# Re: transformer theory q

```Original poster: "Darren Freeman by way of Terry Fritz <twftesla-at-qwest-dot-net>" <free0076-at-flinders.edu.au>

>         No way!!!!!  The real limit on the minimum number of primary turns is
>core saturation, which is a function of turns/volt, frequency, core
>area, and saturation flux density of the magnetic material.  By the way,

I agree with the above, it's an accurate overview of the situation...

>core flux density depends only on the applied voltage, not the current
>in the windings as some have suggested here.  There have been a number

... but I don't agree with that in general. Yes, for steady-state AC
excitation this is true. Monitor the current under no load and you will
find that before saturation the current and voltage are roughly
proportional but of course about 90 degrees out of phase since it's just a
big inductor with no load. So yes, you can look at voltage. But the symptom
of saturation is that the current starts to become non-sinusoidal and its
peaks begin to shoot upwards. So for testing a transformer with thin wire

If I change the frequency of the AC, then I change the voltage at which
saturation occurs. If I keep my eye on the current drawn at no load, the
current at saturation will be about the same. So I'd say that's a good
reason to conclude that saturation at no load can be measured from current,
and it certainly doesn't depend just on voltage.

What I think you mean to be saying, is that saturation has nothing to do
with the current drawn under load. Yes, if the voltage is kept constant, I
can draw whatever current I like without saturating the core (although I
might burn the windings). Saturation can be observed on the current drawn
under no-load (called the magnetising current). It has nothing to do with
the current when a load is present, unless you are really saturating it, in
which case the magnetising current could become a significant fraction of
the loaded current. But by then you're really being nasty to your tranny
and generating lots of 3rd harmonics onto your power line and
ever-so-slightly reducing the efficiency of people's induction motors =)
Which would be so slight that nobody would notice unless the transformer in
question was 15kVA in which case you shouldn't have been trying to see how
far you could push it =)

Anyway, it's a small point but I didn't want somebody thinking it had
nothing to do with current, just disconnect the load if you want to try
increasing the voltage - watch the ammeter and see if it shoots up much
faster than it did as you turned up the variac most of the way. Take note
of the voltage you felt was safe - and then ignore the current, apply the
load, and what Ed said becomes right - keep your eye on the input voltage
now that you know what the safe limit actually is. And don't change the
supply frequency and expect that safe voltage to stay the same because it
won't. Ever wondered why 100 kHz transformers in computer PSUs are so small?

>Ed

Have fun,
Darren Freeman

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