Re: transformer theory q

["Tesla list" <tesla-at-pupman-dot-com>]

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 
with no load, keep your eye on the current.

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