Re: TIG inverter project - new thread
- From: Glen Walpert <gwalpert@xxxxxxxxxx>
- Date: Wed, 02 Nov 2005 13:56:25 GMT
On 1 Nov 2005 03:55:03 -0600, The Phantom wrote:
>On Mon, 31 Oct 2005 14:36:38 GMT, Glen Walpert wrote:
>>>>> I think the way to characterize a "full wave" recitifier is that it draws
>>>>>pulses of current from both the positive and negative polarity of the grid
>>>>>voltage, be it single or 3-phase. Thus, the arrangement in the welder
>>>>>which has the similarity to a single phase center-tapped topology is a
>>>>>"full wave" rectifier in this sense, and so is the 6-phase bridge
>>>>>arrangement.
>>
>>I would be inclined say that a full wave rectifier draws current on
>>the positive and negative polarity of the *rectifier input*, not the
>>line input to its transformer. By this definition the 6-phase design
>>is half wave, all of the transformer secondary windings see current in
>>one direction only even though the primary windings see current in
>>both directions. But this is inconsistent with the terminology used
>>in single phase CT xfmr 2-diode config, so perhaps you are right here
>>and both the 6-phase and 3-phase bridge rectifiers should be referred
>>to as full wave.
>
> Does the term "full wave" even get used in the standard descriptions of these
>topologies?
Outside of this thread? Not sure.
>>The 6-phase configuration has 6 different phases present on the
>>rectifier input, measured WRT the transformer secondary neutral. The
>>3-phase bridge has only 3 phases on the rectifier input measured WRT
>>the transformer neutral, regardless of having the same ripple as the
>>6-phase rectifier.
>
> So would you call the ripple at the output of a 3-phase bridge "6-phase"
>ripple, or "3-phase" ripple?
Ripple with a 360 Hz fundamental. Once you have the ripple the
circuit topology which produced no longer matters.
>>>>Agreed. Perhaps worth noting that, like the single phase
>>>>center-tapped transformer 2-diode full wave rectifier, the old
>>>>"6-phase rectifier" circuit is essentially obsolete because it makes
>>>>inefficient use of transformer iron and copper, and the cost of the
>>>>high side drivers is less than the cost of the larger transformer for
>>>>high power apps today. Perhaps this is why the 6-phase and 12-phase
>>>>designs are not mentioned in recent EE textbooks.
>>>
>>> So do people just not do 12-phase rectification any more? Maybe only the
>>>*really* big installations, such as an aluminum plant.
>>
>>Could be, I am not familiar with such really big installations, but in
>>addition to the drawback of inefficient transformer utilization, the
>>more phases the worse the power factor,
>
> Why is the power factor worse with more phases and how much worse is it?
Presumably because of smaller conduction angles - more phases, less
ripple amplitude, less conduction at lower voltages. Somewhere
recently I saw a graph of the power factor vs. load for 3, 6 and 12
phase rectifiers but I can't seem to find it now. IIRC 6 was less
than twice as bad as 3 and 12 was much less than twice as bad as 6;
something like .9 for 3 phase, .84 for 6 and .8 for 12, at some
unrecalled load, better at full load and worse at partial loads but
retaining the same proportions.
.
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