Re: Transformer drawing 1 amp

Bob.Jones5400@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)
.

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