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Re: Transformer question - over volting



Tesla List wrote:
> 
> Original Poster: NickandSim-at-aol-dot-com
> 
> Hi All,
>          recently there have been several posts about running 120V
> transformers or variacs off 240V and I would like to make a point here.
> A transformer can take a certain number of volts per turn at a give power
> level.  For different input voltages you wind a different number of primary
> turns.  If you over volt it the power input you are trying to put through
that
> core is way above what the core can take saturating the core, the saturated
> core sinks a huge amount of power from the mains which completely destroys
> the primary windings and probably melts the secondary as well.
> A transformer can be run below its rated input voltage but *never* above by
> more than 10% or so. If you do try to run a transformer above it rated
> voltage by much you are guaranteed to let the smoke free in big way.
> You can very easily and cheaply buy conversion auto transformers to step up
> or down between 240V and 120V, If anyone in the UK needs one I have pair of
> these rated at about 600VA which I will sell for about 10 each.  Most
> variacs also have a 120V and 240V tap to allow them to be used to step up or
> down between these *and* provide variable voltage output at the same time.
> 
> Keeping the smoke in in GB
> Nick Field

	As a rule-of-thumb, probably conservative for modern steels, for a 60
cycle transformer the turns/volt should be AT LEAST 

	7.5/(core area in square inches)

	This happens to come out of a 1928 copy of the ARRL Radio Amateur's
Handbook, but the laws of physics don't change and one gets essentially
the same answer for modern steels and more exact calculations. Use of 
T/V even 20% below thats value will give a potential core saturation
problem, with all of it joys and sorrows.

	240 volts on a 120 volt transformer will lead to instand smoke, or
blown fuses/breakers, or a combination thereof. Don't do it guys, IT
WON'T WORK!!!!!

Ed