Joe Koberg wrote:

> On 2011-05-11 19:38, Robert Rolf wrote:
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>>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.
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> 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?

  >>Some simple minded lamp dimmers just use a power diode to cut out half
>>the power line cycles to get 50% dimming.
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> But I suspect those would be frowned upon as well.


Sure, but WHY?

>>Some power engineer care to enlighten us?
>>
>>R
>>
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> I found this page that mentions it:=20
> http://sound.westhost.com/articles/xfmr-dc.htm
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>     There are a few older household appliances that can create a DC
>     offset, although most are (probably) no longer permitted due to
>     increasing problems caused by the DC component.
>     ...
>     Most of the time, the DC offset is transient - it appears for a
>     short while, then goes away again. When it is there, toroidal
>     transformers may complain loudly, by making growling or buzzing
>     noises. It is important to understand just how this happens, and
>     what can be done about it if it causes problems.
>     ...
>     With larger transformers (500VA and above), the DC resistance is
>     usually so low that even a very small offset will cause mechanical
>     noise due to saturation.


This suggests that the problem is due to a DC BIAS on the line, causing=20
the core to saturate on one side of the cycles. But if you are only=20
drawing current on one half of the cycles, HOW is that an issue? It's=20
not like the transformer 'averages' the input current. The DI/Dt will=20
still be the same, given that the primary has low impedance to DC.

I just played with an audio transformer driving a diode. I see 'normal'=20
half cycles and clipped other half cycles. The input side looks a bit=20
asymmetric (from reflected load) but so what? We're talking POWER lines=20
here, where I can't see half cycles being an issue. Don't industrial=20
heaters (KW and up) do half cycles?

If this really is an issue I would like understand the WHY,  and if it=20
is something to avoid doing.

Thanks.

R



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