Re: What limits the current in an unloaded transformer?

On Fri, 09 Nov 2007 14:51:32 -0800, FunFred
<alanfredperry@xxxxxxxxxxxxxx> wrote:

On 9 Nov, 16:31, John Larkin
<jjlar...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
On Fri, 09 Nov 2007 03:21:41 -0800, FunFred



<alanfredpe...@xxxxxxxxxxxxxx> wrote:
I got to do some experiments recentley and I realise that my
understanding of transformers has some holes.....

First I got hold of a variac and some resistor loads which I attached
to the transformer.

The transformer got hot, but not too hot, not much new there.
When I wound up the input voltage to the primary it got hotter. OK so
far...

Then as I was in a hurry and I wanted to test some other transformers
I connected just the primmary windings.

This is were it went a bit strange...

Just sitting at standard mains voltage, the transform did not do much,
but when I wound the input voltage up, the input current suddenley
climbed, transformer got a lot hotter and before I knew it the primary
coil had burned out.

I repeated this on a couple of scrap transformers and the same thing
happed.

So the question is... If there is virtualy no current on an unloaded
transformer why does the current suddenley climb when you go over it's
rated voltage?

I can only guess the core saturates and the side effect is the primary
starts conducting current and this causes it to get hotter and finaly
burns out.

Nobody I knows seems to be able to confirm this ... it's been bugging
me!
and googling just throws up hundereds of other issues....

Cheers

FunFred

At higher voltages (or lower frequencies!) the core will hit magnetic
saturation. When that happens, the inductance drops radically and
nothing limits the promary current but the copper resistance.

Most transformers are designed to run pretty close to core saturation
at rated line voltage and frequency, so a bit more pushes them into
their saturation region.

The primary current waveform is interesting. It has the usual
quadrature sine wave caused by primary inductance, and usually some
distortion caused by the core. As you crank the primary voltage up,
the distortion increases, becoming huge spikes as the core saturates.

Note that saturation is caused by the primary voltage, not by a load.

John

The test was to see if the 110 VAC tansformers could have withstood
230 VAC.
This was more of a "what's the worst sombody could do".
The fault was to be detected and a warning given.

Having the loads connected protected the transformer to a degree. But
why?

Loading the secondary increases the primary current, so causes a
little voltage drop in the primary resistance. That reduces the
primary magnetization, so the transformer is somewhat less likely to
saturate.

You can think of the primary as being a perfect, zero-resistance
winding in series with an external resistance equal to the actual
primary resistance. So loading the transformer increases the voltage
drop in that "external" resistor, so the primary sees less net voltage
drive.

The thing that saturates the iron in the primary drive is applied
volt-seconds, namely how much voltage is applied for how long. So
higher primary voltage, or lower frequency, both push the core towards
saturation.

Of course, if you load the secondary too hard, both the primary and
the secondary wire heats up, so again the transformer fries. But it
doesn't saturate!


Does this mean the loads absorbed energy via the magnetic coupling
thus reducing the magneticfield imposed on the core which meant the
transformer is not so saturated and lasts long? ( OK but not for
long )

The result was so non-intuative to me,

The thought was "How could an unload transformer be worse than a fully
load one?" thus I was a little supprised!!!

but then I do not normaly go and drive things this hard or have worked
with transformers at this level.

This sort of behaviour is mentioned in some texts, but not stated,
sort of implied and you have to work it out for your self or do the
Test.

A friend has since suggested that if the frequency was doubled the
voltage could be doubled. I then pointed out that 100 Hz mains was a
little uncommon!! :-)

You're both right.

John


.

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