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Transformer primaries in series, secondaries in parallel -- ok?

Discussion in 'Power Electronics' started by (*steve*), Dec 2, 2017.

  1. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I am after a transformer with a 15-0-15 secondary at several amps. Yesterday a couple of transformers landed on my lap (luckily they didn't land on my foot). Unfortunately they each have a pair of 30V windings.

    Can I connect the primaries in series and the outputs of the two transformers in parallel to give me a pair of 15V outputs?

    I would pair up a winding from each of the transformers to maintain the same load on each.

    The transformers are each rated at 300VA and are toroidal. I expect the combined output will be limited to 300VA in this arrangement (which is heaps).

    Please assume I can manage to get the phase relationship correct and that I'm aware that this is a very extravagant use of two expensive transformers :)

    Since both transformers are rated for the full input voltage, there shouldn't be any concerns WRT insulation resistance.
     
  2. duke37

    duke37

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    If the secondaries are connected in parallel, then the primary voltages will be equal and given the correct connections, all should be well.
    The voltage on each of the primaries will be half the normal so the output voltage will be half the normal.

    The power rating of the transformer depends on the current so will be twice that of one transformer, but at half the voltage so 300VA seems reasonable.
     
    Harald Kapp likes this.
  3. duke37

    duke37

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    With the primaries in series, they will have the same current so the secondary currents will also be the same and power will be shared equally.
     
  4. HellasTechn

    HellasTechn

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    If you connect the primaries in series then voltage on each secondary will be half.
    Now if you connect the secondaries in paralel, then where will you get the 0V from ?

    My opinion is
    With primaries in series you have to connect secondaries in series also. Get the center tap from the point you connect the secondaries. Given that the primaries in series will give half the voltage on secondaries, then when secondaries in series you will get your 15 - 0 - 15 volts with center tap as ref. point.
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Each transformer (call them T1 and T2) has 2 independent 30V windings (call them S1 and S2). I plan to connect T1-P1 and T2-P1 in parallel; and T1-P2 and T2-P2 in parallel. I will end up with two 15VAC outputs which I can connect in series to get a 15-0-15 output.
     
  6. Alec_t

    Alec_t

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    If P1 and P2 are the respective primary coils of T1 and T2 then they need to be in series in order to get 15V out from the secondaries.
     
    HellasTechn likes this.
  7. duke37

    duke37

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    Connecting a pair of output windings in parallel will balance the system. Connecting a second pair in parallel will work only if all the windings are identical. If there is a slight difference then a lot of circulating current will be generated. Better to use one pair only for balance.

    If feeding rectifiers the circulating currents can be avoided by rectifying each winding serarately.
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Good point. I'm pretty sure the windings will be close to identical, but there's no point in risking it.

    I'll place one pair in parallel feeding a single bridge rectifier, and the other two windings each into their own bridge rectifier.
     
  9. HellasTechn

    HellasTechn

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    I can not understand what you mean. could you provide a diagram ?

    look at my diagram below and tell me is it wrong ?
     

    Attached Files:

    Last edited: Dec 6, 2017
  10. duke37

    duke37

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    There are two transformers, each with two output windings. In order to balance the two transformers, the primaries are connected in series as you show. This means the input currents are the same.
    Two secondaries are connected in parallel so the voltages are equal, including the primaries. You now have 15V on this pair of parallel windings.
    There are two spare windings, one on each transformer, if these are identical or very near, then they can be connected in parallel to give 0-15v + 0-15V output. If they are not close enough, then large circulating currents can occur. This can be avoided by not connecting the windings in parallel, but by rectifying each winding separately and then commoning the output.

    The phases of the windings must be correct.
     
  11. HellasTechn

    HellasTechn

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    OK. i just realised thet the two transformers have secondarys with center tap.
    I thought they where without a center tap.
    :)
     
  12. AnalogKid

    AnalogKid

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    Post #1 says each secondary is rated at 30 V, but it does not say:
    1. the voltage rating of each independent primary winding.
    2. the AC line voltage in your country.
    3. is the 30 V secondary rating for series or parallel primary connections

    ak
     
  13. duke37

    duke37

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    The voltage of the primaries are not important so long as they match the supply.
    The voltage of the AC line is unimportant.
    The secondaries are intended for 30V output but will give 15V in this application. They are independant so can be used as desired, not fixed for 30-0-30V
     
  14. AnalogKid

    AnalogKid

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    Says someone in England to someone in the US?
     
  15. duke37

    duke37

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    The point is that the transformers were designed for the supply to which they were subjected.
    In the UK we have a supply of officially 230V to match the rest of europe. In practise it is more like the old standard of 240V (50Hz)

    I believe the US uses a centre tapped supply, I have seen voltages of around 110/220 and 115/230. (60Hz).

    I have had equipment to repair, imported from the US and pluged in without a thought. Where does the US get all the smoke from?
     
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  16. AnalogKid

    AnalogKid

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    coal.
     
  17. Kabelsalat

    Kabelsalat

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    The center pin make it all wrong because it is two separate iron cores. See my drawing then you see what I mean:
     

    Attached Files:

  18. Tha fios agaibh

    Tha fios agaibh

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    Why?
    I see nothing wrong as long as loads are close to equal. This is the same set-up commonly used in North America.
     
    HellasTechn likes this.
  19. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Here is the solution I think we agree on:

    upload_2017-12-9_9-3-52.png

    There are two 240V primaries in series operating from 240V, thus giving about 1/2 the voltage across each. This guarantees an equal current through each transformer primary (but not necessarily an equal voltage across them).

    Each of the secondaries is nominally 30V, in this arrangement giving 15V.

    One primary from each transformer are connected in parallel and rectified, forcing an equal voltage across each. This will force an equal voltage across the primaries as well.

    The other primaries are individually rectified and connected in parallel. Whilst they theoretically have the same voltage across them, any slight difference will cause current to circulate within the pairs of paralleled secondaries.

    Finally the two outputs are connected in such a way as to provide a double-ended output.

    For simplicity, I'm not showing filter caps or any other related circuitry.

    Here's another question... Nobody would have an issue with me paralleling up the outputs of a single transformer -- which presumably would cause the same issue of circulating currents if they were not identical.

    upload_2017-12-9_9-19-47.png

    So why have an issue with paralleling up the secondaries of two transformers like this?

    upload_2017-12-9_9-29-56.png
     
    HellasTechn likes this.
  20. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    p.s. I write 240 VAC because we don't have 220 VAC, or 230 VAC (or incidentally 240 VAC). For historical purposes, we have 254 VAC.
     
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