Connect with us

Closed Delta 120/240V 3-phase service

Discussion in 'Electrical Engineering' started by Beachcomber, Feb 10, 2008.

Scroll to continue with content
  1. Beachcomber

    Beachcomber Guest

    In the case of a commercial building served by 3 phase - closed delta
    240/120 Vservice, what is the best way to detect a ground fault on a
    three phase motor circuit on phase C were it to come into contact with
    the ground?

    Whereas the service transformer on phase AB has the only centered
    tapped (neutral) windings 120V phase-to-neutral and 240V
    phase-to-phase and phase C (not connected to the phase AB transformer)
    is the wild phase.

    It seems to me that (without additional protective equipment), if
    phase C accidently touches the ground there would be a voltage
    imbalance, but the overcurrent protection would not necessarily trip
    off.

    Am I wrong by assuming this?

    Also, what, in general, are the pros and cons of a 120/240V 3 phase
    closed delta connection?

    Is this a good choice for a small commercial building with a 3 story
    hydraulic elevator and mostly 120V. appliance and lighting loads?

    Beachcomber
     
  2. Guest

    | In the case of a commercial building served by 3 phase - closed delta
    | 240/120 Vservice, what is the best way to detect a ground fault on a
    | three phase motor circuit on phase C were it to come into contact with
    | the ground?

    A GFI device? All 4 wires (A,B,C,N) would run through the CT together.


    | Whereas the service transformer on phase AB has the only centered
    | tapped (neutral) windings 120V phase-to-neutral and 240V
    | phase-to-phase and phase C (not connected to the phase AB transformer)
    | is the wild phase.

    It is 208 volts relative to ground.


    | It seems to me that (without additional protective equipment), if
    | phase C accidently touches the ground there would be a voltage
    | imbalance, but the overcurrent protection would not necessarily trip
    | off.
    |
    | Am I wrong by assuming this?

    That depends on how solid the ground path is. If the impedance is high
    to get back to the source, maybe not enough current will flow. That
    could easily be the case if it contacts earth. It could even be the
    case if it contacts a metal part of the building. But if the impedance
    is low enough, you get high current.

    Still, if there is any risk of a ground fault, and especially one that
    could be contacted by a person as part of the ground path, then ground
    fault protection would be a good thing to have. A GFI device?


    | Also, what, in general, are the pros and cons of a 120/240V 3 phase
    | closed delta connection?

    If there is a loss of one phase in this system it can lead to trouble.
    Delta can have back-energizing issues as well as lopsided loading if
    a phase is lost. And 240D/120 would be quickly out of balance if the
    120 volt loads predominate. Alternatively, a Scott-T version of that
    could help in limited 3-phase loading environments.


    | Is this a good choice for a small commercial building with a 3 story
    | hydraulic elevator and mostly 120V. appliance and lighting loads?

    208Y/120. Configure the elevator system for 208 volts. It is the more
    common electrical system in the USA, now. An elevator manufacturer not
    being able to do 208 volts is limit their market severely.
     
  3. Guest

    13:08 -0600, "Ben Miller" <[email protected]>
    | wrote:
    |
    |>Beachcomber wrote:
    |>> In the case of a commercial building served by 3 phase - closed delta
    |>> 240/120 Vservice, what is the best way to detect a ground fault on a
    |>> three phase motor circuit on phase C were it to come into contact with
    |>> the ground?
    |>>
    |>> Whereas the service transformer on phase AB has the only centered
    |>> tapped (neutral) windings 120V phase-to-neutral and 240V
    |>> phase-to-phase and phase C (not connected to the phase AB transformer)
    |>> is the wild phase.
    |>>
    |>> It seems to me that (without additional protective equipment), if
    |>> phase C accidently touches the ground there would be a voltage
    |>> imbalance, but the overcurrent protection would not necessarily trip
    |>> off.
    |>>
    |>> Am I wrong by assuming this?
    |>>
    |>> Also, what, in general, are the pros and cons of a 120/240V 3 phase
    |>> closed delta connection?
    |>>
    |>> Is this a good choice for a small commercial building with a 3 story
    |>> hydraulic elevator and mostly 120V. appliance and lighting loads?
    |>>
    |>> Beachcomber
    |>
    |>
    |>The protective device will trip if phase C is grounded, just as for the
    |>other phases. The delta system will not allow full utilization of the
    |>available kVA, since one phase can not be used for the single-phase loads.
    |>With mostly 120V lighting & appliances, I would go 208/120 4-wire wye, so
    |>you get the benefit of all three phases for the 120V loads. Run the
    |>elevators & A/C on 208 3-phase.
    |>
    |>
    |>Benjamin D Miller, PE
    |>www.bmillerengineering.com
    |>
    |
    | I have only seen center tapped "closed delta" a few times, notably in
    | the alley next to the Hog's Breath Salloon in Key West (look at the
    | pole behind the stage) and I bet from the looks of it they added the
    | 3d transformer as the load increased. Usually it is "open delta" with
    | 2 transformers as a cheap way to get some 3 phase to a customer who
    | has mostly single phase loads.
    | The open delta was actually pretty popular in the light industrial
    | areas of East Naples (Florida).

    I've seen a few setups with one big fat transformer and one small one.
    Usually, the big one has 2 bushings and the small one has 1 bushing.
    On the secondary, the big one has 3 wired lugs and the small one has
    only 2 wired lugs. I'm guessing these are arranged as Scott-T. Where
    I have seen these is in business (not industrial) areas of small towns
    in West Virginia and Ohio, usually servicing stores or restaurants.
    Some of those smaller pole pigs were the smallest pole pigs I've seen.
    My guess is the smallest ones were no more than 3 or 5 kVA.
     
  4. Guest

    07:12 GMT, wrote:
    |
    |>| I have only seen center tapped "closed delta" a few times, notably in
    |>| the alley next to the Hog's Breath Salloon in Key West (look at the
    |>| pole behind the stage) and I bet from the looks of it they added the
    |>| 3d transformer as the load increased. Usually it is "open delta" with
    |>| 2 transformers as a cheap way to get some 3 phase to a customer who
    |>| has mostly single phase loads.
    |>| The open delta was actually pretty popular in the light industrial
    |>| areas of East Naples (Florida).
    |>
    |>I've seen a few setups with one big fat transformer and one small one.
    |>Usually, the big one has 2 bushings and the small one has 1 bushing.
    |>On the secondary, the big one has 3 wired lugs and the small one has
    |>only 2 wired lugs. I'm guessing these are arranged as Scott-T. Where
    |>I have seen these is in business (not industrial) areas of small towns
    |>in West Virginia and Ohio, usually servicing stores or restaurants.
    |>Some of those smaller pole pigs were the smallest pole pigs I've seen.
    |>My guess is the smallest ones were no more than 3 or 5 kVA.
    |
    | The two transfomer delta around here is just 2 of the expected 3 in
    | true delta. They connect A to C and leave B floating off of the AB
    | transformer that is center tapped. As long as you have a well balanced
    | 3 phase load it works fine. We did get in trouble in the computer biz
    | since we had some single phase loads derived from the incoming 3 phase
    | inside the machine. I have had to juggle machines around or "roll the
    | phases" to balance the phase loads.

    What I wonder is how 240 volt L-L loads would do with either the closed or
    open 240D/120 systems, when connected between the high-leg and either pole
    of the 120/240 side. Consider the simple 240 volt water heater, assuming
    it treats both wires as hot, which it must in USA single phase systems.
    Also consider a computer PSU that has two-pole switching and can handle
    a 230 volt German Schuko that has no polarity to it (e.g. either wire it
    gets from being plugged in could be the hot). I do remember reading some
    power utility requirements that nothing but three phase loads is allowed
    to connect to the high-leg. But what is the effect of doing so?
     
  5. Beachcomber

    Beachcomber Guest

    I'm wondering if the simplicity of a single phase 240/120 V service
    for a small elevator building is in some way superior to the slightly
    more complex 3 phase- 208/120 V. wye service.

    Can a small hydraulic passenger elevator serving a maximum of 3 flloor
    run OK on a 240 V. single phase circuit? If wired the other way, this
    would probably be the only 3 phase load.

    My concern with the 3 phase is that if the utility loses a phase, a
    208V 3 phase motor will stop rotating and burn up (without special
    protection). I've seen this happen in a condo building. Also,
    because of the partially powered delta-wye at the transformer and some
    of the 120 V lines might go to something like 68 V or so.

    I know that there are probably protective devices that can be
    installed to protect this. I wonder though, if it might just be
    better to stick with a 120/240 V. single phase service.

    Any thoughts?

    Beachcomber
     
  6. Guest

    |>What I wonder is how 240 volt L-L loads would do with either the closed or
    |>open 240D/120 systems, when connected between the high-leg and either pole
    |>of the 120/240 side. Consider the simple 240 volt water heater, assuming
    |>it treats both wires as hot, which it must in USA single phase systems.
    |>Also consider a computer PSU that has two-pole switching and can handle
    |>a 230 volt German Schuko that has no polarity to it (e.g. either wire it
    |>gets from being plugged in could be the hot). I do remember reading some
    |>power utility requirements that nothing but three phase loads is allowed
    |>to connect to the high-leg. But what is the effect of doing so?
    |>
    |
    | I'm wondering if the simplicity of a single phase 240/120 V service
    | for a small elevator building is in some way superior to the slightly
    | more complex 3 phase- 208/120 V. wye service.

    It would be simpler. For a small building it can be OK. For a larger
    building, the utility would want more balance in the phase loading.

    One option is to use a three phase transformer where all three secondary
    windings are 240 volt center tapped to 120/240 volts. What you get are
    SIX separate hot phase wires, and a neutral. I'll label the phase wires
    going clockwise as A,B,C,D,E,F. You can have 2 three phase systems from
    this, connecting to A,C,E or B,D,F. They would be 208Y/120. And you can
    have 3 single phase systems from this, connecting to A,D and B,E and C,F.
    Divide the tenants that get single phase three equal ways and this way
    they get genuine 120/240. This is actually a bit more complex than the
    common systems (for example you would have 3 separate distributions for
    the single phase). You probably can't meter the whole thing at once.
    But I'd prefer such a system, myself.

    If for some reason you really need 240 volts on the threee phase loads,
    that either forces the 240D/120 system, or just 240D, or a 240Y/139 system
    just for three phase.

    Ultimately, I'd prefer to have 3 120/240 volt systems plus 480Y/277 for
    the three phase loads.
     
  7. Don Kelly

    Don Kelly Guest

    ----------------------------
    He is essentially describing a 6 phase secondary. Each 120 to neutral and
    separated by 60 degrees but you can get 240 between the legs that are 180
    degrees apart. as well as 208 between the legs that are 120 degrees apart.
    So you can have 2 -120/208V Ysystems with common neutral as well as 3
    120/240V single phase circuits as well as some 120V D with one terminal
    tied to neutral.

    There is no real advantage but the chance of problems with mixed up wiring
    will be increased.

    The question then arises- why bother?
     
  8. Guest

    | wrote:
    |> One option is to use a three phase transformer where all three
    |> secondary windings are 240 volt center tapped to 120/240 volts. What
    |> you get are SIX separate hot phase wires, and a neutral. I'll label
    |> the phase wires going clockwise as A,B,C,D,E,F. You can have 2 three
    |> phase systems from this, connecting to A,C,E or B,D,F. They would be
    |> 208Y/120. And you can have 3 single phase systems from this,
    |> connecting to A,D and B,E and C,F. Divide the tenants that get single
    |> phase three equal ways and this way they get genuine 120/240. This
    |> is actually a bit more complex than the common systems (for example
    |> you would have 3 separate distributions for the single phase). You
    |> probably can't meter the whole thing at once.
    |> But I'd prefer such a system, myself.
    |>
    |
    | I am obviously missing something. How do you connect the center taps of
    | three windings together, and then connect them in a wye, and not blow up the
    | transformer? And how do you get 240/120 & 208Y/120 from the same windings?

    Maybe an ASCII art picture will help:

    B C
    \ /
    \ /
    A----N----D
    / \
    / \
    F E

    A-N and N-D are both wound on the first core. B-N and N-E are both wound
    on the second core. C-N and N-F are both wound on the third core. A third
    of the 120/240 loads would be served from A-N-D. A third of the 120/240
    loads would be served from B-N-E. A third of the 120/240 loads would be
    served from C-N-F. Half of the 208Y/120 loads would be served from A,C,E
    and N. Half of the 208Y/120 loads would be served from B,D,F and N.

    You could also get 120 volts from A-B or B-C or C-D or D-E or E-F or F-A,
    but you would not want to.

    You could also ignore the center tap and rewire it for 416Y/240.

    If the windings can be split and wired in parallel, you could rewire it
    for 208Y/120 with double the amperage.

    A transformer with dual secondary 120 volt windings on each of the three
    cores would be quite flexible, being able to be configured for any of
    these three systems (but you would have 12 wires coming off the cores on
    the secondary side).
     
  9. Guest

    | ----------------------------
    | |> wrote:
    |>> One option is to use a three phase transformer where all three
    |>> secondary windings are 240 volt center tapped to 120/240 volts. What
    |>> you get are SIX separate hot phase wires, and a neutral. I'll label
    |>> the phase wires going clockwise as A,B,C,D,E,F. You can have 2 three
    |>> phase systems from this, connecting to A,C,E or B,D,F. They would be
    |>> 208Y/120. And you can have 3 single phase systems from this,
    |>> connecting to A,D and B,E and C,F. Divide the tenants that get single
    |>> phase three equal ways and this way they get genuine 120/240. This
    |>> is actually a bit more complex than the common systems (for example
    |>> you would have 3 separate distributions for the single phase). You
    |>> probably can't meter the whole thing at once.
    |>> But I'd prefer such a system, myself.
    |>>
    |>
    |> I am obviously missing something. How do you connect the center taps of
    |> three windings together, and then connect them in a wye, and not blow up
    |> the transformer? And how do you get 240/120 & 208Y/120 from the same
    |> windings?
    |>
    |>
    |>
    |> --
    |> Benjamin D Miller, PE
    |> www.bmillerengineering.com
    |
    | He is essentially describing a 6 phase secondary. Each 120 to neutral and
    | separated by 60 degrees but you can get 240 between the legs that are 180
    | degrees apart. as well as 208 between the legs that are 120 degrees apart.
    | So you can have 2 -120/208V Ysystems with common neutral as well as 3
    | 120/240V single phase circuits as well as some 120V D with one terminal
    | tied to neutral.
    |
    | There is no real advantage but the chance of problems with mixed up wiring
    | will be increased.

    The advantage as I see it is the combination of keeping loading balanced
    (for a large building) and having genuine 240 volts. I would divide up
    such a building into 3 segments and feed only one phase to each. So the
    mixed up wiring risk would only be from the transformer out to each of
    the 3 single phase main panels. I've never actually seen a transformer
    like that on the market. If it came down to it, I'd just do it with 3
    separate single phase transformers. I am rather adamant about getting
    genuine 120/240 to any and all residential units. If the utility says
    they need to have the phases balanced more that a single phase drop would
    allow (in a big building, I would expect that), I'll suggest the "6 phase"
    answer, either externally or internally (via 480 perhaps). That is, if I
    ever get into the landlord business (not too likely).
     
  10. Guest

    46:10 GMT, wrote:
    |
    |>| wrote:
    |>|> One option is to use a three phase transformer where all three
    |>|> secondary windings are 240 volt center tapped to 120/240 volts. What
    |>|> you get are SIX separate hot phase wires, and a neutral. I'll label
    |>|> the phase wires going clockwise as A,B,C,D,E,F. You can have 2 three
    |>|> phase systems from this, connecting to A,C,E or B,D,F. They would be
    |>|> 208Y/120. And you can have 3 single phase systems from this,
    |>|> connecting to A,D and B,E and C,F. Divide the tenants that get single
    |>|> phase three equal ways and this way they get genuine 120/240. This
    |>|> is actually a bit more complex than the common systems (for example
    |>|> you would have 3 separate distributions for the single phase). You
    |>|> probably can't meter the whole thing at once.
    |>|> But I'd prefer such a system, myself.
    |>|>
    |>|
    |>| I am obviously missing something. How do you connect the center taps of
    |>| three windings together, and then connect them in a wye, and not blow up the
    |>| transformer? And how do you get 240/120 & 208Y/120 from the same windings?
    |>
    |>Maybe an ASCII art picture will help:
    |>
    |> B C
    |> \ /
    |> \ /
    |>A----N----D
    |> / \
    |> / \
    |> F E
    |>
    |>A-N and N-D are both wound on the first core. B-N and N-E are both wound
    |>on the second core. C-N and N-F are both wound on the third core. A third
    |>of the 120/240 loads would be served from A-N-D. A third of the 120/240
    |>loads would be served from B-N-E. A third of the 120/240 loads would be
    |>served from C-N-F. Half of the 208Y/120 loads would be served from A,C,E
    |>and N. Half of the 208Y/120 loads would be served from B,D,F and N.
    |>
    |>You could also get 120 volts from A-B or B-C or C-D or D-E or E-F or F-A,
    |>but you would not want to.
    |>
    |>You could also ignore the center tap and rewire it for 416Y/240.
    |>
    |>If the windings can be split and wired in parallel, you could rewire it
    |>for 208Y/120 with double the amperage.
    |>
    |>A transformer with dual secondary 120 volt windings on each of the three
    |>cores would be quite flexible, being able to be configured for any of
    |>these three systems (but you would have 12 wires coming off the cores on
    |>the secondary side).
    |
    | I agree with Mr Kelly
    |
    | They use delta vee to save money, why would they do this 6 pole thing?

    To serve a lot of 120/240 volt single phase loads AND keep phases balanced?
    It might depend on how large a building is involved. Suppose it is a 10
    floor building with 9 floors that have 12 residential apartments, each.
    Would you in the role of a utility engineer be satisified running single
    phase service into that building? I would think not. Then would delta
    with only one side having all the 120/240 loads really be any better?
    What I would ultimately propose for such a project would be to bring in
    480Y/277 to the building, run that to the elevators, and put single
    phase transformers on floors 3 (serving 2-4), 6 (serving 5-7) and 9
    (serving 8-10), fed from diverse 480 volt connections (480 to 120/240
    volt single phase dry transformers are pretty common). Various outside
    HID lights could also be fed from the 277 volts.

    The risk of mixing the wiring with 6 phases is probably realistic. But
    isn't that what color marking of conductors is for? Besides, the only
    places to get that mixed up is between a common single transformer and
    the place where the phases split into 3 sets of single phase. If this
    concept had been chosen a long time ago and been an accepted standard,
    I'm sure we would have a color code standard, now, for each phase angle.

    If anyone does make a three phase transformer that could be wired for
    either 208Y/120 or 416Y/240, my bet is someone with the know-how could
    wire it for the scheme I suggested (although it might not have enough
    outgoing terminal lugs, or enough conduit space, to do it).
     
  11. Don Kelly

    Don Kelly Guest

    ----------------------------
    The "6 phase " scheme wouldn't cost any more in terms of the transformers
    and could be done using a bank of 3 single phase pole pigs. However, it
    would appear that there would be additional costs involved with no net gain
    as well as a more complex system. Where possible one should avoid adding
    complexity (KISS). This doesn't mean that it can't be done but may mean
    that it shouldn't be done.
    There has been some playing around in the past on the basis that 3 phase is
    good so maybe 6 or 12 phase is better- tain't so except in some special
    cases.
    Where 6 phase has been used is in some urban distribution where there is a
    savings in terms of space necessary at a given line to neutral voltage
    because the line to line voltage between adjacent phases is then the same as
    to neutral. Whether this is still being done is something that I don't know.
    It also has been used for rectifier supplies in order to reduce harmonics
    and ripple.

    Your scheme of 480Y and single phase transformers is far better.
     
  12. Beachcomber

    Beachcomber Guest

    Are we not forgetting about the primary windings for such a scheme?

    For three phase, the textbooks say that wye-wye is not used because
    of undesirable harmonics in the neutral. Would that not apple also to
    this six phase scheme?

    How would the primaries be connected then?

    Also, doesn't six phase distribution mean six phase transmission? Six
    phase transmission for any distance is going to be horrorendously
    expensive just to keep a few apartment buildings at 120/240 V.

    Beachcomber
     
  13. Guest

    | The "6 phase " scheme wouldn't cost any more in terms of the transformers
    | and could be done using a bank of 3 single phase pole pigs. However, it
    | would appear that there would be additional costs involved with no net gain
    | as well as a more complex system. Where possible one should avoid adding
    | complexity (KISS). This doesn't mean that it can't be done but may mean
    | that it shouldn't be done.

    So what would you have instead, keeping to the requirement of genuine
    240 volts at the single phase circuits?


    | There has been some playing around in the past on the basis that 3 phase is
    | good so maybe 6 or 12 phase is better- tain't so except in some special
    | cases.
    | Where 6 phase has been used is in some urban distribution where there is a
    | savings in terms of space necessary at a given line to neutral voltage
    | because the line to line voltage between adjacent phases is then the same as
    | to neutral. Whether this is still being done is something that I don't know.
    | It also has been used for rectifier supplies in order to reduce harmonics
    | and ripple.

    I've wondered if any of those 6-wire transmission lines might be phased
    this way, even if the loads end up being split at the far end.


    | Your scheme of 480Y and single phase transformers is far better.

    So that's the way you would go? What if the utility says they don't want
    to have the loss of an extra set of transformers with MV -> 480 -> 120/240
    and insist on no more than one transformer per MV -> meter path?
     
  14. Guest

    | writes:
    |
    |>Maybe an ASCII art picture will help:
    |
    |> B C
    |> \ /
    |> \ /
    |>A----N----D
    |> / \
    |> / \
    |> F E
    |
    |>A-N and N-D are both wound on the first core. B-N and N-E are both wound
    |>on the second core. C-N and N-F are both wound on the third core. A third
    |>of the 120/240 loads would be served from A-N-D. A third of the 120/240
    |>loads would be served from B-N-E. A third of the 120/240 loads would be
    |>served from C-N-F. Half of the 208Y/120 loads would be served from A,C,E
    |>and N. Half of the 208Y/120 loads would be served from B,D,F and N.
    |
    | I always wondered if either any electric company offered such a service,
    | or if there was any such thing as a breaker panel that would support it.
    | I strongly suspect the answer to both is an absolute No Way, esp. if
    | you think about how the heck you'd design the panel. You'd have to invent
    | nonstandard breakers that would do lots of skipping to be useful. Of
    | course such a service would really be useful only in an apartment building
    | where there would be multiple standard panels, mostly residential split
    | phase (1/3 A-D 1/3 B-E 1/3 C-F) and a 3 phase panel for elevator service,
    | so no need for an actual panel for this 7 wire service.

    I have seen 6-pole disconnects, presumably intended for large motor control.
    But I have seen them up to 600 volt 400 amp. That's a huge motor.

    A 6 phase panel is just too bizarre.
     
  15. Guest

    |
    |>> B C
    |>> \ /
    |>> \ /
    |>>A----N----D
    |>> / \
    |>> / \
    |>> F E
    |>
    |>>A-N and N-D are both wound on the first core. B-N and N-E are both wound
    |>>on the second core. C-N and N-F are both wound on the third core. A third
    |>>of the 120/240 loads would be served from A-N-D. A third of the 120/240
    |>>loads would be served from B-N-E. A third of the 120/240 loads would be
    |>>served from C-N-F. Half of the 208Y/120 loads would be served from A,C,E
    |>>and N. Half of the 208Y/120 loads would be served from B,D,F and N.
    |>
    |>I always wondered if either any electric company offered such a service,
    |>or if there was any such thing as a breaker panel that would support it.
    |
    | Are we not forgetting about the primary windings for such a scheme?
    |
    | For three phase, the textbooks say that wye-wye is not used because
    | of undesirable harmonics in the neutral. Would that not apple also to
    | this six phase scheme?

    Nothing about this scheme prevents a delta primary.

    Of course, if you want to backfeed power for credit into the utility,
    you might need a wye-wye anyway.


    | How would the primaries be connected then?

    Delta.


    | Also, doesn't six phase distribution mean six phase transmission? Six
    | phase transmission for any distance is going to be horrorendously
    | expensive just to keep a few apartment buildings at 120/240 V.

    This would be a traditional E-core transformer. The primaries would be a
    winding on each bar of the E-core at 480 volts (for example) and wired in
    the traditional delta configuration. The secondaries would be a pair of
    120 volt windings. You can wire the 120 volt windings in parallel for a
    traditional 480D -> 208Y/120 transformer. You can wire those same 120 volt
    windings in series for the somewhat unusual 480D -> 416Y/240 (military has
    generators for 416Y/240 and might have to do just that in the case they
    get a 480 volt power source instead). That series wiring could be used
    with the center tap of each secondary connected to neutral instead of one
    end, to make the 6 phase arrangement.
     
  16. Guest

    | wrote:
    |> Maybe an ASCII art picture will help:
    |>
    |> B C
    |> \ /
    |> \ /
    |> A----N----D
    |> / \
    |> / \
    |> F E
    |>
    |> A-N and N-D are both wound on the first core. B-N and N-E are both
    |> wound on the second core. C-N and N-F are both wound on the third
    |> core. A third of the 120/240 loads would be served from A-N-D. A
    |> third of the 120/240 loads would be served from B-N-E. A third of
    |> the 120/240 loads would be served from C-N-F. Half of the 208Y/120
    |> loads would be served from A,C,E and N. Half of the 208Y/120 loads
    |> would be served from B,D,F and N.
    |>
    |> You could also get 120 volts from A-B or B-C or C-D or D-E or E-F or
    |> F-A, but you would not want to.
    |>
    |> You could also ignore the center tap and rewire it for 416Y/240.
    |>
    |> If the windings can be split and wired in parallel, you could rewire
    |> it for 208Y/120 with double the amperage.
    |>
    |> A transformer with dual secondary 120 volt windings on each of the
    |> three cores would be quite flexible, being able to be configured for
    |> any of these three systems (but you would have 12 wires coming off
    |> the cores on the secondary side).
    |>
    |>> ---------------------------------------/----------------------------------|
    |>> Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address
    |>> below | first name lower case at ipal.net /
    |>> |
    |>> ------------------------------------/-------------------------------------|
    |
    |
    | Got it. Thanks. It was the "center tap" that threw me. It is really 6
    | windings in a star configuration.

    If you have a common single phase 480 volt transformer with a pair of 120
    volt windings that can be wired in parallel or series, I suppose you would
    call that a 2 winding secondary. But what if the windings are just tapped
    in the center and cannot be separated for parallel wiring? Would you still
    call that a 2 windings secondary? Either kind of winding could be used on
    each bar of the E-core to make the 240/208/120 6 point star configuration.
     
  17. Guest

    | (Beachcomber) writes:
    |
    |>I'm wondering if the simplicity of a single phase 240/120 V service
    |>for a small elevator building is in some way superior to the slightly
    |>more complex 3 phase- 208/120 V. wye service.
    |
    |>Can a small hydraulic passenger elevator serving a maximum of 3 flloor
    |>run OK on a 240 V. single phase circuit? If wired the other way, this
    |>would probably be the only 3 phase load.
    |
    | Do the elevator companies even offer a single phase elevator system in
    | the size you need? (how many HP is such an elevator motor). Since
    | the system has to control the motor, possibly reversing it, you may
    | not be able to simply replace the motor with a single phase motor.
    |
    | Do you have a need for 240V (not 208V) residential devices? If not,

    Yes.

    | there's no real advantage of 240V delta or open delta over standard
    | 208Y/120 service. The only real place I see where 240V delta has a place
    | is residential service with a small 3 phase load, such as a workshop or
    | large A/C unit. That's simply because homes have 240V and 120/240V single
    | phase appliances.

    As do condos in very large buildings where not all big appliances are
    provided as part of the condo arrangement.

    Suppose you buy a condo with 208Y/120 service, of which you get 2 lines
    of that. It has appliances included in the sale because the developer
    bought special 208 volt versions. You live there a few years and one of
    them fails and you need to buy a replacement. Go to Sears or some place
    like that and ask for 208 volt versions as if you were in this situation
    and see how easy it is.

    Some modern ranges could easily handle 200 to 240 volts by means using a
    thermostat that manges the heat even so that at 208 volts there are some
    times the element goes off to maintain the correct temperature. It would
    be on for 208 volts more than for 240 volts.


    | I have seen open delta 3 phase in residential areas of Baton Rouge LA for
    | whole house A/C. 1 large and one tiny transformer.

    I'm sure when A/C systems started showing up, with many using 3-phase, they
    were among the first to get them down in the very hot south.

    I wonder how many of those big+little transformer setups were Scott-T instead.
     
  18. Guest

    | writes:
    |
    |>What I wonder is how 240 volt L-L loads would do with either the closed or
    |>open 240D/120 systems, when connected between the high-leg and either pole
    |>of the 120/240 side. Consider the simple 240 volt water heater, assuming
    |>it treats both wires as hot, which it must in USA single phase systems.
    |
    | I would assume no effect, as long as the insulation was rated for 208VAC
    | to ground vs. 120VAC to ground, and you don't overload the possibly small
    | high leg transformer. The insulation won't be an issue for anything
    | designed for European use where either leg may be hot, since either leg
    | must handle 240V. One overload problem is when something is connected
    | across the missing leg of an open delta system. The transformers have to
    | deal with a higher VA per delivered watt, plus more copper losses.
    | Combine this with a small high leg transformer and you may see voltage
    | sag.

    Would a computer overload it?
     
  19. Guest

    |>As do condos in very large buildings where not all big appliances are
    |>provided as part of the condo arrangement.
    |
    | [snip explanation why you'd want 120/240V service in a condo)
    |
    | I agree that 120/240V service should always be supplied for residential
    | service.
    |
    | I'll amend my earlier answer to say if the configuration is a small
    | apartment building small enough to power all the residences off a single
    | transformer providing 120V/240V service, an open delta to provide for an
    | elevator or other small 3 phase load would be appropiate.

    Or a Scott-T if the original 120/240 volt pole pig is a two-bushing type.
    Then a small one bushing transformer connected to the 3rd phase would
    give you the 90 degrees difference that Scott-T needs.


    |>| I have seen open delta 3 phase in residential areas of Baton Rouge LA for
    |>| whole house A/C. 1 large and one tiny transformer.
    |
    |>I'm sure when A/C systems started showing up, with many using 3-phase, they
    |>were among the first to get them down in the very hot south.
    |
    |>I wonder how many of those big+little transformer setups were Scott-T instead.
    |
    | I was real curious what was going on in the Baton Rouge case so I did
    | study the wiring enough to figure out it was open delta. I don't think
    | I've ever seen a Scott-T type setup using two pole pigs.

    What you would see in a Scott-T is that one transformer, preferrably the
    big one, would have 2 bushings, and the other would have 1 bushing. Each
    of the bushings would be connected to separate MV phases to get the 90
    degree phase relationship.


    | I take that back, sort of.
    |
    | One weird setup I've seen, and I'm not totally sure exactly what it is:
    | 1 large 3 phase transformer (single can) with a small transformer can
    | below it on a pole. The small transformer has NO MV connection nor even a
    | MV bushing! It sits below the drops to the homes/businesses. The only
    | thing I can think it could be is the big can is 240V delta, and the high
    | leg powers the small can, with is a 208V/240VCT. The big can powers half
    | the residential services plus any 3 phase services, the small can powers
    | the other residential services. It's sort of Scott-T relative to the big
    | can. But this is a guess, I actually don't know how it's wired. I even
    | took a photo to try and figure it out. There are quite a few of these
    | setups in Philadelphia.

    If the big can is already 3 phase, why would anyone need to also have a
    Scott-T? That doesn't make sense. It must be something else. How many
    MV bushings on each can? How many secondary lugs? While I have seen a
    few single-can three phase transformers in pole pig in pictures from other
    countries, I've never seen them in the USA. Pad mounts are different and
    do come in big three phase tanks.

    See if you can get multiple photos each at the same distance taken in a
    partial circle around the transformer setup so it can be seen in 3-D by
    pairing selected photos. Take them on a dismal cloudy gray overcast
    day. If you have exposure setting, take a second set overexposed by 2
    stops (to get more detail in the wiring itself). If using film, Tech Pan
    is the best choice unless something in color provides clues.
     
  20. Guest

    | wrote:
    |
    |> I'm sure when A/C systems started showing up, with many using 3-phase, they
    |> were among the first to get them down in the very hot south.
    |>
    |> I wonder how many of those big+little transformer setups were Scott-T instead.
    |
    | A Scott-T is used to get two phase from three phase. There are very few
    | two phase requirements any more.

    It can produce the equivalent of 240 delta when connected appropriately:

    * * * *
    / \ / \ | /
    / \ / \ | /
    / \ / \ | /
    *-------* *---*---* *---*---* *---*---*

    240D 240DCT Scott-T 240VCT

    What you need is 208 volts at 90 degrees, one end connected to the neutral
    which is the center tap of the 120/240 single phase. The effective voltage
    phasing gives the same relative voltage as the 240 delta. You get a 90
    degree phase angle when one primary is connected L-L and the other is
    connected L-N where the latter L is different (e.g. A-B and C-N).


    | Some erroneously call the 240/120V system "two phase" when it's really a
    | single phase with a center tap. (Or two 120V windings connected in series.)

    Personally, I would have preferred to use the term "phase" for each of the
    different vectors around the reference (generally the grounded conductor).
    But that's not how things worked out. So I need to use some other term.
    Often "pole" is used, but that can get confused with those big wooden sticks
    that hold wires up high. I'd say "vector" but no one would understand.


    | The last time I encountered a true Scott-T was in my exam for
    | Professional Engineer, back in 1952.

    I've found Scott-T dry-type transformers online.
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-