Olin Lathrop wrote:

> Robert Rolf wrote:
>=20
>>>>Why would DC draw be bad? Full wave rectifier transformers draw DC
>>>>on each half of the secondary, so that can't be an issue.
>>>
>>>But that doesn't lead to a DC bias on the line side, does it?
>>
>>Does it matter WHICH transformer sees only half cycle loading?
>=20
>=20
> It matters whether the AC line sees the DC, since that's what we're talki=
ng
> about.  If the DC is in the secondary of a transformer which is not direc=
tly
> connected to the AC line, then it is irrelevant.

> Once again, the question was whether it's bad to draw net DC **from the A=
C
> power line**.  How exactly do you imagine a transformer-coupled full wave
> rectifier has any bearing on this?

My example of a transformer with rectifier was to compare what happens=20
to IT, with what happens to a power line transformer.
In the case of FWR, each HALF of the transformer winding sees only=20
unipolar currents, so DC in a winding cannot be the issue.

The case of half wave rectification (ONE DIODE) results in current being=20
drawn for only half a cycle, as would happen with the OP 50% duty cycle=20
with power line heaters, so again, the transformer secondary only sees=20
unipolar currents, as with HWR, and presumably the primary sees only=20
half cycle currents too.

Am I to understand that this unipolar current can somehow be reflected=20
back across to the primary, even thought there is no ADDITIONAL flux=20
change created by it being unipolar, and DC fluxes don't couple. And=20
that unipolar current draw is somehow a problem in a transformer=20
secondary, be it power line or rectifier?

> To be clear (since you weren't I'm going to pick something), the AC line =
is
> driving the transformer primary.  The secondary is center tapped with the
> center tied to ground and each end of the secondary drives the positive
> supply thru a diode.  Please explain how that presents a net DC load on t=
he
> AC line, or is even relevant to the discussion.  Invoking dead fish or ph=
ase
> of moon is not allowed.

I may be dense, but how does drawing current for only a half cycle=20
result in DC in a transformer so as to shift the flux operating point to=20
where is may saturate? I see it as being no different (flux wise) than=20
running the transformer with current in both directions. The flux STILL=20
gets to zero at twice the mains frequency. With half cycle use, it just=20
never gets to see much reverse flux. So what?


Which goes back to my original question, why is 1/2 cycle power=20
utilization a bad thing? How does it create problems in a transformer,=20
given that it IS NOT steady state DC which would shift the flux=20
operating point?

R




--=20
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