DIYers are often confronted to unknown salvaged transformers.
The overall VA rating of a transformer can be estimated by a number of well-known methods, like size, weight or primary resistance.
Methods to compute the share of each winding when the transformer has a number of heterogeneous windings seem less common.
Here is a deterministic approach to do just that.
Note that the method is mathematically accurate, but the result may be not 100% accurate, because of practicalities: placement of the winding in the winding window, finite number of preferred wire gauges, etc.
It is better than nothing though, and it gives usable results.
The method:
You first have to measure the DC resistance of each winding of the transformer, using 4-wires techniques or equivalent when required.
You then measure the open-circuit voltage of each winding. Note that the voltage needs to be actually measured, even when there is an indication of the theoretical or nominal voltage.
You then compute the power figure for each winding: the V²/R quotient. This figure represents the power of the winding in question, with an unknown coefficient. The coefficient is the same for all the windings on the same core.
When you have all the individual coefficients for the secondaries, you can compute the sum: Σ
With this information you are able to compute the percentage of power for each winding, and if you know the global power, you can calculate the power and current for each winding.
The primary power figure can also be computed: it acts as a checksum, it has to be equal to the Σ secondary.
If it does not coincide, there may be reasons: if the difference is smaller than 10%, it is a normal variation.
If the secondary Σ is much larger, that means the transformer is unbalanced, either as a result of poor design or on purpose: sometimes, not all the windings are meant to be used at the same time.
If the primary Σ is larger, that is probably down to poor design (overdesign of the primary). I see no good reason for doing it
The overall VA rating of a transformer can be estimated by a number of well-known methods, like size, weight or primary resistance.
Methods to compute the share of each winding when the transformer has a number of heterogeneous windings seem less common.
Here is a deterministic approach to do just that.
Note that the method is mathematically accurate, but the result may be not 100% accurate, because of practicalities: placement of the winding in the winding window, finite number of preferred wire gauges, etc.
It is better than nothing though, and it gives usable results.
The method:
You first have to measure the DC resistance of each winding of the transformer, using 4-wires techniques or equivalent when required.
You then measure the open-circuit voltage of each winding. Note that the voltage needs to be actually measured, even when there is an indication of the theoretical or nominal voltage.
You then compute the power figure for each winding: the V²/R quotient. This figure represents the power of the winding in question, with an unknown coefficient. The coefficient is the same for all the windings on the same core.
When you have all the individual coefficients for the secondaries, you can compute the sum: Σ
With this information you are able to compute the percentage of power for each winding, and if you know the global power, you can calculate the power and current for each winding.
The primary power figure can also be computed: it acts as a checksum, it has to be equal to the Σ secondary.
If it does not coincide, there may be reasons: if the difference is smaller than 10%, it is a normal variation.
If the secondary Σ is much larger, that means the transformer is unbalanced, either as a result of poor design or on purpose: sometimes, not all the windings are meant to be used at the same time.
If the primary Σ is larger, that is probably down to poor design (overdesign of the primary). I see no good reason for doing it
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Generally the manufacturer specifies their transformer by secondary VA.
All transformers are less than 100% efficient at full power.
Therefore the primary VA MUST be greater than the secondary VA when loaded to maximum output.
It might just be 5% greater, i.e. within the 10% tolerance given by Elvee. BUT, if the transformer is temperature VA rated and has a small primary copper (less VA than the secondary) then this will result in the primary being the limitation to VA rating.
Many transformers have more copper in the secondary than in the primary.
This extra copper results in an apparent total secondary VA that is greater than what the transformer can deliver. The effect is to reduce the transformer regulation.
Some manufacturers will make a transformer where the VA is specified by using maximum operating temperature.
Some will have an option to specify VA by regulation instead of temperature.
These transformers generally have a high proportion of copper and run at a lower core flux.
Do not expect the winding VA for these regulation specified transformers to fit with Elvee's model.
All transformers are less than 100% efficient at full power.
Therefore the primary VA MUST be greater than the secondary VA when loaded to maximum output.
It might just be 5% greater, i.e. within the 10% tolerance given by Elvee. BUT, if the transformer is temperature VA rated and has a small primary copper (less VA than the secondary) then this will result in the primary being the limitation to VA rating.
Many transformers have more copper in the secondary than in the primary.
This extra copper results in an apparent total secondary VA that is greater than what the transformer can deliver. The effect is to reduce the transformer regulation.
Some manufacturers will make a transformer where the VA is specified by using maximum operating temperature.
Some will have an option to specify VA by regulation instead of temperature.
These transformers generally have a high proportion of copper and run at a lower core flux.
Do not expect the winding VA for these regulation specified transformers to fit with Elvee's model.
This consideration does not apply to this method: that is the reason why I indicated the necessity of measuring the no-load voltages. Only the magnetizing current would need to be taken into account, but it is normally negligible except for very small transformers
I don't see why your explanation over-rides the efficiency of the transformer.
If the output is 200VA.
If the efficiency while delivering that VA is 94%.
The input must be 200/0.94 = 212.755VA
The primary must not overheat when it passes that ~212.8VA
The primary VA must be more than the rated secondary VA.
If the output is 200VA.
If the efficiency while delivering that VA is 94%.
The input must be 200/0.94 = 212.755VA
The primary must not overheat when it passes that ~212.8VA
The primary VA must be more than the rated secondary VA.
I always use the loaded secondaries method.
If you can, measure the wire size of each secondary and use that to ascertain the MAXIMUM current that that winding could supply.
Then, load each secondary simultaneously until the loaded voltage droops by about 10% from the steady state voltage.
Again, it's not an exact method, just another method.
If you can, measure the wire size of each secondary and use that to ascertain the MAXIMUM current that that winding could supply.
Then, load each secondary simultaneously until the loaded voltage droops by about 10% from the steady state voltage.
Again, it's not an exact method, just another method.
Because it has nothing to do with practical considerations: that is simply the way the parameters are processed mathematically in this method.
To make your point valid, you would need to use the nominally loaded voltages for each secondary. This has two serious inconvenients: you have to know the power of the windings beforehand, which is unfortunate because that is precisely what you are trying to know, and it makes everything hugely more complicated (might be counted as an advantage by some people though).
With the method as described, the only notable cause of errors is the magnetizing current. There are also higher order effects, like the interaction of the leakage inductance with other parameters, but these are truly microscopic compared with the overall accuracy achievable anyway.
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There is one "legitimate" case where the primary power figure is significantly larger than the secondary Σ: old style transformer having a primary tap for 120V instead of two identical 115V windings. This requires more copper than strictly necessary and boosts the power figure.
In addition, old style transformers have superposed primary and secondary rather than side by side for modern EI transformers.
Since the primary is generally the innermost winding, its resistance is lower and this also boosts the power figure.
Toroidals also have superposed windings, but the mean turn lengths vary relatively little, and the effect is much less pronounced.
For 50Hz transformers, the conversion coefficient between the power figure and the VA rating is approximately 1/20th: for example, if the power figure for a winding is 140, its VA rating is ~7VA
In addition, old style transformers have superposed primary and secondary rather than side by side for modern EI transformers.
Since the primary is generally the innermost winding, its resistance is lower and this also boosts the power figure.
Toroidals also have superposed windings, but the mean turn lengths vary relatively little, and the effect is much less pronounced.
For 50Hz transformers, the conversion coefficient between the power figure and the VA rating is approximately 1/20th: for example, if the power figure for a winding is 140, its VA rating is ~7VA
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