Just wondering, since transformer VA rating roughly correlates to weight, would a given autoformer VA rating roughly match twice the VA rating of an equal weight transformer? Or some other ratio such as 1:1?
If you're wanting background, this is because I'm trying to very roughly gauge the rating of some surplus 120/240 autoformers.
If you're wanting background, this is because I'm trying to very roughly gauge the rating of some surplus 120/240 autoformers.
basically it's a current rating, determined by the wire used for the windings.
If you use a 240Vac transformer that is tapped at 120Vac and the whole winding is 3Aac rated then the most you can take from it is always 3Aac.
Much more likely is that the windings use different wire diameters/areas and you must then ensure you use the autoformer the right way round to get the extra current capacity of the thicker winding.
If the voltage tapping is very low eg 230/240 volt (10V tapping) then the whole transformer can supply ~10times the power of the main winding IF THE 10V tapping has SUFFICIENT copper area.
If you use a 240Vac transformer that is tapped at 120Vac and the whole winding is 3Aac rated then the most you can take from it is always 3Aac.
Much more likely is that the windings use different wire diameters/areas and you must then ensure you use the autoformer the right way round to get the extra current capacity of the thicker winding.
If the voltage tapping is very low eg 230/240 volt (10V tapping) then the whole transformer can supply ~10times the power of the main winding IF THE 10V tapping has SUFFICIENT copper area.
A 2:1 (for example 240/120V) autotransformer does have double the power rating of what an isolating transformer on the same core would have had. Different ratios give different results, however.
Hi,
according to the Radiotron Designer's Handbook, chapter 5 on page 235, VA equals the square of the product of core area in square inches times a constant 5.58....
so if you know the center leg and stacking height of your traffo, you can compute for the VA rating.
according to the Radiotron Designer's Handbook, chapter 5 on page 235, VA equals the square of the product of core area in square inches times a constant 5.58....
so if you know the center leg and stacking height of your traffo, you can compute for the VA rating.
hi, auto transformers have both conduction and induction at their outputs so they are definitely smaller than isolated winding ones for the same VA rating
Yes you are right. You can make a 200 VA auto transformer (120V /240V) in core which is suitable for making 100 VA isolated transformer.
http://www.plitron.com/technotes.asp
Autoformers
For applications requiring either a simple step-up or step-down of voltage, and where there is no need for electrical isolation between windings, an autoformer can be used. Significant size and weight reductions can result over isolated transformers. The power rating of an autotformer is given by the expression:
VA rated = POUT x (V HI - V LOW) / V HI
agreed, an auto transformer can be much smaller for the VA rating.
I do not agree.
The step down auto transformer will have a low voltage tapping that passes the whole output current. This winding must be rated for the desired output current.
For 2:1 step down the low voltage winding will have approximately double the wire area to pass double the current.
If the output winding is the same area as the whole primary winding then that wastes copper area in the high voltage side. this increases weight and volume and may force the manufacturer to adopt a bigger core just to fit all that copper in.
To get the full benefit of the smallness of the auto transformer all the windings must be current rated for the current they are required to pass.
Half the output current flows "down" from input to output while the same amount flows "up" through the "common" part of the winding. This is the point of using an autotransformer. Transformer winding current is thus only half of the output current for a 2:1 autotransformer. If output is 120V 1A (120VA), only 60VA needs to be transferred through the transformer from upper to lower winding.
Code:
2:1 autotransformer example.
0.5A
------>
--------+
|
|| *)
|| ) |
|| ) | 0.5A
|| ) |
|| ) +-> 1A
|| | ---->
|| +-----------¤
|| |
|| *) +->
|| ) |
|| ) | 0.5A
|| ) |
|| )
|
0.5A | 1A
<---- | <-----
--------+------------¤
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explain this for me.
How can a winding that all goes in the "same" direction pass currents in opposite phase to the output?
Consider the law of conservation of energy.
The winding ratio in this case is 1:1 so the same current must flow into the dot end of the upper winding as flows out of the dot end of the lower winding, save for magnetizing current. If it didn't, energy would not be conserved and that sure would be sensational! 🙂
The same thing is true for isolating transformers: current flowing out of the dot end of a secondary gives rise to a current flowing into the dot end of the primary. The amp-turns (mmf) of all windings on a transformer cancel, except for the small difference which drives magnetizing current.
The winding ratio in this case is 1:1 so the same current must flow into the dot end of the upper winding as flows out of the dot end of the lower winding, save for magnetizing current. If it didn't, energy would not be conserved and that sure would be sensational! 🙂
The same thing is true for isolating transformers: current flowing out of the dot end of a secondary gives rise to a current flowing into the dot end of the primary. The amp-turns (mmf) of all windings on a transformer cancel, except for the small difference which drives magnetizing current.
I'm beginning to understand the 2:1 auto T.
Could you simulate a 11:10 auto T for me/others? i.e. tappings at 0Vac, 200Vac, 220Vac.
Could you simulate a 11:10 auto T for me/others? i.e. tappings at 0Vac, 200Vac, 220Vac.
that makes sense, but the big thing I did not appreciate was that the 220V tapping supplies reverse current that adds to the 20V tapping to give the total output current.
You can see in this example that the low voltage tapping carries ~10times the current of the high voltage tapping and that L1 will need ~10times the copper area.
The 220V tapping and it's current approximately determine the VA rating of the main winding and of the core.
it seems for "the opposite current direction" claim, there is an assumption that the load is reasonably heavy that the current drawn by the load is bigger enough than the idle current. Say the load is very light (10Mega Ohm, e.g.) which is pretty similar to unloaded/idle condition, the currents in the upper and lower part of the windings will be in the same direction. I did simulation in multisim for the same to verify this, but i don't know how to upload images...
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It is indeed the case: even a transformer has to obey Kirchoff's laws, and if the upper winding takes a magnetizing current, this current has no option but return through the lower winding.
In an auto connection, it is true that Kirchoff's Law holds true, but magnetizing current does not follow transformer law (Np/Ns = Ip/Is). Transformer law is an approximation based on an ideal device.
Load current essentially follows transformer law, and magnetizing current completely disobeys it. Kirchoff holds true for both. This is most easily seen with a typical isolation transformer energized but unloaded. The primary carries magnetizing current, the secondary has none. Connect this same xfmr as a step-up auto, and the same situation applies. A step down auto is the same deal, but less obvious. Just wanted to clarify the statement about magnetizing current.
Not sure if this was made clear or not, but the VA rating of a transformer stays the same and holds true whether the unit is connected as an auto or iso. The benefit of the auto is the device only has to transform a portion of the whole, determined by the step-up or down ratio. The rest of the VA is simply passed through at the expense of lost galvanic isolation.
Load current essentially follows transformer law, and magnetizing current completely disobeys it. Kirchoff holds true for both. This is most easily seen with a typical isolation transformer energized but unloaded. The primary carries magnetizing current, the secondary has none. Connect this same xfmr as a step-up auto, and the same situation applies. A step down auto is the same deal, but less obvious. Just wanted to clarify the statement about magnetizing current.
Not sure if this was made clear or not, but the VA rating of a transformer stays the same and holds true whether the unit is connected as an auto or iso. The benefit of the auto is the device only has to transform a portion of the whole, determined by the step-up or down ratio. The rest of the VA is simply passed through at the expense of lost galvanic isolation.
when I read what you state, I find it does not agree with what has been published.
Can you restate, so that I can understand better and make it fit with previous knowledge?
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