Re: Transformer drawing 1 amp

On Mar 15, 12:07 am, John Popelish <jpopel...@xxxxxxxx> wrote:
Bob.Jones5...@xxxxxxxxx wrote:
Thanks guys,

Could someone explain why and how a transformer limits its current?

When there is no secondary load, the inductance of th
primary limits its current. That inductance drops
precipitously, twice a cycle, if the core flux reaches
saturation. This occurs if you apply excessive voltage to
the primary.

The transformer limits current to the secondary by the
resistance of both primary and secondary windings (that
waste some of the available voltage, so less is there to
drive current through a given load resistance). But if
there is a significant flux path around the primary that
does not also pass through the secondary, then there is an
additional inductive current limiting effect that acts like
having an inductor in series with the secondary.

Some transformers intended to survive large secondary
overloads (like welders, neon sign, microwave oven and large
battery chargers) provide this flux path by separating the
primary and secondary coils and adding blocks of core
material between them, with a small air gap.

In this picture of a microwave oven transformer, the
secondary has been removed (it wound through the holes where
the coin lays) and you can see the two blocks of laminations
that almost close the flux path around the primary winding.http://www.abiengr.com/~sysop/images/MOT-primary2.jpg

They all look the same to me. I know there is saturation of the core
that limits stuff but I don't know any details about it. It seems that
the only parameters that one can change(the main ones at least) are
the winding ratio and total windings. (assuming standard transformer
design)

The critical spec that involves saturation is volts per
turn, so for a given number of primary turns, the voltage
you apply to the primary. Are you sure the winding you are
connecting to the line is really one designed for that much
voltage? If you have a variac you can use to turn down the
line voltage, smoothly, you can use it to see if the high no
load primary current abruptly goes away at some reduced
voltage. That would prove that the current is not a result
of a short (that would cause the current to drop in rough
proportion to the applied voltage).

I suppose if you just have one winding on the secondary and
100 on the primary its not going to function as well as if you have
100 windings on the secondary and 10000 on the primary.

And you are back to the volts per turn problem.

In any case, I'm testing the transformer as we speak. Its been running
for about 40 mins and now has a temperature of about 37C. For the
first 20 mins its was quite cool and maybe rose about 5 degree's above
ambient. My meter is telling me that its drawing 0.8A but another
meter said it was only about 0.6A when I tested it. Not sure if the
meters are bad, if it has something to do with the inductance, or I
just made a mistake... or maybe the meters are not calibrated
properly.

If the current is a result of a few turns shorted, that
current will fall as these turns get hot (till that hot spot
causes more turns to short).

(snip)

With the variac I have the following

20V 0.1 A
40V 0.13 A
60V 0.21 A
80V 0.31 A
100V 0.47 A
120V 1.07 A
125V 1.48 A
130V 1.78 A

This looks almost exponential although I can't tell as the first 5
measurements are approximately linear and there is a huge jump when
going from 100 to 120.

What does this mean? There definitely seems to be something happening
about 115V. (looks almost linear up to that point then linear again
but with a much larger slope)

I can't tell what the "core" is inside the bobbin but strangely there
is a wood chip(looks like a wedge) that is sticking out on one side up
above the bottom. It looks like someone wedged it in there but it was
made like that(its obvious). Not sure if this could mean the core is
changed or what. Although looking at the bobbin The two coils look
like the are stacked on top of each other and are of the same width.

It sounds like this transformer might be current limited like you guys
were mentioning. If so, does this mean that I cannot use it for my
purposes? The reason I want to use it is because its 15 amps and my
current ps is only about 0.75A and every time I use it above 1A I get
nervous. (it has a fan in the case though and seems to stay cool but I
never run it long above 1A... sometimes though I run it at 2-3 amps
for short periods of time( in seconds)). I don't actually need the
full 15 amps out of the thing and ATM I only have a 4A bridge
rectifier but I was plan on using that and if I ever have the need up
it then I'll just replace the rectifier. Also I couldn't find any
power mosfets larger than about 8A so I'm limited by that ATM too.

I just don't see any reason to waste the transformer if it will work
for my needs. Later on I want to try and build my own transformer when
I get some more time. I've got a book on transformer theory and
design but haven't got around to reading it yet. (trying to get this
PS built so I can play with some motors).

Does this current limiting have anything to do with resonance? If I
were to keep increasing the voltage with the variac would I eventually
see a resonance curve?

Thanks for your help. I appreciate you taking the time out to explain
things(and the other guys too. Everyone has been most helpful).

Bob

.

Relevant Pages

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