The majority of loss in a loaded transformer is due to winding resistance (copper losses) so the maximum power out is more or less proportional to the input voltage, at least up to the point where saturation comes into play. Half volts in equals (same current) half power out.
Regulation will be worse at lower voltages because copper losses are relatvely higher.
Barry
Regulation will be worse at lower voltages because copper losses are relatvely higher.
Barry
Which also means the output power will be less than half:
At its nominal voltage, the transformer takes a total P=Pout+Pcu.
The useful output is Pout=P-Pcu.
At half that voltage, the current remains the same and P'=P/2.
Pcu remains the same, therefore P/2=P'out+Pcu, and P'out=P/2-Pcu.
It is easy to see that 2*P'out < Pout:
P-2*Pcu < P-Pcu.
Most medium to large power transformers have around 5% impedance at rated power meaning the output voltage sags by 5%. If the input voltage is halved then the voltage still sags by 5% but the actual sag now becomes 10% of the output voltage. The impedance is only partially related to loss, even large transformers in the MVA range have similar impedances but their loss is in the 1 to 2% range.
Transformers used in electrical distribution and transmission have high X/R ratios, on the order of 10 - 20. This is because the goal is to have low losses, and the majority of impedance is due to leakage inductance.
Not so with the small power transformers used in audio circuits. You can measure primary and secondary DCR's of each winding and end up with a very accurate model of how the transformer will regulate. In other words X/R is small.
So the smaller the transformer the more impedance is related to copper loss.
Not so with the small power transformers used in audio circuits. You can measure primary and secondary DCR's of each winding and end up with a very accurate model of how the transformer will regulate. In other words X/R is small.
So the smaller the transformer the more impedance is related to copper loss.
assuming thats a cheap 1-2 pound E-! transformer, and consumes 5-10 watts no load with a 30C temp rise no load and a 70C temp rise full load..
cutting the voltage in half will drop that no load temp rise to say 5C, so you can practically double the copper losses which means 41% more current.
regulation will drop to one third/fourth what it was originally...
cutting the voltage in half will drop that no load temp rise to say 5C, so you can practically double the copper losses which means 41% more current.
regulation will drop to one third/fourth what it was originally...
I think you got mixed up somewhere, the regulation of a transformer is related to its impedance which is the vector sum of Leakage inductance and DC resistance not the loss which is a function of DC resistance + iron loss. Halve the voltage halve the regulation. Look at the equivalent circuit of a transformer if that does not make sense.
Barry As for 30C core temperature rise, not the best design but quite common on cheap consumer transformers and also a few welding transformers which usually have class H insulation so 30C is not a big rise for them. I doubt an off the shelf transformer larger than 100VA would run that hot at idle.
This is what I was told by an engineer at a large transformer manufacturer when I asked a similar question.
Neer As I Kin Figger...
Near as I can figure, Guys, the secondary voltage would be halved because of 110 volts being on a 220 volt primary. The current rating would be about the same as normal. Volt-ampere rating would be halved because of half voltage on secondary, with no current increase available. Copper loss would be much higher because of thin wire being wound twice as many turns per volt as 110 volts on the primary requires. Probably 2-4 times the copper loss of a correctly wound 110 volt primary. Regulation would have to suffer somewhat. All that said, I actually had a method-driven madness to use a transformer that way to power an FM Audio Processor that I designed and built. I used a 10 VA trafo to power a system that used a watt or two. The halved level of flux-leakage gave about 6db less magnetically-induced hum in the low-level stages!
Near as I can figure, Guys, the secondary voltage would be halved because of 110 volts being on a 220 volt primary. The current rating would be about the same as normal. Volt-ampere rating would be halved because of half voltage on secondary, with no current increase available. Copper loss would be much higher because of thin wire being wound twice as many turns per volt as 110 volts on the primary requires. Probably 2-4 times the copper loss of a correctly wound 110 volt primary. Regulation would have to suffer somewhat. All that said, I actually had a method-driven madness to use a transformer that way to power an FM Audio Processor that I designed and built. I used a 10 VA trafo to power a system that used a watt or two. The halved level of flux-leakage gave about 6db less magnetically-induced hum in the low-level stages!
Hello,
Small VA transformers, the kind we use in DIY projects are voltage or regulation limited. What that means is that as the transformer is loaded the voltage can drop up to 20% from no load to where the voltage drops to below name plate. Larger VA transformers have better regulation meaning there is a smaller voltage drop from no load to nominal voltage before temperature becomes the limiting factor.
This is my rule of thumb for DIY audio; never select at transformer to operate over ½ of the name plate VA. Never mind the 120 / 240 thing. Hook them up with the secondary as primary if you like. Pick a resistor to model your anticipated load. Apply power slowly with a variac measure voltage and calculate VA. With the load applied monitor the rate of temperature change with a Fluke infrared thermometer. I do not like to run a transformer hotter than I can hold my hand on or about 140 F or 55 C.
Save all that theory for when you roll your own.
DT
All just for fun!
Small VA transformers, the kind we use in DIY projects are voltage or regulation limited. What that means is that as the transformer is loaded the voltage can drop up to 20% from no load to where the voltage drops to below name plate. Larger VA transformers have better regulation meaning there is a smaller voltage drop from no load to nominal voltage before temperature becomes the limiting factor.
This is my rule of thumb for DIY audio; never select at transformer to operate over ½ of the name plate VA. Never mind the 120 / 240 thing. Hook them up with the secondary as primary if you like. Pick a resistor to model your anticipated load. Apply power slowly with a variac measure voltage and calculate VA. With the load applied monitor the rate of temperature change with a Fluke infrared thermometer. I do not like to run a transformer hotter than I can hold my hand on or about 140 F or 55 C.
Save all that theory for when you roll your own.
DT
All just for fun!
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