Transformer questions

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** With no secondary load, primary dissipation can be roughly doubled so current goes up by 41%. Using the primary CT to feed a load results in the same current in each half but double in the load.

So the new VA rating is 1.41 times the original.

** The thing gets very hot.

** A 70C or 80C bi-metal cut off would likely do the job.

No way know for sure as wire temp grades vary a lot.

.... Phil

Why not ask a transformer manufacturer this question? Hammond Manufacturing for example. They are happy to answer engineering questions by email or phone.

I understand that they traditionally underrated their transformers, but I can't vouch for that - just something I was told a long time ago (mid

60s) when I was active in an antique radio club in Toronto - close to the Hammond factory, and Fred (? - recollection of name) Hammond was referred to by some members who knew of him and his company. As far as I can tell the company still builds amongst the best xformers on the planet.

John :-#)#

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** Cannot say much about transformers they did not make.

OTOH I can, because I see and analyse so many of the damn things from all over the world.

** Err - what is the "traditional" amount then ?

IME their ratings are in line with industry norms.

.... Phil

They also aren't going to make up new ratings for us for tapped primary operation.

NT

Random ones will have 120V inputs and 240V inputs without taps or split windings. For those, N/A.

Come to think of it, I have very few transformers in my collection that have this. Kind of an odd question...

If you mean that the random sample is conditionally selected for having split or CT primary, then yes, the autoformer tap can deliver twice the nominal primary input current. Which can be increased slightly above nameplate rating, for the reason Phil gives.

To protect against a 2:1 overload, use a fuse. The transformer will take tens of minutes to heat up, in which time the windings are already toast.

Tim

-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website:

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OK, I've put the hammer & nails down

ave

most of mine are centre tapped primary fwiw.

Maybe thermal protection needs to be on the inside. On 240v:

50VA is 208mA 12VA is 50mA 6VA is 25mA I'm not sure how I'm going to protect the smaller ones with a mains fuse. L ooks like it would need to be a 20mm glass fuse plus a higher value ceramic mains fuse. Even so I'm not going to find 25mA or 50mA fuses (or whatever the value is with magnetising current).

Maybe the solution is to not make such low power 120v transformers, start a t 50VA.

I'm trying to imagine how parallelling centre taps would go ;)

NT

Yes, transformers with thermal protection have the fuse wrapped inside the primary winding.

As is the usual case with fuses, it's a fire prevention method. The component (transformer) is still trash afterwards. It's sometimes possible to repair (if you can yank the fuse out of the winding and replace it), but that's a rather extreme measure, not at all economical.

I'm guessing "extreme" might be on the table, based on your "random pile" and the nature of your posts, so FYI.

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
Website: https://www.seventransistorlabs.com/

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The idea is how to make formally safe a wide variety of laminated core tran sformers converted to 120v output. I suspect the way forward is a fuse for medium iron, and a fusible resistor for small ratings. Rebuilding the trans formers would be much better avoided if possible of course.

Replies have been quite informative. Thanks everyone.

NT

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** A "thermal fuse" is NOT a fuse at all - it's a one shot, temp cut off.

Self re-setting cut offs are also used the same way as are thermistors that activate an external cut off system. In all cases, the trip temp is set below the max temp rating of the winding wire and insulation used.

Its not for fire protection, but rather for *user safety* where a safety ground does not exist.

** Nonsense.

But because the one shot device is deeply buried in the primary winding, its non always practical to replace it.

.... Phil

You might be able to use the secondary as a PTC temperature sensor. Inject DC and filter out the AC component. But a large enough DC current would probably saturate the core. If you have two equal secondary windings they could be put in series opposing to cancel the AC voltage and the net magnetizing current would be zero. Might not be practical, but it's a thought...

Paul

A fuse is a one shot, current cut off. How is that not a fuse?

There are even "mechanical fuses", which cut off above a certain tension or torque. Much better than to, say, snap a bolt, than shear a gear tooth in a transmission. Precisely the same purpose.

"Our Thermal Cut-Offs (Organic Thermal Element Type) are used to _prevent fires_ caused by abnormal heat generation from circuits and other heat producing electrical products. They are a non-resettable thermal _fuse_ which open electrical contacts when temperatures exceed the specified level." [my emphasis]

Further down, they give a curve of temp rise vs. current. The temp rating is reduced at higher current levels. Not very significantly for this particular part, but there are many types available. Current and temperature are not independent.

That's precisely what I went on to say...

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
Website: https://www.seventransistorlabs.com/

I don't think UL/CE will have any problem with properly sized fuses and, when available, thermally protected transformers.

The smallest transformers are typically "impedance protected" on account of their small size, but this won't be the case when using a split primary autoformer, because there is very little leakage inductance.

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
Website: https://www.seventransistorlabs.com/

Depending on the transformer class, you may be able to attach a PTC to the transformer outside and then it's resettable. Done that for UL/ETL. Also done the internal fuse... Just gotta make sure it doesn't blow unless the transformer is going to be toast anyway. Harder to do right IMO.

boB

I have designed large transformers with a hollow fiberglass tube in the windings in which a thermistor or thermostatic switch can be inserted. These were large transformers with multiple sections 10 or 20 kVA each, but it could be done for smaller units. If the windings are fairly loose, or if there is some space between the windings and the core, a small thermistor could be jammed in. You would need to make a sensor circuit with an SSR or other means to remove power. You could also add indicators to show temperature. Maybe red, yellow, and green LEDs.

Paul

Ahhh yes ... Fiberglass tubing Done that with 1kVA to 5 kVA inverter transformers. Had to also place a Klixon thermal cutout (UL Recognized) inside one of them. Others had thermistors taped and glued to the outside going to the ptocessor for de-rating.

boB