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RE- variac rewind





---------- Forwarded message ----------
Date: Fri, 10 Oct 1997 21:50:00 GMT
From: Robert Michaels <robert.michaels-at-online.sme-dot-org>
To: tesla-at-pupman-dot-com
Subject: RE- variac rewind

        Variac rewinding is like pissing against the wind. (You said
        you wanted  =any=  insight  [your words, my emphasis] ).  In
        your particular case, however, it more like pissing against
        a hurricane.

        Firstly -- you either are having very bad luck measuring
        wire these days or someone somewhere forgot how to design
        a Variac.  (How sure are you that it's a Variac??   It's
        hard to imagine a firm such as General Radio making that
        kind of mistake).

                No. 12 wire  (I assume that's # 12 - AWG,  I'm
                not up on Botswanian [Or Hotmail-ian] wiring standards
                these days) -- No. 12 wire is supposed to carry no
                more than 25 amperes.   And that is in  =free air=.
                All wound up inside a transformer it surely would
                have to be de-rated.   It can take 50-amperes
                and still not be hot enough to toast a decent
                slice of bread, but such is like going out on
                Saturday night without a condom -- risky business.


        Secondly -- In a Variac, the portion of the windings
        which are under the wiper are ground flat.  This to ensure
        smooth, almost spark-less travel of the wiper over those
        windings.   A righteous re-wind job requires either the
        use of square-cross-section wire, or else the machining
        of the wire surface under the wiper if round-cross-section
        wire is to be used.

                The machining puts the double-whammy (followed
                closely by a kibosh) on the whole thing if done
                home style:   The machining is essential to
                smooth, spark-free operation of the wiper.  But --
                the very act of machining causes metal particles
                to be introduced into the windings.  Nicht gut,
                mein herr.  It's a kibosh-22 situation.

        Thirdly -- yours is a high-power Variac.  Such means it
        must be wound/rewound with heavy wire.  Such means that
        if the wire is not machined flat under the wiper, the
        wiper has a particularly rough surface over which to
        travel.  Ergo - more sparks.

                Tesla coiling tip -- sparks should occur at
                your secondary terminus, not inside your Variac.

        Such sparks will rapidly cause pitting of the windings
        (not to say the wiper itself) which will increase the
        sparking, which will increase the pitting, which will
        increase the sparking, which will increase the pitting,
        which will increase the sparking, which will increase
        the pitting, which will increase the sparking, which
        will make you sorry you ever thought of rewinding (not
        to say writing -- your post).

                             - - - - - - -

        Make it easy on yourself:  Get a job slinging hamburgers
        for a few days.  Then you can buy a real Variac (clone
        thereof more likely), and use your # 13 wire for something
        you can't buy so readily if at all  (Hint: It starts with
        T  [and ends with c-o-i-l] ).

                In this burg (pardon the expression) you can
                easily make $25 per shift slinging  (that's
                after taxes).  In four shifts you can buy
                any of a number of used Variac (clones), and
                within a month you can rule the world (Variac
                wise).  It's the American way.  (Don't know
                about Botswania - or wherever that "Hotmail"
                Domain is).

                                        When it comes to pissin',
                                        I prefer porcelain, in
                                        -- Detroit, USA

                                        Robert Michaels,
                                        Sane Coiler




TL>From: Mad Coiler <tesla_coiler-at-hotmail-dot-com>
TL>Subject: variac rewind

TL>List,

TL>One of my friends just got a hold of a reletively new looking but burnt
TL>out 'Light dimmer' from a school. Several windings on one end were all
TL>melted, and there were arc marks where the contact touched the winding.
TL>This contact wasnt making good conection. Anyways, it is a 50A unit,
TL>wound with #12 wire. I have two lbs of #13 enamel mag wire. Would the
TL>#13 wire be OK so long as it isnt run for long periods at high current
TL>levels? That is provided that the thing works at all. If I remember
TL>correctly the name plate says 50A / 8kVA. The next coil that I might use
TL>this variac on will be using probably three 12kV 60mA NST's. This
TL>shouldnt be near the current limit of the variac anyways. But then again
TL>with resonant charging I have blown a 15A breaker with a single 15kV
TL>60mA neon. (not neccesarily a good thing!)

TL>Any insight appreciated,
TL>Mad Coiler



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