17-09-2015, 03:39 PM
You're spot-on with the secondary windings. The primary windings aren't so straightforward.
To aid understanding, it's probably best to consider a more standard transformer.
Most of them come with dual 115V windings. For 230V use, you put them in series. For 115V use, you connect them in parallel.
At 115V, you could use just one winding. But then, you end up with the other winding having 115V across it (just regular transformer action) - which could be problematic. Or, a clever designer that lives in a 115V region might take advantage of that? Note also that if doing this, you might not be able to get maximum output power from the transformer because of resistive losses in the primary - just depends...
Some transformers have a single primary winding that has multiple taps. Perhaps 220 or 240 in the old days? In which case, this is very easy to understand...
So your transformer appears to have 2 primary windings. Did you understand my point/question about the magnetising current? That is key to determining what is happening here, so perhaps it's worth a quite summary:
On the face of it, a transformer is a simple device to understand - it's all about the turns ratio, right?
So what makes a 240V primary a 240V primary? Is this a convention - because 240V mains is the intended application? Let's take a simple transformer that takes 240V in and gives 12V out - those labels are great, but instead, you could label it 20:1. Why don't we do that?
It's to do with core saturation. Any magnetic material will have a maximum flux density - once the core is fully magnetised, the winding changes its behaviour, and the current in the winding will shoot up. It's why you can only put so much signal level on a tape before it starts to distort...
Given that copper and steel are expensive and heavy, mains transformers are optimised to use just enough metal for the application. In fact, they are normally just below the optimum amount - every little helps! It's why many large transformers buzz audibly. When I need a large transformer for an audio amplifier, I have to specify a lower flux density in the core, which means that the transformer is physically larger than an off-the-shelf equivalent of the same power rating.
So connect a transformer to a variac with a current meter in series, and you'll find the saturation point when the current suddenly rises (and this is with nothing connected to the secondary windings). It'll be close to the rated voltage of the primary. Does that make sense?
Back to your transformer...
I'd (carefully!) do a saturation test on windings 1 and 2. I probably wouldn't go much about 130-150V initially, and I'd definitely watch out for winding 3! Use your lamp limiter, just in case.
If windings 1 and 2 saturate at the half-mains-ish voltages we expect, then that's good news. Next step is to work out how to wire them up.
For our use, I'd say that we need to connect them in series. Connect N to winding 1. Join 1 to 2. Connect L to the other end of 2. Having done that, the thing should run from 245V from your variac safely. Check the primary current, as always. Then (carefully!) check all the secondary windings (that's why you can never have enough Flukes
).
If all seems good so far, take the time to measure all the voltages again and report back. Working out how the various input voltages were obtained from the transformer should then be quite feasible...
Earlier I mentioned 115V use with that example modern transformer, and what to do with the other winding. Here we have a similar thing to consider. In the US, would we attempt to put 1 and 2 in parallel? Probably not, but if you do, you can only do it on the winding 2 tappings that give an identical number of turns on both. Perhaps the transformer was wound on the expectation that low voltage inputs would use a single winding? Interesting, but not an immediate concern here in the UK...
Finally, I've been talking about putting windings in series and parallel, but there is an important point regarding the phase of the windings. That has to be right. On an unknown transformer like this, there are a couple of approaches:
1. Use a lamp limiter (pick the lowest power bulb you can find). Monitor the current and if it suddenly rises at just a tiny input voltage, then reverse the phase of one of the windings.
2. Apply a low voltage sine wave to a winding, and use a dual trace 'scope to compare the phase of the primary to the other primary.
Whatever method you use, make a careful note of the phase. On a schematic diagram, a dot is used to indicate the start of the winding.
Hopefully this all makes sense. Apologies if I did the "teaching you to suck eggs" thing at any point
All the best,
Mark[/b]
To aid understanding, it's probably best to consider a more standard transformer.
Most of them come with dual 115V windings. For 230V use, you put them in series. For 115V use, you connect them in parallel.
At 115V, you could use just one winding. But then, you end up with the other winding having 115V across it (just regular transformer action) - which could be problematic. Or, a clever designer that lives in a 115V region might take advantage of that? Note also that if doing this, you might not be able to get maximum output power from the transformer because of resistive losses in the primary - just depends...
Some transformers have a single primary winding that has multiple taps. Perhaps 220 or 240 in the old days? In which case, this is very easy to understand...
So your transformer appears to have 2 primary windings. Did you understand my point/question about the magnetising current? That is key to determining what is happening here, so perhaps it's worth a quite summary:
On the face of it, a transformer is a simple device to understand - it's all about the turns ratio, right?
So what makes a 240V primary a 240V primary? Is this a convention - because 240V mains is the intended application? Let's take a simple transformer that takes 240V in and gives 12V out - those labels are great, but instead, you could label it 20:1. Why don't we do that?
It's to do with core saturation. Any magnetic material will have a maximum flux density - once the core is fully magnetised, the winding changes its behaviour, and the current in the winding will shoot up. It's why you can only put so much signal level on a tape before it starts to distort...
Given that copper and steel are expensive and heavy, mains transformers are optimised to use just enough metal for the application. In fact, they are normally just below the optimum amount - every little helps! It's why many large transformers buzz audibly. When I need a large transformer for an audio amplifier, I have to specify a lower flux density in the core, which means that the transformer is physically larger than an off-the-shelf equivalent of the same power rating.
So connect a transformer to a variac with a current meter in series, and you'll find the saturation point when the current suddenly rises (and this is with nothing connected to the secondary windings). It'll be close to the rated voltage of the primary. Does that make sense?
Back to your transformer...
I'd (carefully!) do a saturation test on windings 1 and 2. I probably wouldn't go much about 130-150V initially, and I'd definitely watch out for winding 3! Use your lamp limiter, just in case.
If windings 1 and 2 saturate at the half-mains-ish voltages we expect, then that's good news. Next step is to work out how to wire them up.
For our use, I'd say that we need to connect them in series. Connect N to winding 1. Join 1 to 2. Connect L to the other end of 2. Having done that, the thing should run from 245V from your variac safely. Check the primary current, as always. Then (carefully!) check all the secondary windings (that's why you can never have enough Flukes
).If all seems good so far, take the time to measure all the voltages again and report back. Working out how the various input voltages were obtained from the transformer should then be quite feasible...
Earlier I mentioned 115V use with that example modern transformer, and what to do with the other winding. Here we have a similar thing to consider. In the US, would we attempt to put 1 and 2 in parallel? Probably not, but if you do, you can only do it on the winding 2 tappings that give an identical number of turns on both. Perhaps the transformer was wound on the expectation that low voltage inputs would use a single winding? Interesting, but not an immediate concern here in the UK...
Finally, I've been talking about putting windings in series and parallel, but there is an important point regarding the phase of the windings. That has to be right. On an unknown transformer like this, there are a couple of approaches:
1. Use a lamp limiter (pick the lowest power bulb you can find). Monitor the current and if it suddenly rises at just a tiny input voltage, then reverse the phase of one of the windings.
2. Apply a low voltage sine wave to a winding, and use a dual trace 'scope to compare the phase of the primary to the other primary.
Whatever method you use, make a careful note of the phase. On a schematic diagram, a dot is used to indicate the start of the winding.
Hopefully this all makes sense. Apologies if I did the "teaching you to suck eggs" thing at any point
All the best,
Mark[/b]
