Re: 3 phase distribution transformer question

["Bert Hickman" <bert.hickman-at-aquila-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Tesla list wrote:
> 
> Original poster: ghub005-at-xtra.co.nz
> 
> I have been offered a three phase 15kVA, 11kV, 240/415 volt
> distribution transformer - free if picked up.
> 
> What would happen if this unit was run on single phase 240V? Apart
> from derating the output voltage by sqrt(3), can a three phase core
> cope without all of the separate magnetising currents?
> 
> I would appreciate any information or hearing about the experiences
> that others have had with single phase operation of three phase
> transformers.
> 
> Safe coiling,
> 
> Gavin Hubbard
> 
> P.S. Mark - this transformer is intended for you ;-) Needs to be
> picked up from the Waikato region though (Hamilton?)

Gavin,

Nice catch! You should be able to use one or two of the three LV
windings, but you'll need to leave the third set of windings
open-circuited. You COULD energize the transformer from a single LV
winding and pick off HV off two of the secondary windings at reduced
power.

To get maximum power from a single phase source, you'll want to use two
of the LV windings. However, you will need to insure that they are
connected so that the magnetic flux from one is in the same direction
(via the magnetic circuit through the core) as the flux from the other
winding. Refer to the ASCII art representation of a 3-phase transformer
below. Suppose you use primary windings A and C. You'll need to phase A
and C so that the resulting fluxes aid each other. You also can see that
there's no connection strategy that will allow you to use the third
winding from the same single-phase source. The flux from winding A goes
"down" the center leg, while the flux from winding C goes "up" the
center leg, so there's no way that winding B can be connected to aid
both A and C simultaneously. The HV windings of A and C can then be
connected either in parallel or series (with proper phasing) to get the
desired output voltage and current.   

        ---------------------------
        -->-->-->-->-->-->->-->-->-
        ---------------------------
     AO ||| O   B O ||| O   C O ||| O
      O |^| O     O ||| O     O ||| O
      O ||| O     O ||| O     O ||| O
      O |^| O     O ||| O     O ||| O
      O ||| O     O ||| O     O ||| O
        ---------------------------
        -<--<--<--<--<--<--<--<--<-
        ---------------------------

Safe coilin' to you, Gavin!

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-com
Web Site: http://www.teslamania-dot-com