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>

At 12:15  15/11/01, you wrote:
>Original poster: "Finn Hammer by way of Terry Fritz <twftesla-at-qwest-dot-net>" 
><f-hammer-at-post5.tele.dk>
>
>Peter!
>
>Too few primary turns, and Your primary inductance will become too  low,
>so that the current in the primary will become too high. Connected
>unloaded to the mains, the transformer will melt or burn, instead of
>idle along nicely.

That's not all that happens - with too few turns the core will begin to 
saturate and the current will become nonsinusoidal. The peaks of the wave 
will become rather tall and if only a slight further reduction of turns (or 
increase of voltage) occurs, it can become enormous. This is why the tranny 
cooks faster than you might expect. Professional power transformers are 
designed to operate close to saturation, to allow efficient use of the 
available iron and to reduce heating in the copper wire. Don't overvolt 
them too much without an ammeter to check for saturation or you might be 
unpleasantly surprised. Of course a little saturation is fine - judge what 
you can get away with for a no-load situation. The magnetising current 
shouldn't change too much with full load, so if it's bearable and you need 
to overvolt it a little bit then go right ahead.

<EXPLANATION>
When a magnetic material such as iron is placed into a magnetic circuit 
such as an inductor or transformer, it increases the magnetic flux density 
for a given magnetomotive force (MMF). Now we should all know that magnetic 
flux density, also called magnetic field strength, is measured in Teslas 
(thankfully =), capital 'T', but also MMF is measured in Ampere-Turns, A-t. 
So for a given number of Amperes times the number of turns in the coil, a 
given MMF is provided by the source connected to the coil. It will result 
in a magnetic field being produced, and for a vacuum this magnetic field 
would be linearly proportional to the applied MMF. However, if any magnetic 
material is present nereby there will be some effect on the magnetic flux 
density for that same applied MMF, and we rely on this in the transformer. 
The iron core results in tens of thousands of times greater magnetic flux 
circling around the turns of wire, and this means that the inductance is 
much greater. For AC, an inductor impedes the flow of current, and the 
current that flows due to magnetising the core (remember an unloaded 
transformer is just an inductor with spare wire hanging off) is called the 
magnetising current. If the magnetic flux density in the core gets up into 
the 1.2 T - 1.5 T range, the core begins to saturate and further increases 
in MMF can't gain as much flux as before. So at the peaks of the current 
flowing in the cycle, the inductance seems lower than at the straighter 
parts of the cycle. If the core saturates too far, these peaks become huge. 
Further still, the current becomes zero and you start swearing and possibly 
opening some windows =)
</EXPLANATION>

>Calculating the primary turns is all right, but nothing beats actually
>testing it: Get a ballpark number by calculating, then wind that number
>of turns with _any_ kind of wire you have laying around. Connect to
>mains to check how much current flows. The idle current should not
>exceed, say 5%, of full current draw.
>If the current is too high, wind another 10-15 turns, and try again.

If you can view the current waveform, also check to see how much it looks 
like a sinewave. If it looks nice and sinusoidal, take some turns off. Then 
overvolt it, since we all know you will overvolt it one day, and check that 
it doesn't look too much more distorted. If you notice that the peaks start 
to really grow, you're probably doing it about right, remember that you 
want to do like the professionals and get the most out of your available 
iron (less turns = less winding resistance). Just overvolt it to the limits 
that you want to go, and give your transformer that voltage rating. Stick 
to it like it's the law, because YOU made it that way and YOU know what 
happens =)

>When you get it right, for this particular core, wind a nice and
>beautifull primary, with the right wire and all, and you will have a
>great start on a transformer.

... and know a hell of a lot more about making transformers than you did 
before you started!!!

>Cheers, Finn Hammer


Have fun,
Darren Freeman