Transformer drawing 1 amp

That sure looks like the onset of saturation.

The wedge is probably just an assembly aid, to hold things together.

If it was a current limiting design, there would be a space between the primary bobbins, and a chunk of core material slid into the space between them, to detour some flux around the secondary. Yours just sounds like a fairly high leakage inductance design to make the output voltage sag a bit under the full load of an uncharged battery, and made with the absolute minimum core material and copper to minimize the cost.

I don't think so.

(snip)

No, it would just catch fire.

Speaking of which, I think you can get some use out of this transformer, as long as you fuse the primary at about 2 amps (in case the heat shorts out the winding) and don't mount it near anything flammable. If it is insulated for high temperature, it may work well enough for your purpose. If there is some room in the primary bobbin, and you could slide a few more turns of magnet wire in there, to raise the turns count a little (lower the volts per turn), you might be able to lower the idle current quite a bit. This will also reduce the secondary voltage a little.

I think it is better described as a cheap transformer (minimum copper and iron, intended to run hot) that has a soft output regulation. In other words, you can expect a fairly large change (say, 10%) change in secondary voltage between zero load and full load. A true current limiting transformer would have the secondary voltage go to almost zero at something like full rated to twice full rated current. This gets into what "full load rating" means.

Not so much. It does that by shifting the phase between voltage and current. Current through an inductance is 90 degrees delayed with respect to the applied voltage, so that almost half the time, the current is in the reverse direction, compared to the voltage. During those parts of the cycle, the energy previously stored in the magnetic field (when the current and voltage had the same sign) are being sent back to the power line. There is current, but the average power (the average of the positive instantaneous power when the magnetic field energy is increasing and the negative instantaneous power when the magnetic field energy is decreasing) is quite a bit less than the product of the RMS voltage and RMS current (VA). So a 1 amp primary idling current does not mean that the transformer is soaking up all of 120 watts, but only some fraction of that.

If it was a current limiting design, the secondary voltage would severely collapse as you passed through full rated current. If you shorted the secondary (collapse the voltage all the way to zero) the current would not be limited only by the resistance of the windings, but also by the effective inductance the design puts in series with the secondary.

Whoa! Dooood! I've been cited! ;-) ;-) ;-)

Guess I'll throw myself a little party. ;-)

Cheers! Rich

It's probably current-limited. At no load, it's wasting power, probably by saturating the core. But when you draw current off the secondary, the back EMF will limit the flux and it can come out of saturation - or the leakage flux doesn't leak so much, something like that. (Transformers are like Black Magic, AFAIC. ;-) )

Find some kind of load, like some automotive light bulbs - I don't know how many amps a headlight takes, but it'd probably be a dandy load. (Does a headlamp take 15 amps?)

Use the Variac, and measure the voltages and currents on both sides, and write up a chart like the other one, but with various loads (a couple of brake lights, a BMF power resistor, whatever).

Another verrrrry interesting thing you could do, if you have access to a dual-trace scope, would be to watch the primary voltage and current simultaneously. This will tell you how reactive it is. You could be drawing 1 amp with a power factor of, maybe 10% (or whatever - I'm sure someone will correct me if this is wrong.)

Good Luck! Rich

No, it lets you use less iron for the bobbin size. CTs are similar.

You saw in the scope pix I posted that a single shorted turn overwhelms the peaky current peaks caused by saturation. It still is possible that you have a high resistance short, but that wouldn't produce much heat if it were the case.

If you have some small wire you could slip through a small space (if there is any!) and get a turn around the center leg of the transformer, you could do what I did and look at the primary current when you short a single turn. If there's not quite enough space, take a slender wooden shish-ka-bob skewer and try to force open a space for a wire.

Based on the scope pix, I don't think you have a shorted turn. I think you have a cheap transformer.

In addition to running the core into saturation, it's probably a cheap grade of iron, such as M19 instead of M6 (the number refers to the watts dissipated per pound of laminations). This could help account for the high current draw and the high temperature of the core even at no-load.

Ok, Thanks again, Hopefully I'll understand more of this stuff when I play around with transformers. I have some idea from physics but I don't have any working knowledge and I've forgot a lot of stuff.

Bob

Ok, I'll try it when I get some time. There seems to be a gap in some spots that I'm sure I could get some wire through.

Yeah, it probably is just a cheap transformer. I didn't know about the differences in the material but now I do ;) I know they "welded"(looks like its bee soldered) the laminates together so I can't take it apart.

Do you know if its easy to come by the laminates? I'd like to try to make a few transformers for fun but I don't want to spend to much money on it. I've seen bobbins for sell but not laminates. I was thinking about trying to make some but I don't really have that thin metal. well, I have some stainless steel sheets that might work? I'd have to coat them with something so that there are no eddy currents. Maybe I can play around with this and see what works best.

Thanks for your time and energy, You've been quite helpful.

Thanks, Bob

There *will* be saturation. The current in the primary of an unloaded, iron cored transformer is essentially just the magnetizing current. In a properly designed transformer with rated voltage applied to the primary, this current will not be sinusoidal, because the core will be near saturation. See the waveforms Bob Jones and I posted over on ABSE. With waveforms that peaky, an average responding meter will be substantially in error.

Only if you are measuring a sinusoidal current waveform will an average responding meter give the same reading as a true RMS meter.

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If the laminations are welded together, it may cause more eddy currents and heating of the steel. It may be deliberate.

Toroids are good for experimental transformers. You can often take apart an old Powerstat (variac), and wind a secondary on it. I have some 2 kVA units that were trashed because of burned brushes. I have used them with a few turns of very heavy wire (actually bus bar) to get several thousand amperes at a few volts. Usually you can get about 0.25 to 1 volt per turn on a 200 VA to 2 kVA core. They are very efficient, and very quiet (acoustically and electrically). They are a bit harder to wind, however.

I have thought about making a small toroid from a length of steel strapping tape wound up like a clock spring. It should work, but the steel is probably not the best for the purpose. Might be an inexpensive learning tool, however.

You're welcome.

Paul

The suppliers usually want to sell hundreds of pounds at a time, but you could try these guys:

Your best bet would be to try to find some surplus transformers where they haven't welded the lams, or varnished them too heavily. Then you might be able to get the lams out and rewind the transformer. You will also need magnet wire; sometimes local motor rewind shops will sell small quantities. And, you will need insulating materials; Nomex is the material of choice.

Stainless steel won't work well at all. The only thing that really works well is real silicon steel transformer laminations.

Thanks for all the help. I'm going to use the transformer as is for now and get on with building the rest of the circuit. When I get some more time I'll come back to messing around with the transformer specific stuff. I can always drop in a better transformer if I need to. If the only issue at this point is heat then I can deal with it. Once I'm more comfortable with building a high current supply I might invest more into quality components.

Bob

** No law against using a small fan to blow on that tranny.

Easily triples the available power capacity.

....... Phil

Yeah, I already mentioned that I would do that. I didn't know it tripled the power capacity though. Do you mean that if it dissipates X watts then adding a fan will allow it to dissipate 3*X watt? Is this true in general or just for things that heat up slowly and have adequate ventilation for the fan?

Thanks, Bob

** No - I mean it can allow a tranny to deliver up to 3 times rated power to a load.

Equates to 9 times increase in dissipation within the tranny.

Only limited by the ability if the tranny to regulate its secondary voltage.

...... Phil

NOTE: I've removed the magnetic shunts that used to sit between the primary and secondary. (I cut off the secondary with a chop saw, and used a punch to get the shunts out.)

Thanks, Rich