Re: [CAD_CAM_EDM_DRO] Variable Transformer

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