Re: TIG inverter project - new thread
- From: Chris Jones <lugnut808@xxxxxxxxxxxxxxxx>
- Date: Tue, 01 Nov 2005 21:46:11 +0000
The Phantom wrote:
[...]
>>On another note for i:
>>
>>Do not connect the welding return, often referred to as the "ground"
>>connection, to the safety ground or machine frame in the welder. When
>>this is done and the welding "ground" connection falls off a workpiece
>>which is connected to earth (safety) ground (as all welding
>>workbenches should be) while welding, then full welding current will
>>return to the welder through the safety ground wire, overheating it or
>>melting it depending on welding current.
>>
>>This means that your H-bridge should not be connected to ground in any
>>way except when the welder return "ground clamp" is connected to a
>>grounded workpiece. You need to survive the situation where the "hot"
>>lead only is connected to earth ground.
>
> I had thought about this when I was reading some of Igor's postings
> earlier. Igor acknowledges in another posting that the torch and work
> terminals are both insulated from the welding machine. When he gets this
> inverter going and connects it to a workpiece which is itself grounded,
> the entire welding machine guts are going to be flailing around with a
> square wave of voltage at the inverter frequency and the capacitance to
> ground through the main power transformer, etc., will have to be driven by
> his inverter.
>
> Do commercial AC welders do it this way, or do they avoid flailing the
> work terminal by using a different topology?
I wouldn't worry about the 'flailing' of the work terminal with respect to
earth ground, except make sure that the zero volts line (or you could call
it 'ground' but that would be confusing) of all of the internal circuitry
that connects to the welding circuit should float with the work terminal as
should the shielding boxes around those bits of circuitry. The whole lot
would then reside inside the outer casing of the welder, but it would be
insulated from it. The outer casing of the welder would be connected to
real earth ground for safety reasons. The only circuits which can be
connected to real earth ground are ones which are isolated from the welding
circuit by optoisolators, transformers etc. This is my suggestion.
Chris
.
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