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Element heaters Star connection no Neutral

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  • 15-02-2021 9:06pm
    #1
    FintanMcluskey
    Registered Users Posts: 8,430 ✭✭✭


    Hi folks

    On a 3 phase star connected set of heater elements there is no neutral connection.

    Why would this not need a neutral connection?

    Also why would it be wired in star as opposed to delta? There are 2 other heater element sets on the machine that are connected in delta

    Thanks in advance for any help


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    #2
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,596 Mod ✭✭✭✭2011


    FintanMcluskey wrote: »
    Hi folks

    On a 3 phase star connected set of heater elements there is no neutral connection.

    Why would this not need a neutral connection?

    It is a 3 phase balanced load therefore there is no neutral current.
    Therefore no neutral cable necessary.

    This is why 3 phase star connected motors have no neutral cable too.
    Also why would it be wired in star as opposed to delta?
    There are 2 other heater element sets on the machine that are connected in delta

    If you connect in delta then you would have 400V across each element which means the elements would have to be rated for 400V. I assume that your elements are rated for 230V which is why they are connected in star.


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    #3
    FintanMcluskey
    Registered Users Posts: 8,430 ✭✭✭FintanMcluskey


    2011 wrote: »
    It is a 3 phase balanced load therefore there is no neutral current.
    Therefore no neutral cable necessary.

    This is why 3 phase star connected motors have no neutral cable too.

    Ah that makes sense.

    If you connect in delta then you would have 400V across each element which means the elements would have to be rated for 400V. I assume that your elements are rated for 230V which is why they are connected in star

    I’m not onsite but the electrical schematic is showing that the star connected elements are rated at 400v.


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    #4
    Andrea B.
    Registered Users Posts: 1,926 ✭✭✭Andrea B.


    FintanMcluskey wrote: »
    Ah that makes sense.



    I’m not onsite but the electrical schematic is showing that the star connected elements are rated at 400v.

    Might that be 3p 400v connection across phases and 230 between phase and star?


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    #5
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,596 Mod ✭✭✭✭2011


    FintanMcluskey wrote: »
    I’m not onsite but the electrical schematic is showing that the star connected elements are rated at 400v.


    Look carefully. Does it show that an individual element is rated for 400V? Or does it show that 2 elements in series are rated for 400V ?
    When connected in star you effectively have 2 elements in series between any 2 phases.

    Obviously if you connect 400V across a single element the current it draws will increase by root 3. This will mean that the cables and protective device will have to be rated accordingly.

    Do you want to increase the load and have a higher output?


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    #6
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,596 Mod ✭✭✭✭2011


    DIFFERENECE-BETWEEN-STAR-AND-DELTA-FIG-1-1.jpg

    See the above image. Forget the neutral and imagine this is a balance 3 phase load and we have connected this to a 400V 3 phase supply. There would only be 230V across each of the coils (think of the voltage divider rule). This is because there are 2 coils between any 2 phases.


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    #7
    FintanMcluskey
    Registered Users Posts: 8,430 ✭✭✭FintanMcluskey


    Will check in the morning the actual voltage rating of the elements, on the schematic it’s labelled 400v and star connection as is the delta connected elements.

    Will let ye know what I find out.

    Thanks for help


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    #8
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,596 Mod ✭✭✭✭2011


    FintanMcluskey wrote: »
    Will check in the morning the actual voltage rating of the elements, on the schematic it’s labelled 400v and star connection as is the delta connected elements.

    Will let ye know what I find out.

    Thanks for help

    Any update? Just curious


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    #9
    FintanMcluskey
    Registered Users Posts: 8,430 ✭✭✭FintanMcluskey


    2011 wrote: »
    Any update? Just curious

    I have just discovered the elements are wired in Delta not star as per schematic(which wasn’t updated).

    All elements are rated 400V so I don’t know why the schematic would have shown a star configuration!

    In delta, what would happen when an element fails?

    The circuit has a thermal overload, would this trip?


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    #10
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,596 Mod ✭✭✭✭2011


    Gestureapo wrote: »
    If an element fails in delta you get 200v across each of 2 elements in series

    So less heat ,nothing trips

    If I have that right


    Nope.


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    #11
    Gestureapo
    Closed Accounts Posts: 61 ✭✭Gestureapo


    2011 wrote: »
    Nope.

    Apologies , that's Star , was too quick posting

    Thought I deleted it


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    #12
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,596 Mod ✭✭✭✭2011


    Gestureapo wrote: »
    Apologies , that's Star , was too quick posting

    Thought I deleted it

    In star you have 230V across each winding, not 200V.

    400/ √3 =230


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    #13
    Gestureapo
    Closed Accounts Posts: 61 ✭✭Gestureapo


    2011 wrote: »
    In star you have 230V across each winding, not 200V.

    400/ √3 =230

    With an open element , is it not 200?


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    #14
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,596 Mod ✭✭✭✭2011


    Gestureapo wrote: »
    With an open winding , is it not 200?

    No.


    Edit: Actually I forgot there is no neutral so in this case you are correct. Apologies.


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    #15
    Gestureapo
    Closed Accounts Posts: 61 ✭✭Gestureapo


    http://www.chromalox.com/catalog/re...ory-Three-Phase-Equations-Wiring-Diagrams.pdf

    Link there , copy and paste


    Wattage reduced by 33% for open element in delta

    Wattage reduced by 50% for open element in Y

    Not quite sure on the line currents there

    Thermal cutout wouldn't open, presumably you'd be using a TOR and Contactor +MCB on the power circuits?


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    #16
    Andrea B.
    Registered Users Posts: 1,926 ✭✭✭Andrea B.


    3p 400 supplying a delta config with one element open circuit, still leaves 400 between the 3 phases, hence 400 across the remaining elements.


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    #17
    FintanMcluskey
    Registered Users Posts: 8,430 ✭✭✭FintanMcluskey


    If current measures 21 amps on all 3 phases when all elements are healthy, what current would be expected with an element failed?


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    #18
    Dred.
    Closed Accounts Posts: 44 ✭Dred.


    FintanMcluskey wrote: »
    If current measures 21 amps on all 3 phases when all elements are healthy, what current would be expected with an element failed?

    Line current 21amps in delta ?


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    #19
    2011
    Moderators, Home & Garden Moderators, Technology & Internet Moderators, Regional East Moderators Posts: 12,596 Mod ✭✭✭✭2011


    FintanMcluskey wrote: »
    If current measures 21 amps on all 3 phases when all elements are healthy, what current would be expected with an element failed?

    As it is connected in star then if an element failed then no current would flow in the one that it was corrected to.


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    #20
    Dred.
    Closed Accounts Posts: 44 ✭Dred.


    2011 wrote: »
    As it is connected in star then if an element failed then no current would flow in the one that it was corrected to.

    Does he mean delta? Star is easy enough


    You can work out the delta currents from the formulae in the link for Wt and i LL

    Wouldn't be a standard calculation would it?


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    #21
    FintanMcluskey
    Registered Users Posts: 8,430 ✭✭✭FintanMcluskey


    2011 wrote: »
    As it is connected in star then if an element failed then no current would flow in the one that it was corrected to.

    No, it’s delta connected.

    Schematic was incorrect.

    I’m wondering what would happen to phase currents in delta connection when 1 element fails?


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    #22
    FintanMcluskey
    Registered Users Posts: 8,430 ✭✭✭FintanMcluskey


    Dred. wrote: »
    Line current 21amps in delta ?

    Yeah.

    Elements are 5kW each


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    #23
    Dred.
    Closed Accounts Posts: 44 ✭Dred.


    Delta wattage = √3 * 400 * 21 = 14450W

    Open delta wattage 14450 ×0.67 = 9680W

    L1 = L2 = 9680 /800 = 12A

    L3 = 12×√3 = 20A


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