Help identifying transformer windings

[SurreyNick]

Terry/Mark

Let me see if I correctly interpret what you're suggesting.

The seven tags of winding 1 & 2 are used according to the input voltage, i.e. 110, 125, 145, 200, 220, or 245v (Which of the 7 tags should be allocated to mains neutral? Am I right in thinking it can only be one of four i.e. either of winding 1 or the first or last of winding 2)
Winding 3 is then a centre tapped HT
Winding 4 is for a rectifier heater
Winding 5 is a centre tapped heater winding

Is that right?

Nick

[Mark Hennessy]

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]

[ppppenguin]

This guy in Chile gives one of the best explanations of transformer design that I've seen:
http://ludens.cl/Electron/Magnet.html

The rest of his website is well worth a browse. Lots of interesting stuff from a guy who's clearly a good engineer with a gift for explaining what he does.

[SurreyNick]

Thanks for the link to ludens.cl, I have had a quick scan and will come back to it.

I appreciate your help Mark and I can assure you, you can't go simple enough for this dunderhead. I can be slow on the uptake, particularly when trying to pick something up from written explanations only, and so I prefer to be treated as a dunce and have things explained repeatedly, step-by-step than have someone assume I know what they're talking about

I will try and grasp what you're telling me in the morning when my mind is freshest. In the meantime looking at this transformer I am wondering if winding 1 and 2 are, in fact, just one winding with 7 taps.

I have spotted this Philips power transformer on eBay (see pics) and looking closely it also accommodates the same six voltages as mine. It uses a voltage selector to do this, the same  as that shown in pics 3,4, and 5 attached. Forgive me if you are already familiar with these selectors, but it has seven pins of which I assume the centre one is for incoming mains live. By rotating a circular bridge this live in can be connected to one of the remaining six taps, which each correspond to one of the six possible voltages.

Looking very closely at the left side of the transformer in pics 1 and 2 I think all the taps on this side of the transformer relate to one winding i.e. are all to do with voltage selection. The grey wire with a yellow sleeve on it doesn't connect to the voltage selector and could be mains neutral at one end of the winding. Of the 7 wires connected to the selector; red, green and yellow clearly go to outside pins of the voltage selector and connect to taps on this side of the transformer. Blue, black and what I assume is brown and another grey are also connected to the voltage selector with blue, brown and black appearing to go to the other taps on the same end, but opposite side, of the transformer. The grey wire looks like it could be connected to the centre pin of the voltage selector and therefore be live in.

If I'm right and I can work out which are the taps at the start and end of this winding on my transformer it should be relatively easy to establish which taps are for 100, 125, 145, 200, 220 and 245v and at the same time calculate the voltages on the other windings.

Do you agree?

Nick
                           

[Ed_Dinning]

Hi Nick, that is the Philips "simple" voltage selector. They normally don't do anything simply at Philips.

There is another type that has at least 2 links on the selector and there are 2, primaries, each rated for about 130v these can be series and parallel connected for voltages from 100 to 250 py various positions of the voltage selector.
Complicated, but all down to economics of transformer production for various countries/ voltages worldwide.

The technique mentioned earlier of feeding a low voltage into the heater windings will work very well to ID windings. This works best when the valves that were used with the particular transformer are known.
Note that sec windings often had "compensating " turns to overcome losses in the transformer, so a 4v winding, when used as an input may need 4.5 v to get the correct output on another winding.

Ed

[Mark Hennessy]

I will try and grasp what you're telling me in the morning when my mind is freshest. In the meantime looking at this transformer I am wondering if winding 1 and 2 are, in fact, just one winding with 7 taps.

I would have expected that to show up when you did your resistance measurements, but perhaps you managed to occidentally overlook that possibility? Easy enough to check, and it would make quite a bit of sense.

Looking very closely at the left side of the transformer in pics 1 and 2 I think all the taps on this side of the transformer relate to one winding i.e. are all to do with voltage selection.

Yes, and that fits with my observation that windings 1 and 2 are at the same end of your transformer

The grey wire with a yellow sleeve on it doesn't connect to the voltage selector and could be mains neutral at one end of the winding. Of the 7 wires connected to the selector; red, green and yellow clearly go to outside pins of the voltage selector and connect to taps on this side of the transformer. Blue, black and what I assume is brown and another grey are also connected to the voltage selector with blue, brown and black appearing to go to the other taps on the same end, but opposite side, of the transformer. The grey wire looks like it could be connected to the centre pin of the voltage selector and therefore be live in.

If I'm right and I can work out which are the taps at the start and end of this winding on my transformer it should be relatively easy to establish which taps are for 100, 125, 145, 200, 220 and 245v and at the same time calculate the voltages on the other windings.

Do you agree?

Yes indeed. I think we're getting very close now.

Set the multimeter to Ohms and take another look at windings 1 and 2

Cheers,

Mark

[SurreyNick]

Well, as a result of all your help, I do believe we've cracked it!

Sorry Mark, although I understand the principal of the saturation test, I don't know how to connect a current meter in series on a transformer. Oops! embarrassing  
Does it entail taking a current reading off the taps into which I am applying the voltage?

So in ignorance I just went for the 'matter of elimination approach' and I have established (I think) that winding 1 and 2 is just one winding with 7 taps - the primary. By keeping a constant 20v across the taps I measured the output on what was winding 4 and worked out that if taps 1 and 7 (see pic) are the start and end of the winding then winding 4 is a 4v heater winding (now re-labelled winding 3). The high voltage winding is 300-0-300v (now re-labelled winding 2) and the final winding is 2-0-2v.

To see if this was consistent with the Philips 795A I managed to find the Philips Service Manual online and all the valves are 4v heater types (FC4, VP4B, TDD4, PEN A4, 1821) and the maximum voltage across C2 is 275v.

Do you think the values I have calculated make sense?

Nick
   

[Alan Douglas]

Does it entail taking a current reading off the taps into which I am applying the voltage?

You meter the current you're feeding into the transformer.

[ppppenguin]

it's always tricky to identify this sort of conceptual problem at a distance but here goes.

Voltages are always measured ACROSS something. You connect the voltmeter to 2 points in the circuit and you are measuring the voltage between them. Sometimes the very descriptive term "potential difference" is used.

Currents flow THROUGH something. To measure a current you have to break the circuit and insert the ammeter.

This diagram http://www.ryerson.ca/~kantorek/Circuitbasic.gif shows an ammeter and voltmeter correctly conencted. In this case the source of power is DC from a battery and the load is a resistance. I found the diagram on this page http://www.ryerson.ca/~kantorek/EES512.html where there's a nice simple explanation of what's happening.

Nick, hope this helps.

[SurreyNick]

Got it!
Thanks
Nick