Hello,
What manufacture says to run their transformer unloaded?
Unloaded it is not a transformer, it is a inductive heater. I bet that it will run cooler with a 20% load than with none.
DT
All just for fun!
I Suspect, But Haven't Seen It Written....
I don't think I've seen it written anywhere, but I suspect that transformers designed for heavy, continuous load are wound with lower turns-per-volt, than, say, a transformer designed for a widely variable load. I've seen a microwave oven transformer, secondary totally removed, draw 3A at 120V, and warm the core very rapidly after power was applied. I think the flux in the core decreases as load increases and high constant load transformers are wound with fewer turns-per-volt because the load keeps core flux at a substantially lower level than at idle. I also suspect that a high load transformer can use a much smaller core than a variable load transformer of the same VA rating.
I don't think I've seen it written anywhere, but I suspect that transformers designed for heavy, continuous load are wound with lower turns-per-volt, than, say, a transformer designed for a widely variable load. I've seen a microwave oven transformer, secondary totally removed, draw 3A at 120V, and warm the core very rapidly after power was applied. I think the flux in the core decreases as load increases and high constant load transformers are wound with fewer turns-per-volt because the load keeps core flux at a substantially lower level than at idle. I also suspect that a high load transformer can use a much smaller core than a variable load transformer of the same VA rating.
Copper loss in watts is similar at 110V and 220V because of the halved VA the copper loss as a percentage of power output is approximately double. There will be some increase in primary copper loss at 220V due to increased magnetising current
W=I2R, I is the same and R is the same so Watts is the same.
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Hi,
how hot are the inner windings and their insulation?
How hot is the iron core and it's insulation?
Meltdown!
The core saturates heavily and draws several times the normal magnetizing, also called excitation, current, and if unfused, overheats very quickly and burns out.
You don't even get double the normal secondary voltage while it's burning out.
The core saturates heavily and draws several times the normal magnetizing, also called excitation, current, and if unfused, overheats very quickly and burns out.
You don't even get double the normal secondary voltage while it's burning out.
Core flux is predominantly determined by applied volts, not load or current. Everything else you said sounds about right.
For 800VA or larger a 20A circuit breaker trips within minutes with even 20% over voltage. A small transformer without proper protection will cook.
You can put 240V into a 120V transformer if the frequency is a bit more than double (needs to be a bit higher to reduce iron loss) The insulation needs to be up to the job, this technique is carried further with aircraft power where 400Hz and 1000Hz are used resulting in very light power transformers and power supplies. I once stripped an 10Khz induction heater which had a 1 MVA water cooled 4:1 step down transformer, The transformer weighed about 50 Kg the windings were interleaved like a good quality audio output transformer and .004" laminations were used
Ok, so I dug out a suitable transformer, Toroid 240 V in 24 V out at 1A.
With 110 in I got 13.1 V no load
9.9 V 12 R load
9.05 V 9 R load
8.5 V 7R5 load
The last reading is 1.13 A so exceeding the rating.
This ran for 2 hours perfectly happily, got warm about 35 degrees with a 15 degree ambient.
With 110 in I got 13.1 V no load
9.9 V 12 R load
9.05 V 9 R load
8.5 V 7R5 load
The last reading is 1.13 A so exceeding the rating.
This ran for 2 hours perfectly happily, got warm about 35 degrees with a 15 degree ambient.
I didn't realise it was as bad as that !
A universal 115+115Vac : dual secondary, 50/60Hz is rated to operate on 240Vac. This can rise to 254 (240+6% or 230+10%) here in the UK.
Where does the +20% start from? 230, or 240 or 254Vac?
24VA transformer run at ~46% of rated voltage does not overheat when pulling 9.6W out of it. That's just 40% of the rated VA and output voltage is crippled already. Regulation @ 110Vac is 54% for 9.6VA.
Could you apply 240Vac and draw the same currents and post the voltages? This way we can see how regulation varies with load and with input voltage.
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Voltage over here is 240V +/- 6% but I put 300V on the transformer so claiming 20% was a bit misleading, At my home the voltage is often 250V/ 435V. I was surprised as well when the breaker tripped but if the transformer was operating on the knee of the saturation curve, 20% would have put it into mild saturation.
Just a bit on running transformers at half their rated VA, this may be a US thing, digging though my transformer collection yeilded 1.2 and a 2kva, US (GE) transformers I could carry one in each hand, they were the smallest mains transformers of that rating I have seen. The Japanese, Australian, German and British transformers I have in the 700Va to 1.2Kva range are at least as heavy as the 2KVA US built unit. They were all EI style
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so +20% on our 254Vac gives ~304Vac and at this voltage the core saturation allows ~20Aac to pass into an 800VA transformer and trip the MCB.
the last 10 posts or so introduced 10 questions addressing nearly every variable into the mix!
so, first off i don't think i need to show manufacture data showing 30C temp rise no load.. this is normal for cheap chinease sh!t, in fact this is why its common for the 105C thermal cut out to break under slight overload!
in fact, by running at moderate saturation you can "regulate" away some of the primary resistive voltage loss... this is how you get a "constant voltage" transformer. they run pretty hot for obvious reasons.
What i meant by regulation is (full load-no load)/no load voltage.
so if you run it at half voltage and pull 41% more current I just figured about one third the regulation you'd get from full voltage and standard load.
i'll have to see if i can duplicate it.
you'd have to cut core loss more than 4% (assuming at full load cu loss= fe loss) and the only way that happens is because the primary voltage drop reduces flux density.. so its possible, but only if its something like a microwave oven transfo..
so, first off i don't think i need to show manufacture data showing 30C temp rise no load.. this is normal for cheap chinease sh!t, in fact this is why its common for the 105C thermal cut out to break under slight overload!
in fact, by running at moderate saturation you can "regulate" away some of the primary resistive voltage loss... this is how you get a "constant voltage" transformer. they run pretty hot for obvious reasons.
What i meant by regulation is (full load-no load)/no load voltage.
so if you run it at half voltage and pull 41% more current I just figured about one third the regulation you'd get from full voltage and standard load.
true, but where the thin wire exits the bobbin i've seen lots of fused wires on sub 20Va tx's.
that's nonsense, but plausible in some cases.
i'll have to see if i can duplicate it.
you'd have to cut core loss more than 4% (assuming at full load cu loss= fe loss) and the only way that happens is because the primary voltage drop reduces flux density.. so its possible, but only if its something like a microwave oven transfo..
I never said that the secondary would fuse. I simply stated that the secondary is designed to supply a certain current. If you wish to exceed this current then you run the risk of the secondary overheating and eventually failing.
The magnetic core may well be able to store and transfer more energy at a different voltage on the primary but ultimately it is the secondary that is providing the output current.
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