Question about autotransformers...

Discussion in 'General Electronics Chat' started by germeten, Apr 18, 2014.

  1. germeten

    Thread Starter New Member

    Apr 18, 2014
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    I have a question about autotransformers. I understand they can be used to step voltage up or down, but can we obtain a corresponding increase or decrease in current by so doing (tapping to the autotransformer) similar to standard transformer?
     
  2. ericgibbs

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  3. inwo

    Well-Known Member

    Nov 7, 2013
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    Yes, if tapping the secondary.

    The smaller gauge primary is for powering the transformer, and generally does not carry the load current.

    Example being a 208/12/24/32 volt transformer connected to supply 240 or 176vac using the 32 volt winding.

    The 12 and 24 volt taps could be used to supply 12, 24, 220, 232, 196, or 184 vac,
     
  4. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    autos are rated in VA, the same as isolation, cost less and are physically smaller. The only trade off is isolation.
     
  5. germeten

    Thread Starter New Member

    Apr 18, 2014
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    Hi; thank you all for the reply.

    ericgiggs: I read the tutorial and it wasn't very clear about current. I understand the part about voltage stepping up & down but I don't see a corresponding (inverse) increase or decrease in current, like standard transformers, but rather that current decreases as voltage decreases, or increases as voltage increases. That's useful but not what I'm looking for at present. For example, a variac doesn't provide gobs of current at the lowest V setting, does it? A standard transformer puts out roughly the same POWER in watts from the secondary as put into the primary (less the efficiency losses.) Anyway I'd like to see a tutorial that covers the V/I relationship of autotransformers in more detail, not just V issues.
     
  6. inwo

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    A variac will supply the rated current throughout it's full range.
     
  7. crutschow

    Expert

    Mar 14, 2008
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    Autotransformers don't have a secondary, just a tapped primary.

    The maximum output current is generally limited to the maximum input current, thus you can't get a big increase in current at low output voltages as you would with a standard transformer.
     
  8. inwo

    Well-Known Member

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    My reference is in electrical buck-boost.

    A normal primary/secondary transformer is connected as an auto transformer.

    Sorry for the confusion. I stand corrected.:p
     
  9. germeten

    Thread Starter New Member

    Apr 18, 2014
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    crutschow: Thank you; although sucks for my purposes, is what I suspected, will be useful to know for other purposes ...someday.
     
  10. crutschow

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    Mar 14, 2008
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    I should add the my statement is true for Variac type autotransformers. Those with fixed taps sometimes use larger wire for the low voltage taps (even though the windings are in series) allowing the output VA rating to equal the input VA rating.
     
  11. germeten

    Thread Starter New Member

    Apr 18, 2014
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    Dude, now you confused me.

    Will I get more current from the lower taps, or not? If current is the same
    throughout the autotransformer, then why would larger wire be needed at
    the lower voltage taps? Again, since this website provides tutorials on
    such topics, I wish there was more clarity re: current relationship to
    voltage in explanations about autotransformers.
     
  12. pujulde

    Member

    Jul 24, 2013
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    I think this information will help you to calculate current on the secondary winding. This site was pointed out to you above: http://www.electronics-tutorials.ws/transformer/auto-transformer.html. There are vivid formulas and scheme example.
     
  13. inwo

    Well-Known Member

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    An auto-transformer, designed as such, can supply full current from any tap.

    The wire gauge is sized for this.

    It is possible to use smaller wire for part of the winding. The part that will not see a load. For example a 120v auto-transformer that will only supply 0-20v out. There will be no taps above the 20v point so the rest of the windings only carry "primary" current.

    The same can be true when connecting a standard separately wound transformer as an auto-transformer. (buck-boost)

    A 120/20v transformer can be connected as normal- primary to 120 volts.
    The 20v secondary can be connected in series with the line voltage to the load.

    This connection will supply 100v or 140v, dependent on phasing, and at a current equal to 20v secondary current rating..
     
  14. germeten

    Thread Starter New Member

    Apr 18, 2014
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    I'm not too good deciphering values that way, I do better with ratios and total watts. Does it work in reverse? Can I get a lower V and higher amps (than input) out of a lower tap on an autotransformer?
     
  15. inwo

    Well-Known Member

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    Yes.........................................................................................................
     
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  16. crutschow

    Expert

    Mar 14, 2008
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    The VA output at a lower voltage output of a fixed tap autotransformer can be the same as the VA rating of the input if the lower voltage part of the winding has larger wire to handle the higher current (i.e. the output current can be greater than the input current). The reason a variac output current is limited to the input current is due to the total winding having the same size wire, not that it's an autotransformer.
     
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