RE: [CAD_CAM_EDM_DRO] Variable Transformer

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

-----Original Message-----
From: Tony Jeffree [mailto:tony@...]
Sent: Tuesday, June 28, 2005 6:50 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: RE: [CAD_CAM_EDM_DRO] Variable Transformer

Andy -

I've apologized to him offline - if I had realized that his message in here
had been sent at the same time as the message in Gecko group, but had been
delayed, I would simply have ignored it.

Actually, I think Mariss's reply, as well as being a warning, gave plenty
of an answer for most mortals - or should I say, for those mortals that
don't want to prove they are mortal too soon!

Regards,
Tony




At 22:58 28/06/2005, you wrote:

>Tony:
>
>Maybe, like me, he wanted an "answer" instead of a "warning".
>
>Reminds me of the bumper stickers that say "If you have to ask, you

wouldn't

>understand".
>
>I thought the groups were about getting answers to questions, and not just
>being ridiculed or beaten into submission.
>
>I don't mean to say that that is what you are doing, but there does seem to
>be an undercurrent of that in the threads about the autotransformer.
>
>Andy Wander
>Verrex Corporation
>
> -----Original Message-----
>From: Tony Jeffree [mailto:tony@...]
>Sent: Tuesday, June 28, 2005 5:30 PM
>To: CAD_CAM_EDM_DRO@yahoogroups.com
>Subject: Re: [CAD_CAM_EDM_DRO] Variable Transformer
>
>At 04:11 28/06/2005, you wrote:
> >I posted this question in the Gecko group as well.
> >
> >I was wondering if a variable transformer would work in building a
> >power supply for the Gecko drives.
> >
> >The one I would like to use is 110 VAC input 0-115 VAC output and 10
> >Amps. I know the Gecko is rated at 80 VDC so you would not want to set
> >the AC output above say 53 Volts. I can set a lock pin on the dial to
> >stop it from an accidental position change.
> >
> >Would this work?
>
>Only in the Darwinian sense of giving a helping hand to the process of
>natural selection ;-)
>
>You had some comprehensive replies to this on the Gecko drive group -
>didn't you believe the answers you got?
>
>In particular, Mariss, who just happens to be the guy that designs and
>builds the Gecko product line, told you it was a bad idea - his opening
>words were "Don't even think about it!"
>
>What part of what he said didn't you understand?
>
>
>Regards,
>Tony
>
>
>
>
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Regards,
Tony




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OFF Topic POSTS: General Machining
If you wish to post on unlimited OT subjects goto:
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you have trouble.
http://www.metalworking.com/news_servers.html

http://groups.yahoo.com/group/jobshophomeshop I consider this to be a
sister site to the CCED group, as many of the same members are there, for OT
subjects, that are not allowed on the CCED list.

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