Re: [CAD_CAM_EDM_DRO] Variable Transformer

Raymond,

Are the toridial (sp? - the donut shaped transformers)
type transformers considered isolation transformers?

Doug

--- Raymond Heckert <jnr@...> wrote:

> Having designed autoformers for a few years, here's
> the answer I'd give
> (and, I think I've given it on other related
> groups): After determining the
> current required by the secondary, calculate the
> current requirements of the
> primary-to secondary leg by the relationship: Volts
> * amps OUT = Volts *
> amps IN (ignoring losses, since these are
> approximations). You must size the
> wire for the high-current carrying leg of the
> primary (we used to use, as a
> rule of thumb, 1000 circ-mils per amp). The
> secondary lead (yes, only the
> leadwire) needs only to be sized to carry the
> required secondary current.
> Then, the winding section from the 'tapped' leadwire
> to COMMON only needs to
> carry the difference in current between the primary
> and secondary. Since
> this current is usually miniscule compared to the
> output, the wire size is
> substantially smaller than the primary-to-secondary
> section. (Autoformers
> are made this way, because it's cheaper to build,
> i.e. less copper, and
> laminations). This small wire can be burned out
> easily, if extended overload
> conditions exist, or high inrush currents are
> generated, caused by feeding
> an extremely low-impedance load (typical of a
> rectifier/capacitor power
> supply) on startup. Now, here's the DANGER part...
> if that section of wire
> between the 'tap' and COMMON ever burns out, your
> secondary load will
> IMMEDIATELY see the full line voltage, so instead of
> 'seeing' perhaps 24
> volts AC, you might see 120 volts AC, or perhaps
> even 240 volts AC. If you
> happen to touch a 24 volt ac output, you'll probably
> feel just a tickle,
> depending on how well you are grounded, etc. But if
> you touch 120 volts, you
> will most certainly feel more than a tickle... it
> might be the last thing
> you ever feel. Don't blame the designer for how the
> autoformer was
> designed, after all, you (the customer) opted for
> the 'el-cheapo' unit
> because you weren't interested in spending the extra
> money that an isolation
> transformer gives!
>
> RayHex
>
> p.s. it doesn't hurt to 'trim' previous posts to
> relevant material
>
> -----Original Message-----
> From: Andy Wander <awander@...>
>
>
>
> <snip>
> >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
>
>
>
>
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