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Old 13th Dec 2018, 10:47 am   #1
ukcol
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Default Thermal protection of mains transformers

https://www.bowood-electronics.co.uk/ do a good range of thermal fuses but what temperature rating should I chose to give the best compromise between reliability and protection of a transformer against overheating damage.
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Old 13th Dec 2018, 11:19 am   #2
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Default Re: Thermal protection of mains transformers

Hi Colin,
A transformer running with a surface temperature of 70 deg C will feel 'ouch' hot to the touch. I'm sure the maximum temperature rating of the windings of most transformers will be much higher than this. I would not wish a transformer to even get to this temperature. In use, most transformers, especially in radio's, will run fairly cool. What I'm trying (very badly) to say, is that the lowest rating of thermal fuse (100 or 125 deg) would still be way hotter than I would want a transformer to run.

I bought some flat, square ones rated at (I think 85 or 90 deg) for a project. The transformer runs very cool so if it got to 85 deg there would definitely be a fault.
Don't forget, we can only mount these on the surface- the core temperature will be much higher. Also as a fuse, they don't protect from over current, but they do have a maximum current rating.

Hope that helps
All the best
Nick
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Old 13th Dec 2018, 11:37 am   #3
Steve G4WCS
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Default Re: Thermal protection of mains transformers

funnily enough I pulled apart a chinese wall plug transformer from a modern clock last night. it was rated at 5V 45mA to drive the LED backlight, and appeared to have gone open circuit. there was a small square 125 degrees thermal fuse taped to the primary winding. I've replaced it with an 105 degree one that I happened to have a supply of.
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Old 13th Dec 2018, 12:07 pm   #4
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Default Re: Thermal protection of mains transformers

Personally, if I was winding a transformer and I wanted thermal protection, I'd bury a one-shot thermal fuse within the winding, and it would be a 150°C one. That's because, I'd be impregnating it with varnish which needs 140°C baking to full cure it (no point in rendering the transformer useless before it's even been installed!). Using modern varnishes, insulating materials, etc, I'd be confident that if a fault occurred, the thing would still open well before the transformer caught fire.

But if you are trying to protect an already-existing transformer, maybe a wax-impregnated one with paper interleaving? That's tricky! I have seen such transformers fail with inter-turn shorts, such that an internal hot spot develops and smoke pours out, before the surface temperature has started to significantly rise. And I'd bet that if the fault occurred at switch-on, the thing would catch fire before the surface temperature had risen to even the normal running temperature.

In short, I don't think you would be able to retro-fit any meaningful thermal protection to an ancient transformer, against the transformer itself failing. You'd probably do better by appropriate fusing - maybe add a 1.25" cartridge fuse somewhere in the mains input winding (you can buy 1" cartridge fuses, for mains plugs, right down to 1A, from Farnell).

But, if you are looking at protecting the transformer failing due to a downstream medium overload (such as output valve drawing excessive current due to 'that' capacitor going leaky), then Nick's advice above is sound.
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Old 13th Dec 2018, 12:09 pm   #5
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Default Re: Thermal protection of mains transformers

I always fitted 60 deg C thermal fuses to the transformers of sets I restored commercially, with no false alarms resulting.

Steve
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Old 13th Dec 2018, 1:57 pm   #6
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Default Re: Thermal protection of mains transformers

Quote:
Originally Posted by kalee20 View Post

.....if you are trying to protect an already-existing transformer, maybe a wax-impregnated one with paper interleaving? That's tricky! I have seen such transformers fail with inter-turn shorts, such that an internal hot spot develops and smoke pours out, before the surface temperature has started to significantly rise. And I'd bet that if the fault occurred at switch-on, the thing would catch fire before the surface temperature had risen to even the normal running temperature.

In short, I don't think you would be able to retro-fit any meaningful thermal protection to an ancient transformer, against the transformer itself failing. You'd probably do better by appropriate fusing - maybe add a 1.25" cartridge fuse somewhere in the mains input winding (you can buy 1" cartridge fuses, for mains plugs, right down to 1A, from Farnell)....
Is there an argument for also fusing the individual secondary windings with a fuse of an appropriately type and rating?
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Old 13th Dec 2018, 4:19 pm   #7
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Default Re: Thermal protection of mains transformers

Hi Colin,
Fuses offer good protection for shorts where significantly more current than the fuse rating can be drawn. They are not so good for lower value overloads though.

For example, you might have a transformer that is supplying a steady current of 500ma. To cater for switch on surges, charging smoothing caps etc., you might have a 1A fuse. Under fault conditions, you might draw 1A, double the current so 4 times the heating effect in the transformer. The 1A fuse would not blow as it will take quite a bit more than 1A to blow it in a sensible time. So the fuse, under these conditions does not offer much protection!

I take Kalee's point that the thermal inertia of the transformer means that the thermal fuse mounted to the outside of the windings will take a while to open if the thing develops shorted turns. I still like the idea of mounting a thermal fuse- it will open eventually!

There is no ideal method, unfortunately!

Cheers
Nick
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Old 13th Dec 2018, 4:48 pm   #8
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Default Re: Thermal protection of mains transformers

Pye in the 697 chassis CTV had a thermal fuse in its transformer to supply the LT voltages. This small replaceable fuse fitted into a slot that’s was fabricated inside the winding. The fuse was snug push fit into the slot. It was very effective.

The design replaced a standard transformer that was only fused with a 2 amp antisurge fuse that also fed the rest of the set. Shorted Bridge rectifier diodes would burn out the transformer before the fuse blew. The replacement transformer was a vast improvement.

I think the fuse/transformer was a Philips design.
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Old 13th Dec 2018, 5:06 pm   #9
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Default Re: Thermal protection of mains transformers

Phillips have been using thermal fuses on transformers for a very long time.
Perhaps when they took over Pye they fixed the problem by changing to one of their 1970s pocket thermal fuses.
In the early days Phillips used a heat conducting bar under the primary with a slug of low melt solder and a spring.
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Old 13th Dec 2018, 6:02 pm   #10
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Default Re: Thermal protection of mains transformers

I didn’t know the name but “pocket thermal fuse” exactly fits the device. I carried them on the van, only place I used them was on the Pye range.
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Old 13th Dec 2018, 6:09 pm   #11
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Default Re: Thermal protection of mains transformers

They used to be lurking in Phillips VCRs in the late 1970s.
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Old 13th Dec 2018, 6:34 pm   #12
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Default Re: Thermal protection of mains transformers

The attachment shows the thermal fuse on the mains transformer fitted in the Murphy model A192.

Although the receiver works OK, the transformer thermal fuse needs to be reset after fifteen minutes use. Most likely the transformer has shorted turns?


DFWB.
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Old 13th Dec 2018, 7:27 pm   #13
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Default Re: Thermal protection of mains transformers

I was quite surprised when one transformer manufacturer told me that core temperatures with modern materials can often teach 180ºC without problems!

John
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Old 13th Dec 2018, 7:54 pm   #14
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Default Re: Thermal protection of mains transformers

All the ones I've seen have been 110-125deg or so.

They aren't usually baked into the transformer though, but trapped just under the polyester tape.

On old stuff I've seen little copper or brass pockets so it's easy to replace the fuse.
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Old 13th Dec 2018, 8:42 pm   #15
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Default Re: Thermal protection of mains transformers

Rather than embedded thermal-fuses [which fail-hard and are not resettable, meaning you need to replace the transformer to fix the fault] there have been for a few decades a range of thermal-control elements that have a seriously-positive temperature-coefficient and a very-relaxed return-to-low-resistance once the overheat is removed.

The only downside to these is that after each successive overtemperature-event they don't ever fully return to their previous low-resistance state - it's a sort of 'ratchet' effect. Fail-safe! And I sold quite a few 'replacement' transformers to clients on the back of this.
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Old 16th Dec 2018, 9:32 am   #16
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Default Re: Thermal protection of mains transformers

I may fit the lowest temperature one that Bowood do (77 deg C) but it looks like the consensus is that it is not a very effective retrofit answer.
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Old 16th Dec 2018, 7:20 pm   #17
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Default Re: Thermal protection of mains transformers

One important tip for working with thermal fuses: Buy some spares! It's easy to pop one by mistake when soldering (there is actually a proper tool for making solderless connections to thermal fuses, by crimping a ferrule over the TF lead and another wire; but it's not worth buying for a one-off job). Douse the thermal fuse with freezer spray and use a pair of pliers between the joint and the body as a heat sink.

Make sure the thermal fuse is sandwiched tightly against the windings, and cover it up to keep the heat in.
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Old 17th Dec 2018, 1:58 am   #18
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Default Re: Thermal protection of mains transformers

I chop the spade off a spade terminal and crimp the remaining bit onto a wire and the fuse.
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Old 17th Dec 2018, 2:58 pm   #19
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Default Re: Thermal protection of mains transformers

Quote:
Originally Posted by John Caswell View Post
I was quite surprised when one transformer manufacturer told me that core temperatures with modern materials can often teach 180ºC without problems!
And the rest!

I've been working on a 400Hz transformer which is built very much down to a size, 7kW in a 5kg weight. Although around 95% efficient, that's still 370W of power lost, as a consequence, even sitting in a howling gale from a small fan, it still runs rather hot!

Normal grades of wire enamel are spec'd up to 180°C. If you pay around 10x the price (£70/kg rather than £7/kg) , you can get enamel which is rated to 240°C. Insulation between windings in the transformer is Kapton film (Google it); the bobbin is silicone impregnated glass-fibre sheet; the thing is impregnated with varnish rated to 220°C; the internal joints are soldered with HMP solder which does not melt till 296°C.

There is a buried thermistor, which indicates that at full-load it is reaching 210°C. There is an overload requirement - it's supposed to operate at 25% overload for 10 minutes, and doing this it does exceed the continuous temperature rating of the varnish, but as this is expected to be an exceptional condition, occurring 10 times in the transformer's life, it has been signed-off.

The design would never have got off the drawing board without access to semi-exotic materials. But, they are available, even if not exactly commonplace!
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Old 17th Dec 2018, 4:09 pm   #20
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Default Re: Thermal protection of mains transformers

These days, cheap consumer appliances seem to have transformers which run way too hot for my liking. Whether they've skimped on copper or iron, I don't know. An example: clock radio - hot enough to warm your coffee if you place it over the transformer!

I exaggerate of course, but not by much.
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