RE: [CAD_CAM_EDM_DRO] Variable Transformer

Doug:

The thing that makes a transformer an isolation transformer is not the shape
of the core(which is what makes a toroidal transformer), but whether or not
the primary and secondary windings actually "share copper", or are connected
directlt to each other.

If you can measure a finite resistance between the primary and secondary
leads(when the transformer is not connected to anything else, please!) then
you do not have an isolation transformer. If the resistance between all
primary leads and all secondary leads is infinite, then it is an isolation
transformer.


Andy Wander
Verrex Corporation

-----Original Message-----
From: Doug M [mailto:mccolld1@...]
Sent: Thursday, June 30, 2005 10:18 AM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: 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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