltage control within
a
very limited range around 120 volts. The obvious option was a 0-140 volt
variable transformer. But I wanted to make sure I had a setup that could
be better limited, for example, to not allow an accidental too low
voltage.
I also didn't want to run all the power through the variable. So what I
was going to do was get a smaller variable transformer, and two buck-boost
transformers. One transformer would be wired 120-16 in buck mode to drop
the voltage down to 104. The other transformer would be wired 120-24 and
supplied via the 0-140 variable transformer, giving me a 0-28 variable
boost.
The end result is 104-132 over the full range of variable transformer
control
(assuming the boost transformer has no issues with being overfed at 140V).
So I might envision a transformer where the taps can be part of a boost
transformer added to the main transformer. The first buck transformer in
my above example would not be needed because the main transformer would be
designed with a 1st secondary at the lowest voltage of the adjustable
range.
A 2nd secondary on the same main transformer would have the adjustable
taps
and it would feed a separate boost transformer which has a secondary wired
in series with the 1st secondary of the main. So the taps would only be
dealing directly with a fraction of the power (assuming there is no back
feed issue involved) based on the needed adjustment range.
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| Phil Howard KA9WGN (email for humans: first name in lower case at
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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.
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.
Don Kelly
remove the X to answer