Re: [CAD_CAM_EDM_DRO] Variable Transformer

I'm sorry, I didn't mean to be rude, I was simply relating the sum of
what I knew. I'm after the same detailed explanation you are, and I
want to reiterate that "don't try it" with nothing added is a waste of
everyone's time.
Michael

Andy Wander wrote:

>Tony:
>
>I guess I should have learned my lesson last time I was in an exchange on
>this subject, but I sometimes have trouble holding my tongue.
>
>With no disrespect meant to you, Mariss, Jan Rowland, or any of the others
>who have "answered" the question, I still feel that it has NOT been answered
>at all.
>Your reply, below, is clever, and made me chuckle, but it doesn't give any
>real info on why the use of a non-isolated power supply is dangerous.
>
>Mariss' answer, which I quote here,
>"Do not even think about it! We have gotten drives back where even the
>1/8" aluminum mounting plate had corners melted from using a variable
>autotransformer. 115VAC at your circuit breaker current rating (>50A)
>can flow thru the drive to ground. That and you can get killed just by
>touching it."
>also just basically says "Don't do it". He says that they have gotten drives
>back that were damaged , but there is no info on whether they were hooked up
>wrong, or if (as I suspect) the use of a DC supply which is "balanced" with
>respect to ground is the real culprit(ok, I realize that a "balanced" DC
>supply WILL probably be the result of using a variac as the AC voltage
>source, but if that's true, why not say so in the answer?). a 50A circuit
>breaker feeding a 750-1500VA isolation transformer will be plenty dangerous
>also, just from the standpoint of the current it can supply. The part about
>"you can get killed just by touching it", could equally apply to an isolated
>high-voltage DC supply, or even isolated 120VAC.
>
>Other answers I have seen simply state that "isolation is necessary",
>without explaining why, and when I ask, I am ignored, or just told to
>believe it.
>
>I have also been told that "it's not the voltage, it's the current that
>kills you", which ignores the fact that current won't occur without the
>voltage to drive it.
>
>I am sure Mariss knows a lot more than me about it, and many others here
>might also(and probably do). But then why can't anyone actually explain it?
>
>
>Now, for my own answer as to why it is not advisable to use a variac with no
>isolation, it seems that the dangers are related to "common practice", and
>to what happens when something is wired incorrectly, or gets damaged or
>loose. Those are certainly valid reasons for doing it with an isolation
>transformer, but I don't see them presented as answers here-all I see are
>dire warnings of "danger", but no explanation(that I can see, anyway) of
>why.
>
>Waiting(hoping) for enlightenment,
>
>Andy Wander
>Verrex Corporation
>
>