Re: TIG inverter project - new thread

On Tue, 01 Nov 2005 13:59:47 GMT, Ignoramus4939
<ignoramus4939@xxxxxxxxxxxxxxxxxxx> wrote:

Snip
>>>>
>>>> Do commercial AC welders do it this way, or do they avoid flailing the
>>>>work terminal by using a different topology?
>>>
>>>AFIK they all do it this way. The "work" and "torch" terminal
>>>connections are interchangable with an internal switch, why would it
>>>matter which one of these "flails around" WRT earth ground?
>>
>> I don't know that it does matter which *one*; it's just that they're both
>> flailing in Igor's proposed arrangement. One more than is absolutely needful.
>
>Neither work nor torch terminals in my welder have a connection to
>equipment ground. I verified that.
>
>> But the internal parasitic capacitances are on one side of Igor's inverter,
>> and the welding load is on the other side. Also, he is in the position of
>> having semiconductor switches between two inductors, L1 internally and cable
>> inductance externally. This is a very bad thing and therein lies a problem.
>>
>> By the way, I was once concerned about the effect of even small inductances
>> when multi-hundred amp ripple currents are present (this was battery current
>> feeding a 4000 watt inverter from a 24 volt battery). So I measured the
>> inductance of the battery cables; the number was 3 uH per 10 feet of 4/0 cable
>> pair, with the cables tie-wrapped together. If they aren't tied together, it's
>> more.
>
>My cables are: 40 feet of work lead 1 gauge, and 25 feet of TIG torch,
>perhaps 8 gauge or so. (it's conductor is water cooled and thusly can
>be made very thin).

Pardon my ignorance of the details of TIG welding, but why does the torch lead
need to be thin?

>Not tied together. The unused part of work lead is
>hanging off a hanger that I welded together, in loops.

The inductance of a relatively straight wire goes up as the wire gets thinner,
and hanging wire in loops also increases the inductance a lot. I bet you have
easily 10 uH and probably quite a bit more with the loops.

>(the neighbors
>prolly get a nice wave of HF from that loop)
>
>>>but these parasitic capicitances are probably part of the reason
>>>fast IGBTs are not recommended for this application.
>>
>> Page 8 of AN-1045 discusses the reasons; the main one seems to be the lower
>> Vce(on) of the slower devices. Also, more favorable tempco.
>>
>
>
>You are fully right, but, I think, I should try to do with what I
>have.
>
>i

.

Relevant Pages

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