Autoformer vs Transformer Rating

[leadbelly]

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.

[AndrewT]

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.

[megajocke]

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.

[Tony]

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.

[Tony]

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

[Elvee]

Originally Posted by megajocke

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.

This is correct.

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.

But this is not, for this reason:

hi, auto transformers have both conduction and induction at their outputs

[JPS]

Originally Posted by leadbelly
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 transforme

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

[AndrewT]

Originally Posted by Tony

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

agreed, an auto transformer can be much smaller for the VA rating.

Originally Posted by Elvee

But this is not, for this reason:

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.

[megajocke]

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.

2:1 autotransformer example. 
 
    0.5A
 ------>
--------+
        |
    || *) 
    ||  ) |
    ||  ) | 0.5A
    ||  ) |
    ||  ) +->      1A
    ||  |        ---->   
    ||  +-----------¤
    ||  |
    || *) +->
    ||  ) | 
    ||  ) | 0.5A
    ||  ) |     
    ||  ) 
        |
  0.5A  |       1A
 <----  |    <-----
--------+------------&#164;

[AndrewT]

Originally Posted by megajocke

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.

explain this for me.
How can a winding that all goes in the "same" direction pass currents in opposite phase to the output?