Cascading Transformers

Subject:       Cascading Transformers
       Date:   Mon, 21 Apr 1997 20:03:57 GMT
       From:   robert.michaels-at-online.sme-dot-org (Robert Michaels)
Organization:  Society of Manufacturing Engineers
         To:   tesla-at-pupman-dot-com

                    Cascading Mil. Spec. Power Transformers

        To some of you this may be blitheringly obvious.  To others
        --- well --- perhaps not, judging by some of the posts I've
        seen here recently.

        The essential idea is this:   =If= one has a power (or plate)
        transformer which was manuf. to military specifications, then
        the ratings on the spec. plate of such a transformer can be
        taken as =starting point= or minimum values.

        A case in point:  Imagine such a transformer rated at 120-V.
        in and 1000-V. center-tapped out.
                (The center tap is irrelevant here but nearly all
                 power transformers are so-designed.  [ Hence the
                 secondary voltage may be give as 500-0-500 ] )
        Two such transformers may be connected back-to-back to obtain
        approx. 8.33-kV with relative impunity.

        This connection is illustrated below.  The secondary of the
        first transformer is used to power the primary of the second.

        Given the transformation ratio of 8.333  (1000-V/120-V =
        and -- given that the primary of the second transformer is being
        supplied with with 1000-V. from the first, Then approx. 8-kV.
        (less the losses incurred) may be expected from the secondary
        of the second transformer -- 1000-V x 8.333 = 8.33-kV.

                             - - - - - - - - - - -

        One might wonder how it's possible to get away with such a
        The answer lies in using =only= mil. spec. transformers.  They
        are typically underspec'd. by a =very= wide margin.

        Mil. spec. transformers are ferociously expensive when bought
        new.  Most fortunately, they are the stock in trade for most
        surplus and electrical junk dealers.  Also most fortunately,
        the vacuum tube equipment for which they were designed is going
        out of style at a high rate of speed.   So, such transformers
        go for very little money and seem to be a glut on the market.

                             - - - - - - - - - - -

        Note 1:   The above presupposes two identical transformers. 
                  need not be the case.  If the first transformer were
                  rated at 500-V, then the final output would still be
                  a respectable and =much= more conservative 4.17-Kv.
                  and well within the actual specs. of many a 1000-V.

                        Most any mil. spec. transformer worthy of the
                        appellation and rated for 1000-V. output would
                        be insulated for at least 5000-V.

                  Alternatively -- the center-tap of the first trans-
                  former may be used to power the primary of the sec-
                  ond for the same conservative results.

        Note 2:   Only the second of the two transformers is
                  so only it needs to be mil. spec.  The first
                  could just as easily be commercially rated, or in fact
                  merely something pulled from an old tv.

        Note 3:   This license to play fast and loose with transformer
                  specs. applies mostly to voltage ratings.  If you kick
                  up the voltage of a 1000-V. transformer to 8000-V. you
                  must take care to reduce the current draw in near
                  proportion.  So, if that 1000-V. transformer is rated
                  at 500-mA., then you've got 60-mA. available
                  per spec. or perhaps 100-mA. if you want to gamble a
                  little (a little =more=).

                  In using two disparate transformers, it's the power
                  rating of the lowest-rated transformer which
                  how much current can be drawn from the pair.

               ||c----------------     ||c------------------->
               ||c                |___
   -----------C||c                    C||c                   output
  120-V.      C||c--c.t.              C||c--- c.t.           approx.
     in       C||c                    C||c                   8.33-kV.
   -----------C||c                 ___C||c
               ||c                |    ||c
               ||c----------------     ||c------------------>

      First Transformer               Second Transformer
      120-V.  primary                 120 V.  primary
      1000-v. secondary               1000-V. secondary

                        c.t. = secondary center-tap.

                               Damn the specs., full Tesla-ing ahead,

                               Robert Michaels - Detroit, USA