Good morning all, just wondering if there has ever been any data published on what temperature a power transformer should reach in a five valve radio. Bowler

I am not aware of any such data. The bottom line is, a power transformer should run almost stone cold in most cases.

Back in the day, there were different mains voltages, frequencies and current flows, depending on the location one lived in and valve radios either had to be prepared to run on them all or there had to be sufficient models available to cater for the various situations.

In the outback, things mostly ran on 32 volts DC. Many radios in the 1930s and 1940s came with an option for 32VDC.

AWA made the 1940s Radiolette with a rather large transformer (larger than needed in most cases) to allow it to run on 40Hz without getting hot. This was despite most of these radios being plugged into a 50Hz supply. The same model could work with 260VAC too, to allow for wide variations in nominal voltages at the time.

The Airzone Radiostar of the late 1930s came as an AC-only model, DC-only model and a rather unsafe by modern standards AC/DC model with a live chassis which meant that if it was plugged into a GPO with the polarity reversed then touching the back of the radio could have been quite shocking if you get my drift.

Whatever one chose though, all power components should run fairly cool, with most residual heat being radiated across from the super hot rectifier valve.

As above, specifications vary and sometimes design short cuts were taken to reduce size and cost.

That said, generally cool to warm is my experience for a healthy setup. Noticeably hot would be a worry, as would any smells issuing from the transformer -- in which case a short (internal or external) or overload should be looked for.

My observation is that wax encapsulated transformers from the '30s operate about 20C above ambient, temperature measured on the top of the lamination pack. 50s lacquer encapsulated transformers seem to operate a little hotter, maybe 25C above ambient.

The better wire lacquers available in the 50s would account for the higher temperature tolerance.

The windings would operate much hotter than this but I haven't tried this measurement - chance of getting a bite. Might try with an infrared meter. In the case of the wax encapsulated transformer the winding temp can't get above the melting point of the wax, which can be microcrystalline waxes with resin added to make them stickier. The melting point of microcrystalline wax can be up to 95C, but if it is blended to give just sufficient melting point for the application, would be lower.

I have tried to find the specification for these waxes but evidently many were proprietary and the compositions kept close to the manufacturers chest, especially for RF coils where the effect on inductive/capacitive properties were critical.

I have used a cheap inside/outside thermometer for these measurements, the outside probe stuck to the laminations with bluetack, and the inside result giving the ambient temperature. It generally takes about an hour for the temperature to max out.

Transformers built properly have a duty cycle and that is related to the end use. I have seen a PMG one which is twice the size of many of the same output. The difference is that the PMG one was to run 24/7 without exceeding a specified temp.

I would concur that much of the modern copper wire insulation is far superior than the lacquer used on many older ones. Some I have seen is quite brittle now. and not suitable for winding nor rewinding.

Marc

I should say that the measurements made have not been exhaustive - just done while working on a radio to keep an eye on how the mains electrical side is going - sudden changes mean pull the plug and investigate since the mains transformer has a high unobtainium content.

I have collected a few dead radios and the odd power transformer. Plus a few speaker ones as they especially in Krieslers & some HMV Nippers (tarred) have a high attrition rate. The early transformers were made to a spec. and nothing in them was supposed to sustain combustion.

I think there is data in the 1937 Trade Annual, but due to the insulation newer transformers can run hotter. But that comes down again to "heat, duty cycle" & materials in it. I will see if heat is mentioned.

Ok! It is there along with what the standard test is. Not to exceed 55 degrees C. A lot of Wall warts & others do have a thermal fuse in the wrap. That normally cant be got at.

I ended up putting one of those in a UPS, as it liked destroying batteries inside 3 years and getting that hot you cold burn your hand on it. Devices that can do that should be banned, or recalled.

Marc

Good morning, thanks to all who have contributed to my question of temperature in power transformers. I have tested a few transformers in radios that have operated for about 1 hour, and would agree that approx 20 degrees C above ambient is close to the mark. The only exception was an awa model 527 whose rectifier and output valve are very close to the transformer on the chasis. Bowler

Where you have to be a little cautious, especially around here, is with the ambient temp. It is not unusual for the buildings here to get to ambient temp which on occasions will get close to 50 degrees and most enclosed ones, including the house have been modified, or built with gabled rooves and venting. Note that a bathroom fan can send a lot of air into a roof: Tiles stay hot.

The main ramification here is that all that extra heat does affect the electrical system and many have not realised that many circuit breakers & such in severe heat will downrate & shut off. I ended up insulating & modifying the venting in the pump shed because of that. So watch where you put the fuse box.

The distributor, & studio work shop boxes are outside all the rest (4) are inside. Solar & Workshop have a pelmet / Verandah over them to reduce exposure to direct sun & 30mm hail stones. Main distributor is being blended into a medium shrub.