Transformer Question: Using 220 taps in the US

[luvdunhill]

Hey, I was thinking about using the 220 taps on the primary side here in the US in order to half the voltage on the secondary side of a transformer. Can anyone think of any reason not to do this?

[Pars]

I can't think of one, but...

[naamanf]

I can't think of any reason why not off the top of my head.

[luvdunhill]

I can't think of one, but...

ah, the menacing popcorn smiley... oh course, if someone got a hold of it and used 220V, that would be really exciting

[n_maher]

I've heard of it being done before and I seem to remember something about paying careful attention to the amount of current that you try and output. But this is all a very fuzzy memory of a conversation long forgotten.

[naamanf]

I would think that the secondary current could be twice what it normally was seeing as the voltage was half.

[n_maher]

Wouldn't the 220V taps be rated for less current since typically the secondaries are rated for the same current regardless of input voltage? I guess what I'm getting at is that if you have a 12V/1A secondary the primaries would be 120V/.1A and 220V/0.05A, or something like that, right? I might have this all wrong, if so, please feel free to tell me. I feel like an atheist preaching about God right now.

[naamanf]

Transformers are rated in VA so as the voltage goes down the current handling would go up. At least this is the way I understand it.

[spritzer]

The same size of wire is usually used on the entire winding (one size for primary, one for secondary) so current handling in that way shouldn't be a problem.

[n_maher]

Just figured I'd throw out what I remembered on the off chance it was useful. Sounds like it isn't an issue.

[spritzer]

Not all transformers are created equal so this might be an issue with some really cheap stuff. Knowing Marc's tastes I doubt that is an issue here...

[luvdunhill]

Not all transformers are created equal so this might be an issue with some really cheap stuff. Knowing Marc's tastes I doubt that is an issue here...

...only problem using this on a design is that it basically means all 220V people are out of luck But, it's nice to be able to rearrange some wires and get double the voltage output,

[krrm]

As naamanf points out transformers are rated in VA, and as V goes up (110 -> 220), A must go down. The issue is heat

From Transformer Ratings:

"The power-handling capacity of a transformer is dependent upon its ability to dissipate heat. If the heat can safely be removed, the power-handling capacity of the transformer can be increased. This is sometimes accomplished by immersing the transformer in oil, or by the use of cooling fins. The power-handling capacity of a transformer is measured in either the volt-ampere unit or the watt unit."

[luvdunhill]

nope, it doesn't matter how the primaries are configured, you can get the same amount of the current and voltage out of the secondaries. That's the beauty of universal primaries.

[spritzer]

...only problem using this on a design is that it basically means all 220V people are out of luck But, it's nice to be able to rearrange some wires and get double the voltage output,

From that point of view you are now my mortal enemy and I have dispatched Stax mafia agents to your house... If there was a mafia that is...

Is this for a project that will be used on 230v...?

As naamanf points out transformers are rated in VA, and as V goes up (110 -> 220), A must go down. The issue is heat

From Transformer Ratings:

"The power-handling capacity of a transformer is dependent upon its ability to dissipate heat. If the heat can safely be removed, the power-handling capacity of the transformer can be increased. This is sometimes accomplished by immersing the transformer in oil, or by the use of cooling fins. The power-handling capacity of a transformer is measured in either the volt-ampere unit or the watt unit."

That is true but the size of the windings also matters since a given size of wire can only handle so much current before it burns up. Due to the cost of copper they are often wound with wires just big enough.

[Beefy]

Won't a non-connected primary essentially become another secondary winding?

[naamanf]

It will have the same voltage as the connected primary. But I am guessing he will be running the primaries in series so they both should be connected.

[aerius]

Let's say you have a transformer with a single 220V primary and 22V secondary, rated for 22VA when used in a 220V country, that is, 1A on the secondary and 0.1A on the primary to make nice easy numbers.

Now if it were used in a 110V country, you still have the 0.1A and 1A limit on the primary & secondary, respectively, so the maximum power you can get out of it is 11VA. If having the rated output of the transformer cut in half isn't a problem, then go ahead and do it, but it's a bit of a waste since the transformer has to be oversized quite a bit.

Ideally, you use something like this so you can change the connections around for different input & output voltages without affecting the power rating. Keeps things smaller & lighter.

[naamanf]

Now if it were used in a 110V country, you still have the 0.1A and 1A limit on the primary & secondary, respectively, so the maximum power you can get out of it is 11VA. If having the rated output of the transformer cut in half isn't a problem, then go ahead and do it, but it's a bit of a waste since the transformer has to be oversized quite a bit.

Why would the current still be limited to what it was at 120v when the transformer is rated for power? Wouldn't it's power limit still be that same? Obviously taking into account the wire gauge was sufficient for the current.

[aerius]

Nope. There's a few things which determine how much current how much current you can put through a transformer; the wire size, the size of the core, and the number of turns of wire on the core. The first is obvious, the other two are related to the magnetic saturation of the transformer core, which if it happens will result in magic smoke and other bad things.

On saturation, the transformer core material, in this case some kind of silicon steel alloy, has a magnetic flux density limit. Which means if you want to put more current throught the transformer you need a bigger core so it doesn't hit the limit. Bigger transformer, more power. Related to this is the number of turns of wire on the primary, there's a formula for calculating the magnetic field strength which involves ampere-turns, that is how many turns of wire around the core and how many amps of current in the wire. Pull too much current and the core saturates, and bad things happen.

One thing to remember is that many power transformers run close to saturation at their rated max power, saves money on copper & iron that way. Going back to my previous example, let's say there's 100 turns of wire on the primary, so with 220V and 0.1A that's 10 ampere-turns, which then gets plugged into the formula for magnetic field strength, and from there they figure out the size of the core needed so it doesn't saturate, then add say, 20% just in case.

So, let's plug it into a 110V outlet and try to pull 22VA out of it. You only have 11V on the secondary so you need to pull 2A there, and thus 0.2A at the primary. 100 turns on the primary, 0.2A, that's 20 ampere-turns, assuming the wire's good for it you've now blown the saturation limit of the core by a huge margin. The transformer gets real toasty, the output on the secondary is severely distorted, and eventually something will burn or blow up.