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Posted by Ignoramus11754 on May 30, 2006, 10:52 am
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On Tue, 30 May 2006 15:36:11 +0100, Pooh Bear
>
>
> Ignoramus11754 wrote:
>
>> On Tue, 30 May 2006 15:13:32 +0100, Pooh Bear
>> >
>> >
>> > Ignoramus11754 wrote:
>> >
>> >> On Tue, 30 May 2006 14:45:29 +0100, Pooh Bear
>> >> >
>> >> >
>> >> > Ignoramus11754 wrote:
>> >> >
>> >> >> My question is, can I do something such as adding (maybe dynamically)
>> >> >> some power factor correction capacitors, or some such, to correct that
>> >> >> power factor? Is such a thing doable relatively inexpensively?
>> >> >
>> >> > It's a bit complicated.
>> >> >
>> >> > It may not be mandatory for you to use PFC anyway.
>> >>
>> >> Are you saying that I am not required by my power company to do that?
>> >
>> > Not knowing the precise details I couldn't actually say but I
>> > wouldn't fret excessively over it right now.
>>
>> Yes, I am not worrying about the power company.
>>
>> >> I would agree, yes. The main reason for my question is that with some
>> >> PFC, I could use my [marginal] 60A garage circuit more effectively.
>> >
>> > Is the mains ac connected to a classic transformer, rectifier, capacitor DC
supply ?
>>
>> The scheme is as follows:
>>
>> Mains 240VAC 1 phase
>> ====> Phase Converter
>> ====> Welder
>>
>> The "Welder" is actually as follows:
>>
>> 3 phase in
>> ====> 3 phase transformer
>> ====> 3 phase rectifier bridge with SCRs (controlled by a "firing system"
>> ====> Welding reactor (inductor)
>> ====> Welding arc (or short as I tried)
>>
>> So, when I tried getting 200 amps into a short, I would get about 60A
>> current (14 kVa) coming from the single phase mains, even though the
>> power used and wasted in the system was no more than 2-3 kW.
>>
>> I am sure that power factor improves as welding arc voltage increases
>> from almost zero, but it is probably still low. It is hard to read
>> mains amperage when I have a arc going. :)
>>
>> I could get some real benefit in terms of blown breakers, and weld
>> capacity, I think, if I could increase my power factor.
>>
>> Hence my question.
>
> How does the phase converter work ?
It has two rotating idler motors (they work in parallel, you can
visualise it as having one idler). Rotating idler motor produces
voltage on the third leg.
There are also balancing and a small PF correction cap built into
phase converter, so that it does not draw too much current when not
loaded.
The welder, though, seems to have a very low PF at low arc voltage.
> Since your rectification is 3 phase you don't have the classic
> harmonics problem but I'm trying to fathom what the phase converter
> does to the load on a single phase supply.
I think that it is actually not messing up too much itself, it is the
welder that is the problem. SCRs "clip" parts of the incoming
AC sinewaves to get the necessary DC current.
i
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Posted by Pooh Bear on May 30, 2006, 10:59 am
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Ignoramus11754 wrote:
> I think that it is actually not messing up too much itself, it is the
> welder that is the problem. SCRs "clip" parts of the incoming
> AC sinewaves to get the necessary DC current.
Right. It's phase control. Can't see a simple way to fix that one.
Graham
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Posted by Ignoramus11754 on May 30, 2006, 11:22 am
Please log in for more thread options On Tue, 30 May 2006 15:59:42 +0100, Pooh Bear
>
>
> Ignoramus11754 wrote:
>
>> I think that it is actually not messing up too much itself, it is the
>> welder that is the problem. SCRs "clip" parts of the incoming
>> AC sinewaves to get the necessary DC current.
>
> Right. It's phase control. Can't see a simple way to fix that one.
I have been reading about this, and some people say that for welders,
capacitors could help.
i
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Posted by Terry Given on May 30, 2006, 6:26 pm
Please log in for more thread options Ignoramus11754 wrote:
> On Tue, 30 May 2006 15:59:42 +0100, Pooh Bear
>
>>
>>Ignoramus11754 wrote:
>>
>>
>>>I think that it is actually not messing up too much itself, it is the
>>>welder that is the problem. SCRs "clip" parts of the incoming
>>>AC sinewaves to get the necessary DC current.
>>
>>Right. It's phase control. Can't see a simple way to fix that one.
>
>
> I have been reading about this, and some people say that for welders,
> capacitors could help.
>
> i
>
You have two things happening. Firstly, the phase control gives you a
current waveform that is a horrible shape, not sinusoidal. If you do a
fourier transform of that icky shape, you will see it is rich in
harmonics. Those harmonics transfer no real power, only reactive power -
it circles in a loop from supply to welder and back again, doing no net
work - but cause I^2R losses, and of course voltage drop across the
supply inductance (which is probably fairly high with your phase converter).
Secondly, low arc voltages correspond to maximum load, which makes the
current waveform, and hence its harmonics, biggest.
It is possible to build a big fat filter to fix this problem (these
exist from W up to GW), of which a "PF correction cap" is but one element.
basically you build an L-C filter of whatever order you feel like. C's
facing load, L's facing supply. you just have to be very careful about
not trying to filter every harmonic on the ac supply, which is why the
first element wants to be a nice big L.
Higher-order filters have less overall inductance. L-C traps for
individual harmonics are pretty common too.
your transformer leakage could serve as one of the L's. if you dont have
a neutral, wire the caps in delta.
because you are trying to filter out very low frequencies, your L's and
C's will be quite big.
Cheers
Terry
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Posted by Ignoramus11754 on May 30, 2006, 7:00 pm
Please log in for more thread options > Ignoramus11754 wrote:
>> On Tue, 30 May 2006 15:59:42 +0100, Pooh Bear
>>
>>>
>>>Ignoramus11754 wrote:
>>>
>>>
>>>>I think that it is actually not messing up too much itself, it is the
>>>>welder that is the problem. SCRs "clip" parts of the incoming
>>>>AC sinewaves to get the necessary DC current.
>>>
>>>Right. It's phase control. Can't see a simple way to fix that one.
>>
>>
>> I have been reading about this, and some people say that for welders,
>> capacitors could help.
>>
>> i
>>
>
> You have two things happening. Firstly, the phase control gives you a
> current waveform that is a horrible shape, not sinusoidal. If you do a
> fourier transform of that icky shape, you will see it is rich in
> harmonics. Those harmonics transfer no real power, only reactive power -
> it circles in a loop from supply to welder and back again, doing no net
> work - but cause I^2R losses, and of course voltage drop across the
> supply inductance (which is probably fairly high with your phase converter).
In my setup, SCRs clip "last parts" of sinewaves, that is, starting at
some value until it gets down to zero. Right?
.-.
. .
. ,
here, for instance, a, SCR could staart conducting at point - and stop
conducting at point ,
Right?
> Secondly, low arc voltages correspond to maximum load, which makes the
> current waveform, and hence its harmonics, biggest.
Did you mean high arc voltages?
> It is possible to build a big fat filter to fix this problem (these
> exist from W up to GW), of which a "PF correction cap" is but one element.
>
> basically you build an L-C filter of whatever order you feel like. C's
> facing load, L's facing supply. you just have to be very careful about
> not trying to filter every harmonic on the ac supply, which is why the
> first element wants to be a nice big L.
>
> Higher-order filters have less overall inductance. L-C traps for
> individual harmonics are pretty common too.
>
> your transformer leakage could serve as one of the L's. if you dont have
> a neutral, wire the caps in delta.
>
> because you are trying to filter out very low frequencies, your L's and
> C's will be quite big.
Thanks Terry. I will try to think and grok it. I thought that just
having capacitors would already help.
i
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> Ignoramus11754 wrote:
>