Transformer Question

I was looking at the commercial isolation transformers and they are very costly, so I decided to build my own. After all, all they are is a transformer with a power cord on the primary and an outlet (and fuse) on the secondary. And I already have an enclosure to put it in.

I'm looking at a bare transformer to use as an 120v isolation Transformer. (120v in, 120v out). The transformer primary is 480 / 240. The secondary is 240 / 120.

This is for single phase 60 cycle AC. (U.S. power).

Will it work if I connect the 240 lugs on the primary to 120 volts, and use the 240 lugs on the secondary to obtain 120 volts. Electrically, this makes sense, but I am not 100% sure, so I thought I'd ask.

Also, this Transformer is rated at 750 va.

Using the calculator chart on

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750 va should give me 6.25 A output. (Which should be enough amperage for anything I need to test on my bench).

However, since this transformer was intended to be used at 240 / 480 on the primary, will it still give me 750 va (6.25 A) on the secondary if I run it on 120v?

Thanks

Reply to
boomer#6877250
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yes it should work but the VA rating will be 1/2. The windings are rated for current so you can pull the rated current. But since you are at 1/2 the rated voltage, the VA will be 1/2

m
Reply to
makolber

The VA rating on a transformer is derived from the maximum voltage times the maximum current.

Running it on half the voltage won't change the maximum current -- so the actual "VA" that you'll achieve is half of the transformer's rating.

Other than that, and possibly more voltage drop than you'd really like, it should work.

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Reply to
Tim Wescott

sounds safe to me.

not any more, at half voltage it's only good for 375.

at 240V it's good for 3.125 A

at 120V it's still only good for 3.125 A

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Reply to
Jasen Betts

You do need to keep the tally of the losses. Off the cuff, increase the secondary turn count by at least 3-5%. If you plan to run the transformer at the max rate, then by 7-10 %.

Basically, the VA of the transformer is the max voltage you possibly have at the secondary for any type of load, times the max current you can possibly have in the secondary on a permanent basis for any type of load.

Reply to
Judges1318

Lots of 'commercial isolation transformers' are intended for RF blocking, and have fancy shielding specifications and pretty cases. If you just want a transformer, have you checked out your local electrical suppliers (Graybar Electric in my neighborhood)? They don't have online catalogs, but they can get you just about anything, in a plain steel box.

That would work, but it's a bigger transformer than you need.

Reply to
whit3rd

In the case of such a big transformer the drop may be negligible, but if you use two potted 380V->12V 1.2W Hahn transformers with connected secondaries and power the combo from 230V, the transformed voltage would be in the 90V ballpark.

Best regards, Piotr

Reply to
Piotr Wyderski

I always thought it was the nominal V & I. And the rating is on the basis of a certain level of MTTF, one can certainly exceed it when less MTTF is acceptable.

NT

Reply to
tabbypurr

If you really want to spend very little, get a couple of microwave ovens and use two transformers to make one isolation transformer.

Dan

Reply to
dcaster

That might be disappointing; microwave transformers never operate without a full load, so the possibility exists that a light load on the composite would saturate one, and get very lossy.

Magnetic parts of a transformer have to be sized according to the MINIMUM load current.

Reply to
whit3rd

** Completely crazy idea that cannot work.

Too many reasons to bother going into here, but it is both stupid and dangerous.

..... Phil

Reply to
Phil Allison

You should not make an isolation transformer your self if you don't have extensive experience

Isolation transformers must have double insulated design which involves special copper wire, creepage and clearance distances and using approved materials for the bobbin etc

Regards

Klaus

Reply to
klaus.kragelund

Why do you think so? To the first approximation, the flux does not depend on the load and it is the flux that saturates the core.

Best regards, Piotr

Reply to
Piotr Wyderski

** Because it is 100% correct for most applcations - d*****ad.

** Fuck the "first approximation" madness.

** And is at its very worst with no load.

You don't know anything about transformers.

.... Phil

Reply to
Phil Allison

The core is magnetized by AC current in the primary, minus the AC current of the secondary. Drawing secondary current means the magnetic core is subjected to lower fields (lower peak magnetization), which keeps it out of saturation.

It's a first-order effect.

When no current is delivered by the secondary windings, the minimal transformer that works well in a microwave oven might pop fuses at every transient on the AC line, or overheat. The magnetron heater always draws current, and the HV always draws current after a second or three of heating, when the microwave oven is ON and the transformer under power.

Reply to
whit3rd

Go and learn something, Allison, it is never too late.

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In the steady state operation the magnetizing flux in the core is almost constant with respect to the load changes. In practice it is indeed highest at no load, but the difference is several percent, so meaningless. If your transformer saturates at no load, it is already broken by design.

What can be a problem is the inrush current, but it is not related to the copper, but to the wrong direction of the remanent flux in the core, which depends on the material.

And now you may kiss my ass.

Reply to
Piotr Wyderski

Yes, but the value of the primary current in the linear region depends directly on the secondary current (the Lenz rule) and the fluxes oppose each other, so their difference in the ideal transformer remains constant and equal to the flux induced by the primary's magnetizing current. It's the MMF that changes with load, not the flux.

In practice there is a dependence (and has the direction in agreement with your statement), but it is small for a transformer with reasonably low winding resistances, several percent.

Now if you want to design a real transformer intended to be connected to mains in the Europe, you must conform to IEC 60038:1999, which says that the input voltage is in the range of 230V+6%-10%. And since the flux directly depends on the coil's voltage, you must already design it to survive 253V AC, with zero safety margin. In practice the margin will be at least another 5%, which puts the load-related flux changes in the noise.

But no sane person would design a transformer to be 3% below the saturation knee of the B-H curve at full load. The saturation is mostly caused by bad luck with the sign of the remnant flux with respect to the current phase of the mains voltage.

Or do I underestomate the Chinese 'savings'?

Best regards, Piotr

Reply to
Piotr Wyderski

** Fraid that is not correct.

Core magnetisation depends on the applied primary AC voltage, minus any IR drop due to magnetisation current OR reflected secondary current.

So the greater the load current, the less the magnetisation current.

Microwave trannies depend on this fact.

.... Phil

Reply to
Phil Allison

Hmm, I would say zeroeth approximation.

The first order correction then includes primary winding DCR. Simple linear stuff. Higher orders are then more complicated.

Load current is reflected in the primary circuit, which causes a voltage drop across DCR, therefore reduces the EMF applied to the core.

Higher order corrections would be asking, is the core nonlinear under this condition? (Yes: reducing the applied voltage by, say, 5%, will result in mu_eff being slightly different, so the magnetizing current won't be proportionally smaller, but may be larger or smaller.) Is the DCR nonlinear (no -- or at least, no 2nd or 3rd order reason to suspect it), or is DCR an accurate representation? (It isn't -- ACR is necessary, particularly for very thick wire, or at higher frequencies, or harmonics.)

Tim

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Reply to
Tim Williams

My experience is that it is not dangerous. If you do a little searching on the internet, you will find numerous articles on making arc and spot welders using microwave transformers. So apparently others have had the same experience as -I have had.

Dan

Reply to
dcaster

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