Is there any reason that a 120VAC to 120VAC isolation transformer would draw 2.54A on its primary when there is no load present on it's secondary?
Part reference:
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datasheet:
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I finally got around to wiring this transformer, and I noticed something that doesn't seem quite right to me. I have it wired for 120VAC (H1 connected to H3 and H2 connected to H4) and it seems to be working as I get about 120VAC on the secondary, ( wired X1 to X3 and X2 to X4) but with no load on the secondary, the transformer is drawing 2.54 Amps.
Looking for a sanity check I guess. I'm beginning to think the thing may be defective... Email to the Temco has produced no response as of yet.
As a quick test, disconnect all the windings from each other and then power up any ONE winding and see what the current draw is. I see that it is a 15kVA rated, so it is not unreasonable that the magnetizing curent is that high, but the actual power dissipation would be much less than that current suggests. It is probably working correctly, but a bit of an overkill for the average workbench.
It's not unusual for a transformer to draw some amps when unloaded. That's what inductors do. The current should be out of phase with the voltage. What you care about is POWER. Measure the RMS power consumed by the transformer. That's what counts.
If it's actually dissipating 300W, it will get very warm.
Sure you do. It's on the outside of your house and goes round and round...They send you a bill every month. My base load is 200W, so an additional 300W would be easy to see. The newer ones simulate round and round with an lcd display and have a flashing IR led that can be used to measure actual power quite accurately. If you have an old Palm III vintage PDA, I can send you a program that lets you point the IR window at your power meter and graph consumption...but a stopwatch counting the display works as well.
It's best to turn off everything you can to reduce the base load and improve the precision of your measurement with and without the transformer connected.
If you wanna spend a few bucks, the P3 Kill A Watt meters are very handy in this application.
As a go/no-go test, you can put an incandescent light bulb in series with the transformer and see how bright it gets.
But at 300W, you shouldn't have any trouble sensing transformer temperature rise with your hand. OR Wrap the transformer in insulation and plot the temperature vs time...weigh the transformer to guess at heat capacity and calculate the power from that.
Sounds VERY reasonable for a 15kVA transformer. 2.5 A suggests coupling ratio on the order of 0.982, not bad for an AC mains transformer that weighs that much.
If it bothers you, you can add a high quality AC cap in parallel around 55 uF. That should 'resonate' out the reactive current assuming
120Vac, 60Hz yields around 127 mH.
Or, ignore it and let your house wiring dissipate a bit of power less than 1W ?
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The symptoms described sound very similar to using a 60Hz transformer on a 50Hz supply (as might happen if you are using American equipment in Europe), and the transformer hasn't got enough iron in it - so it's saturating. However, that's not what you are doing.
If saturation IS the problem, you can usually confirm it (under no-load conditions) by winding the supply voltage up on a variac, and measuring the current the transformer draws. It will rise suddenly when the core starts to saturate. Although the problem is much more likely to be shorted turns, a quick test for saturation might be interesting.
Not sure how repeating myself helps, but here goes. I'd plug it into a Kill A Watt meter. That's how I got the other experimental results I mentioned in this thread.
I also disclosed several other techniques.
If you would read the parts you snipped, I also gave you several other ways to estimate the power consumption of a transformer.
If you don't have a readable utility meter or a stopwatch or a light bulb or a voltmeter or a scale or a thermometer or a hand, you probably should look elsewhere for advice. I'm all out.
Wait..... I think I figured out how to do it with a used tea bag, the chime from Big Ben and a plate of fish and chips. Nope, you're still gonna need the hand.
Phil makes a good point about the terms being thrown around here. Magnetizing current is NOT core inductance current. It is magnetizing current. The better the core material the less this current will be.
Back to the core' inductance...The core's inductance is usually 5 to
10 times the impedance of full load.
You use a pair of windings to get to 1kVA, therefore per winding is
500VA, so at full load each winding is capable of 120Vac at 4.17A into a RESISTIVE load of 28.8 ohms.
Now, the core winding's inductive reactance is at least 3 times that usually more than 5 times that or, 144j ohms [the j signifies reactive impedance of the core and that the current through the inductor will be out of phase with any load by 90 degrees.] At 60Hz that core's inductance will be about 382 mH.
When wired in parallel to achieve the full 1kVA capability and NO LOAD each winding will have approx 0.834A for a total of 1.67A current. Of course this is reactive current and does not consume significant power.
To get rid of such inductive reactive currents, which cause a 'lagging' power factor, it is possible to add a parallel capacitor essentially in resonance taking it to near zero, thus 'correcting' the power factor. Industrial consumers with lots of motors often have a rack [building] full of such capacitors that are switched in and out depending on the correction they need.
It is of note that the current is ALWAYS there, even at full load, in parallel with your load. If the transformer is made properly, the waveform will be fairly linear, if the core is starting to saturate, the current at the peaks will increase due to that saturation
A reasonable model predicts lower current than you saw, but 2.5A is still possible, just seems high.
Further the winding resistance is often less than 1/10 of load, which implies less than 3 ohms, probably more like 1.5 ohms per winding. since they're in series to the load.
The magnetizing current is not really the same as the core inductance current with the core inductance current in parallel with the load, albeit 90 degrees shifted. Magnetizing current is actually what it takes to 'turn on' the magnetic material. Really shows up if you try to measure the core's inductance with a little meter and only put
0.1Vac across a winding, you'll find the inductance you measure is almost nil. All caused because you haven't supplied the required 'magnetizing current' to overcome the material's coercivity.
Phil makes a good point about the terms being thrown around here.
** The one doing the chucking about is YOU - pal.
Magnetizing current is NOT core inductance current.
** But includes it.
Back to the core' inductance...The core's inductance is usually 5 to
10 times the impedance of full load.
** Wrong - it is way more than that.
The NON LINEAR magnetising current increases with applied voltage and rise sharply as the max rating is approached.
Now, the core winding's inductive reactance .....
** Is so high the resulting current flow barely matters.
At 60Hz that core's inductance will be about 382 mH.
** It is far more likely to be many Henries.
To get rid of such inductive reactive currents, which cause a 'lagging' power factor, it is possible to add a parallel capacitor essentially in resonance taking it to near zero, thus 'correcting' the power factor.
** FFS - give up on this crap.
You cannot correct the PF of an unloaded AC supply tranny with a parallel cap !!!
It is of note that the current is ALWAYS there, even at full load, in parallel with your load. If the transformer is made properly, the waveform will be fairly linear, if the core is starting to saturate, the current at the peaks will increase due to that saturation
** In reality, the ONLY significant current flow occurs around each voltage zero crossing.
BTW:
Toroidal AC supply transformers are different.
Inductance is way high and I mag is almost non existent up to the rated input voltage at the rated frequency.
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