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Old 9th Jul 2012, 7:01 am   #1
Neil Purling
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Default Power Transformer gets hot

Its a 4+1 superhet with a fully isolated PSU.

The transformer is not hot enough to hurt if one touches it, but after an hour or so in operation it is more than warm.
I was experiencing hum issue, so I suspected HT load, maybe fault or defect in the smoothing/reservoir can.
I removed all the valves excluding the rectifier and then the rectifier too & it still got heated.
I remember something about eddy currents in transformers from school physics lessons,.
I would not expect a transformer to remain cold if energised without any Secondaries being loaded.
Is this heating effect therefore to be expected? I was concerned at first & then somewhat puzzled after I took the steps I have outlined.
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Old 9th Jul 2012, 7:35 am   #2
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Default Re: Power Transformer gets hot

Neil,

I suspect you have shorted turns in one of the windings. I wouldn't expect a transformer to get warm if it had no load on its secondaries - though you think the opposite.

I presume you have checked the obvious, such as the mains input tap being set for the incoming voltage?

Try the tests in the sticky at the top of this forum.

Richard
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Old 9th Jul 2012, 7:55 am   #3
Neil Purling
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Default Re: Power Transformer gets hot

I will remove every connection to it apart from the connection to the Primary, which will prove the origin of the problem.
The hum concerned me at first, especially as the reservoir/smoother double can is a modern example by F&T. It did not hum at first test. I thought I had disturbed something when I was poking about under the chassis setting the alignment, which was excessively loading the HT supply.

The transformer itself does 'buzz' somewhat more than before. Said component has powered various home-brew projects over the years.
I wonder if this new role has over-loaded it? I don't want to kill off the replacement too.

Transformer is marked "MC413063". The Secondary windings are marked: 252-0-252 58mA, 6.3 3A & 6.3 0.6A. The last one is for the rectifier.

Looking at the max anode & screen currents for the valves in the circuit I get 60mA. Hmm. Wonder what the true total HT load is? This is relevant to choosing a suitable replacement of course.

Last edited by Neil Purling; 9th Jul 2012 at 8:14 am.
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Old 9th Jul 2012, 9:06 am   #4
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Default Re: Power Transformer gets hot

Neil,

transformers generally hum because the laminations are moving very very slightly in the varying magnetic field. If yours hasn't been varnished or dipped in some sort of gunge (it used to be pitch on the early ones), then that may well explain the hum. If you are overloading it, then that too would make the hum worse - but some unvarnished transformers will happily hum away with no load on them at all.

Chances are that your valves are not all running at the max anode and screen currents - so your 60mA figure is only theoretical. An AVO should quickly reveal what the load actually is.

Richard
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Old 9th Jul 2012, 9:28 am   #5
Neil Purling
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Default Re: Power Transformer gets hot

I got my soldering iron out and lifted every connection, apart from the mains AC input to the Primary winding. I have left it energised since then & it's still heating up.
That means it's mullered?
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Old 9th Jul 2012, 9:44 am   #6
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Default Re: Power Transformer gets hot

Afraid so.

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Old 9th Jul 2012, 10:54 am   #7
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Default Re: Power Transformer gets hot

Is there any possibility that something local has added some DC to the mains? That might give the same symptoms, although more of a problem with toroidal transformers. A shorted turn would heat a winding before the core. A cheap transformer could be on the edge of saturation when off-load, as it relies on primary voltage drop to get it under the limit when loaded. Looking at secondary voltage waveforms with a scope might show this - smooth sine wave means no saturation.

Having said all that, a shorted turn is probably still the most likely cause.
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Old 9th Jul 2012, 11:58 am   #8
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Default Re: Power Transformer gets hot

You could try measuring the two 252 volt secondaries. Any marked difference would prove it, but it depends on the accuracy of the winding, and one shorted turn would not make much difference.
As Dave says, some transformers do run warm, and taking an hour to warm up does not seem to be catastrophic.
Does it stay warm, or does it keep getting ever hotter?
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Old 9th Jul 2012, 12:38 pm   #9
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Default Re: Power Transformer gets hot

Quote:
Originally Posted by trh01uk View Post
I wouldn't expect a transformer to get warm if it had no load on its secondaries
Some transformers do get warm with no load whatsoever. But it shouldn't be very much.

A transformer designed for best efficiency in a given size, has roughly equal copper losses and iron losses at full load. So, completely off-load, copper losses are very low, but cores losses (which are nealy load-independent) will still exist. Net result is that temperature rise is very roughly half of what it would be at full load.

Time to reach thermal equilibrium is typically a couple of hours, for a transformer up to 100W or so. So, if it is still comfortable after this time, it's probably OK.

Last edited by kalee20; 9th Jul 2012 at 12:39 pm. Reason: Made clearer and sorted spelling!
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Old 9th Jul 2012, 12:48 pm   #10
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Default Re: Power Transformer gets hot

I would expect a sound but unloaded transformer to get warm but not hot.
Up to about blood heat is fine, and little above blood heat is probably ok, but more than that suggests shorted turns or other defect.
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Old 9th Jul 2012, 12:51 pm   #11
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Default Re: Power Transformer gets hot

What is the current taken by the primary winding with everything else disconnected?

Do you have a variac, so that you can reduce the mains supply by, say 20-30 volts?
If so, or you can do it by adding series resistors, you can quickly see if it is running close to saturation as a fairly small reduction should see the current fall quite sharply.

Is the transformer old or new, and is it definitely for 230/240V rather than 220V?
If it is of unknown provenance, you could check the voltages from the heater windings; if they are all a bit high, it could be that it is intended for a lower primary voltage than your mains supply.
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Old 9th Jul 2012, 5:16 pm   #12
Neil Purling
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Default Re: Power Transformer gets hot

The transformer was an old one. It is a drop-through fitting & has one soap dish with a voltage selector plug on top. I have always had it in the 235-250V selection.
Does anyone recognise the "MC413063" code? It doesn't seem like a military stock code & the fact the ratings were marked on the transformer suggests it was a off-the-shelf product from somebody. I bought it in 2007. It may have been from a Forum member.
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Old 9th Jul 2012, 6:14 pm   #13
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Default Re: Power Transformer gets hot

Hi Neil

The Part number is almost certainly a Cossor number it will have come from something like a 524 Melody Maker.

Cheers

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Old 9th Jul 2012, 7:44 pm   #14
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Default Re: Power Transformer gets hot

Hi Neil
I had a few mins so I had a quick look and Cossor Models 522A Radiogram and Melody Maker 523A use this transformer it may have also been used in other sets.

I have a 524 apart at the moment and it's an almost identical transformer.

Hope this all helps.

Cheers

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Old 9th Jul 2012, 8:01 pm   #15
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Default Re: Power Transformer gets hot

Hi Neil, check that the clamping bolts for the laminations are not creating a "shorted turn" with the laminations. You can remoce them and see if the transformer runs cooler.

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Old 9th Jul 2012, 9:00 pm   #16
Neil Purling
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Default Re: Power Transformer gets hot

The figures for resistance are interesting. How much difference is significant?
I get 115.6 & 110.9 ohms for each side of the HT winding.
The transformer has remained undisturbed for two years. It has powered various projects and now it chooses to throw a sickie.
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Old 9th Jul 2012, 9:35 pm   #17
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Default Re: Power Transformer gets hot

Hi Neil

A mismatch of 5 Ohms doesn't seem unreasonable. The wire length is not the same for the two halves of the winding as the diameter of the winding increases.
If it is a shorted turn you are better off using a shorted turn tester which is really simple and has been covered in this forum on more than one occasion.

One such thread is here

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Old 9th Jul 2012, 10:22 pm   #18
Neil Purling
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Default Re: Power Transformer gets hot

Doesn't seem to make any difference in the off-load AC voltage either side of the centre-tap.
If I re-connected the transformer & started to use it on-load can the problem become worse?
I will see what the HT current draw is & go from there.
I can reduce the HT load if necessary by using a external AF amplifier.
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Old 10th Jul 2012, 10:37 pm   #19
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Default Re: Power Transformer gets hot

In my own past experience if shorting turns occur its only a matter of time before the whole transformer self destructs.
Local heating breaks down the insulation and eventually you end up with a toasted mess, a bad smell and smoke!
The undertaker usually has a good selection of transformers at affordable prices at most BVWS meets.
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Old 10th Jul 2012, 11:36 pm   #20
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Default Re: Power Transformer gets hot

One time, I turned a blind eye ("denial" and all that) to the fact that the mains transformer in a CR100 was ruinning rather warmer than it ought to have been, it wasn't long (a few dozen hours running, maybe) until a loud whooshing, rather like a Roman candle, alerted me to how serious things had become. A peek under the lid released a spectacular geyser of thick, white, waxy smoke. Not a lot of doubt there.

I'd be interested to know what the prospects of rewinding/rebuilding such a burn-up are- I appreciate from the outset that "an arm and a leg" would come into the reckoning but, in some cases, it might be worth it to some people. Would one present a pile of cleaned-up laminations to a re-winder for new bobbin/winding, or is it a case of new everything for a custom wind? In the above instance, I was lucky to find an almost exactly suiltable transformer (electrically and mechanically) in an otherwise thoroughly ravaged '60's Japanese amplifier chassis from a random visit to the tip, otherwise I would have made such a case a "free to collector" item.

Colin.
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