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Thread: Question on particular transformer wiring

  1. #1
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    Question on particular transformer wiring

    I have two of the same transformers that are wired differently. Each of these transformers are used as a step up transformer and they serve only 1 motor each.
    Attached is a picture of the name plate and a detail of the inside.

    So we'll start off with the first one- On the secondary HV side, HO HO HO are daisy chained together, then you have H1 H2 H3 leaving the transformer out to the motor.

    On the second transformer we have- On the secondary HV side the terminals go HO H1 HO H2 HO H3, just like in the picture. It is wired like follows- left HO to H2, H2 out to motor. Center HO to H3, H3 out to motor. Right HO to H1, H1 out to motor.

    To me it looks like the first one is just to make the secondary a Y connection. without connecting the 3 HO's together then it would just be open. Correct me if I am wrong.

    Can someone please explain the function of each way these are wired?
    Thanks!
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  2. #2
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    Quote Originally Posted by paul_arc View Post
    So we'll start off with the first one- On the secondary HV side, HO HO HO are daisy chained together, then you have H1 H2 H3 leaving the transformer out to the motor.

    To me it looks like the first one is just to make the secondary a Y connection. without connecting the 3 HO's together then it would just be open. Correct me if I am wrong.
    Yes, this is a floating-neutral secondary. Without the neutral accessible (or not being used), it is basically a Delta to the outside world.

    On the second transformer we have- On the secondary HV side the terminals go HO H1 HO H2 HO H3, just like in the picture. It is wired like follows- left HO to H2, H2 out to motor. Center HO to H3, H3 out to motor. Right HO to H1, H1 out to motor.
    That describes a Delta connection. Each jumper is a corner of the Delta, each of which is also one line of the output.

    Can someone please explain the function of each way these are wired?
    Thanks!
    The only reason I can see for each type of secondary wiring is to match the output to the needs of the given load. Either could theoretically be wired the way the other one is.


    Added: What I first thought were connections in the second image are actually the drawing of the transformer core.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  3. #3
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    Thanks Larry.

    Also, here are a couple more questions-

    If the motor was a wye motor and you don't wire the 3 HO terminals together like described, would it just send the voltage to the motor but the motor wouldn't actually spin? Just wiring H1 H2 H3 out to the motor and don't do anything with any of the HO terminals in the transformer.

    And-

    Wiring the secondary side delta like explained, would the output voltage still be the same as whats on the name plate? Or does the voltage drop on each tap and not the same as name plate anymore?

    Thanks

  4. #4
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    Quote Originally Posted by paul_arc View Post
    If the motor was a wye motor and you don't wire the 3 HO terminals together like described, would it just send the voltage to the motor but the motor wouldn't actually spin? Just wiring H1 H2 H3 out to the motor and don't do anything with any of the HO terminals in the transformer.
    With only one end of each secondary connected, zero current would flow. No output voltage at all.

    Wiring the secondary side delta like explained, would the output voltage still be the same as whats on the name plate? Or does the voltage drop on each tap and not the same as name plate anymore
    Do you mean if you were to rewire the Y secondary to be a Delta secondary?

    When rewiring a given 3ph secondary from Y to Delta, the line-to-line voltage change would be 0.577 of the original. What is now the line-to-neutral voltage would become the line-to-line voltage.

    Inversely, if rewiring a 3ph Delta secondary to Y, the line-to-line voltage becomes the line-to-neutral voltage, increasing the output line-to-line voltage by a factor of 1.732.

    These numbers are represented when connecting three 120v secondaries in a Y configuration to achieve 208v line-to-line. The reciprocal of 1.732 is 0.577.
    Code references based on 2005 NEC
    Larry B. Fine
    Master Electrician
    Electrical Contractor
    Richmond, VA

  5. #5
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    Quote Originally Posted by LarryFine View Post
    With only one end of each secondary connected, zero current would flow. No output voltage at all.


    Do you mean if you were to rewire the Y secondary to be a Delta secondary?

    When rewiring a given 3ph secondary from Y to Delta, the line-to-line voltage change would be 0.577 of the original. What is now the line-to-neutral voltage would become the line-to-line voltage.

    Inversely, if rewiring a 3ph Delta secondary to Y, the line-to-line voltage becomes the line-to-neutral voltage, increasing the output line-to-line voltage by a factor of 1.732.

    These numbers are represented when connecting three 120v secondaries in a Y configuration to achieve 208v line-to-line. The reciprocal of 1.732 is 0.577.
    Great, thanks a lot Larry! That answers a lot of my questions I had on these. Thanks again!

  6. #6
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    Quote Originally Posted by paul_arc View Post
    I have two of the same transformers that are wired differently. Each of these transformers are used as a step up transformer and they serve only 1 motor each.
    Attached is a picture of the name plate and a detail of the inside.

    So we'll start off with the first one- On the secondary HV side, HO HO HO are daisy chained together, then you have H1 H2 H3 leaving the transformer out to the motor.

    On the second transformer we have- On the secondary HV side the terminals go HO H1 HO H2 HO H3, just like in the picture. It is wired like follows- left HO to H2, H2 out to motor. Center HO to H3, H3 out to motor. Right HO to H1, H1 out to motor.


    To me it looks like the first one is just to make the secondary a Y connection. without connecting the 3 HO's together then it would just be open. Correct me if I am wrong.

    Can someone please explain the function of each way these are wired?
    Thanks!
    What you described appears one has wye connected secondary and the other delta connected secondary.

    Nameplate doesn't indicate anything on how to connect secondary in delta but that is the physical description given. As mentioned the delta connected voltage will be same as what line to neutral voltage is when connected in wye.
    I live for today, I'm just a day behind.

  7. #7
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    Quote Originally Posted by kwired View Post
    What you described appears one has wye connected secondary and the other delta connected secondary.

    Nameplate doesn't indicate anything on how to connect secondary in delta but that is the physical description given. As mentioned the delta connected voltage will be same as what line to neutral voltage is when connected in wye.
    Perfect, thanks for the help. I can see why they are running the different voltages now.

  8. #8
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    That first one is sure an oddball votlage. All I can think of it being used for might be as an input transformer for a 1000VDC drive from a 480V source.
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