Ok, I thought the phase (with respect to the primary) could be varied
by the lead dress off the secondary, and how they come off the core.
That is, the position of the exit with respect to the alignment of the field
at any given instant. If two transformers have different winding exits,
won't they have different phases with respect to the input and each
other? Or is the output phase completely dependent on the primary
phase? My suspicion is that the output may be something besides
0 and 180. Heck this is easy to test with a dual-trace scope on both
Xfmrs (with appropriate resistive voltage dividers).
So if the secondary output voltages are not in phase, the composite
DC voltage (paralleled after the rectifiers) may have a non-symmetrical
ripple component. Although the common capacitor bank will want
to smooth things out, in this condition, the peak current from each
winding will occur out of phase from each other. So we have to
be more respectful of the current rating of the windings (especially
if we reduce the amount of C in the bank).
We have to respect the peak current rating for a center taped full-
wave output because each half of the windings carries full current
on different half cycles.
Jim
> > I should have added that if the DC is paralleled after the rectifies,
> > this phase difference could work to your advantage. If the two are out
> > of phase, the caps will tend to get charged more often, reducing
> > ripple. However, I still think this is a bad idea - not one I would
> > do.
>
> It makes no difference if the rectifiers are full wave.
>
> The caps charge from both rectifiers 120 times per second in the
> USA, and 100 in countries who have slower dynamo's.
> 73, Tom W8JI
> w8ji@contesting.com
Sorry, but I don't agree with that statement either Tom. Why do you
think 3-phase rectification has lower ripple than single phase ? If one
bridge is starting to provide less voltage to the caps, as another one
is providing more, the average voltage will be higher, and so the ripple
less. If you draw it out, I'm sure you would agree.
*If* you could get the two transformers out of phase by 90 deg, and
rectified that with two full wave bridges (on one each transformer), you
would get the caps replenished at 2f (120 Hz in the USA) from one
bridge, and 120 Hz from another, *but at different time*. Hence the caps
get replenished at 4f (240 Hz).
--
Dr. David Kirkby Ph.D,
email: drkirkby@ntlwold.com (formally davek@medphys.ucl.ac.uk)
web page: http://www.david-kirkby.co.uk
Amateur radio callsign: G8WRB
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