At 08:14 AM 1/17/2001 -0600, you wrote:
>Well, I don't automatically agree with the other two gents about this.
>First, the XFMR with the higher current rating fortunately also has the
>higher voltage rating. When you parallel the secondaries, they will
>adjust surprisingly well, compensating for a balance. Of course, you
>must phase them. As you load them, I think they will behave quite well
>at 1 Amp. There will be a slight current flow between the secondaries,
>but I predict it will be negligible and acceptable.
>
This could be a big assumption Roy. This can't be predicted (calculated)
without knowing the impedance of each winding to be paralleled, the
"percent" impedance of each transformer, and the voltage ratio of each
transformer. Because of the differences in these transformers, it's
conceivable that he could end up with a circulating current of 0.1 to 0.3
amps (maybe more) in the 2 windings. This circulating current will be
present at all times - with or without load. This works in opposition to
his goal. Now it is possible by pure chance that the windings aren't too
dissimilar and he could get away with paralleling them. A circulating
current will still be present but if it isn't significant, he'll "get away"
with it.
Dual voltage pole (distribution) transformers have dual primary and
secondary windings. The windings are designed to be run in series or
parallel as needed by the utility. Essentially 2 transformers on a common
core. But these windings are purposely matched for impedance, voltage
ratio, etc. to allow proper parallel operation.
>If not, then a trick to balance them is to install a small "bucking"
>transformer in series with the primary of the higher voltage one, or an
>"adding" transformer in series with the primary of the lower voltage
>one. Often a filament transformer will suffice.
>
A "balance winding or coil" needs to be in the secondary circuit to work
properly. This is definitely a legit method of paralleling two dissimilar
windings and it ensures proper load sharing between the 2 windings. This
works but I didn't mention it the first time around because selecting a
balance coil is more complicated than just knowing the voltage difference
between the 2 windings. You're quite correct that a filament transformer
can be used for buck/boost purposes. But altering the output voltage of one
of the transformers only addresses one of the three factors required for
successful operation. And buck/boost on the primary of one transformer
won't balance the 2 secondary windings at all.
That's probably more than anyone wanted to know about that topic.
Phil
>Good luck,
>
>Roy K6XK/0
>
>
>
>
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>
>
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