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Mains wiring question: Sizing buck-boost transformer?

Discussion in 'Electronic Equipment' started by Gary Walters, Mar 29, 2013.

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  1. Gary Walters

    Gary Walters Guest

    In USA.

    Source: 208v, 60 hz, 2-wire (2 phases from 3 phase "Y" supply). Load: 240v,
    20A.

    I presumed that sizing a buck-boost transformer is simple KVA math (source
    volts * load amps). But...

    This PDF document:

    <http://www.acmepowerdist.com/pdf/Page_104-109.pdf>

    on the last page says:
    - - -
    "An example of an everyday application is always a good way to explain the
    intent of the ³Code.² Example: A 1 kVA transformer Catalog No. T111683 has a
    primary of 120 x 240V and a secondary of 12 x 24V. It is to be connected as
    an autotransformer at the time of installation to raise 208V to 230V single
    phase.

    When this 1 kVA unit is connected as an autotransformer for this voltage
    combination, its kVA rating is increased to 9.58 kVA (may also be expressed
    as 9,580 VA). This is the rating to be used for determining the full load
    input amps and the sizing of the overcurrent protect device (fuse or breaker)
    on the input.

    Full Load Input Amps =
    9,580 Volt Amps / 208 Volts = 46 Amps"
    - - -
    I'm puzzled by the 10x increase of KVA rating. When and how is this true?

    What size B-B transformer do I need?

    Thanks.
     
  2. tm

    tm Guest

    Those are common devices, to go from 208 to 230/240 volts. The secondary
    current determines the rating in KVA.
     
  3. It's true because you're only using the transformer to "create" 24 volts at the current you wish
    to draw at 230v. This extra 24 volts is added back into the line voltage.

    You can switch flip the leads and subtract voltage too, then the transformer is in buck mode.
    If you need 20 amps at 230v and start with 208, you need to boost 22volts (208+22=230) x 20 amps
    = 480VA transformer. A 24 volt transformer rated over 480VA should be fine.

    Autotransformers can be confusing, so pretend it's just DC and some batteries.

    Let's say you need 24 volts at 10 amps and have a 12 volt battery that can already output 10
    amps.

    what size power supply do you need to run in series with this battery to get the 24 volts?

    just another 12 volts, at at least 10 amps, or a 120 watt power supply. Those wired in
    series (your battery and the new power supply) will provide 240 watts.

    If you already had an 18 volt battery, you'd just need a 6 volt, 10 amp or 60 watt power supply.

    The less the voltage adjustment, the smaller then buck/boost transformer rating becomes as it's
    really not doing all that much work.
     
  4. tm

    tm Guest

    Any electrical supply house will have them in stock. They are not too
    expensive either.

    tm
     
  5. Sylvia Else

    Sylvia Else Guest

    You can look at it this way: All the input current flows through the
    primary, and all the output current flows through both the primary and
    the secondary. But the output current is in antiphase with the input
    current, so most of the current in the primary is cancelled. The primary
    has to handle only the difference between the input and output current.

    Sylvia.
     
  6. Don Kelly

    Don Kelly Guest

    "Cydrome Leader" wrote in message
    --QUOTE
    It's true because you're only using the transformer to "create" 24 volts at
    the current you wish
    to draw at 230v. This extra 24 volts is added back into the line voltage.

    You can switch flip the leads and subtract voltage too, then the transformer
    is in buck mode.
    QUOTE
    If you need 20 amps at 230v and start with 208, you need to boost 22volts
    (208+22=230) x 20 amps
    = 480VA transformer. A 24 volt transformer rated over 480VA should be fine.

    Autotransformers can be confusing, so pretend it's just DC and some
    batteries.

    Let's say you need 24 volts at 10 amps and have a 12 volt battery that can
    already output 10
    amps.

    what size power supply do you need to run in series with this battery to get
    the 24 volts?

    just another 12 volts, at at least 10 amps, or a 120 watt power supply.
    Those wired in
    series (your battery and the new power supply) will provide 240 watts.

    If you already had an 18 volt battery, you'd just need a 6 volt, 10 amp or
    60 watt power supply.

    The less the voltage adjustment, the smaller then buck/boost transformer
    rating becomes as it's
    really not doing all that much work.
    =======================================
    lookup "autotransformer".
    In this case you have a 2 winding transformer with a 10:1 ratio
    With 240V applied the secondary will be 24V with a rated current of
    1000/24=41.7A
    If this is connected as a boost autotransformer- the total output voltage
    would be
    240+24 =264V so the output, without exceeding rated output current would be
    11KW. only 1 KW (24V*41.7A) is supplied through transformer action and the
    rest through a direct connection
    Adjusting to 230V output leads to 11*(230/264)=9.58KW
    The input voltage would be 207V excluding any voltage drops in the
    transformer-so 208/230 is close enough.
    Autotransformers are great for turns ratios near one as there are size and
    cost advantages.
    Disadvantage--no isolation between primary and secondary.

    Excuse the lack of "quoting" as I am using windows live mail in an
    emergency- Thunderbird downloads news but then deletes
    the downloads immediately! New problem- correction not yet found.

    Don Kelly
    cross out to reply
     
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