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Thread: Autotransformer Wiring

  1. #1
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    Autotransformer Wiring

    I know this may not be on topic, but I was hoping someone here could shed some light on this for me. I have a 2 kVA auto transformer that I'm wanting to use to supply 110VAC inside a control panel. I've never used an autotransformer and may have goofed when I ordered it for this purpose. The transformer primary will be connected to two phases of 240VAC. I want to get 110VAC on the secondary. The question is where do I connect ground? I'm thinking that it connects to the center tap of the transformer. Is this right or do I need to throw this one back on the shelf and order an isolated transformer?

    Thanks to all in advance,

    Jon

  2. #2
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    jrex -
    What is your source? When you say, "two phases, does that imply you have 120/240 3ph, or 240D 3ph?

    Quote Originally Posted by jrexrode
    ...The transformer primary will be connected to two phases of 240VAC. I want to get 110VAC on the secondary. The question is where do I connect ground? ...
    To the xfm case. I'm not sure why you would want to connect the system ground to any winding.

    Quote Originally Posted by jrexrode
    ... I have a 2 kVA auto transformer that I'm wanting to use to supply 110VAC inside a control panel.
    Just out of curousity, why an auto transformer? Did it save some dollars?

    carl
    Using the code for a design guide is a sign of incompetance

  3. #3
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    That seems to be an unusual application for an autotransformer. What does the wiring diagram for it look like?

    Jim T

  4. #4
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    Quote Originally Posted by jrexrode
    I've never used an autotransformer and may have goofed when I ordered it for this purpose. The transformer primary will be connected to two phases of 240VAC. I want to get 110VAC on the secondary. The question is where do I connect ground?
    If this is actually an autotransformer, then you do not have a separate primary and secondary winding. You have a single winding which is _both_ your input and output.

    Because the output is electrically connected to the input, you do not want to make any grounding connection on the output.

    You will not have a grounded conductor present on the output unless you have a grounded conductor on the input. If you do have a grounded conductor on the input, and you can connect the transformer so that it is on a terminal common with the output, then you can have a grounded conductor on the output.

    In the case of 240V in versus 120V out, with an autotransformer, the only common situation that would let you have a grounded conductor on the output is if you have a corner grounded delta source.

    -Jon

  5. #5
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    What is your source? When you say, "two phases, does that imply you have 120/240 3ph, or 240D 3ph?
    I have two legs of the 120/240 3ph supplying the input.

    Just out of curousity, why an auto transformer? Did it save some dollars?
    It was cheaper than an isolated transformer. I didn't understand fully how this thing operated when I bought it, thinking it would be a suitable substitution for an isolated transformer.

    That seems to be an unusual application for an autotransformer. What does the wiring diagram for it look like?
    The wiring diagram shows the winding with a center tap coming off of it.
    1 3
    |^^^^^^^^^^|
    1 | 3
    2

    Primary volts 240. Connect Primary lines to 1-3.
    Secondary volts. Connect secondary lines
    240 1-3
    120 1-2 or 2-3
    120/240 1-2-3

  6. #6
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    Quote Originally Posted by jrexrode
    ...The question is where do I connect ground? I'm thinking that it connects to the center tap of the transformer. ...
    jrex-

    When you use the term "ground", do you mean the system (120V) neutral?

    carl
    Using the code for a design guide is a sign of incompetance

  7. #7
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    Quote Originally Posted by jrexrode
    The wiring diagram shows the winding with a center tap coming off of it.
    1 3
    |^^^^^^^^^^|
    1 | 3
    2

    Primary volts 240. Connect Primary lines to 1-3.
    Secondary volts. Connect secondary lines
    240 1-3
    120 1-2 or 2-3
    120/240 1-2-3
    jrex -
    I just figured out my last post wasn't particularly useful.

    Here's what I think you have:
    Assume High leg delta, A-N=120, C-N=120, B-N=208. The neutral is likely bonded to earth (ground, building steel)

    You have the two phases (240V) out at the machine, but no neutral. But, you need a 120V control circuit, and you likely want it grounded to maintain code compliance. So, when you connect the autotransformer, you get 120V from the centertap to each end. All this is pretty much as you said, just restated to make sure I understood.

    Now, I think you are asking, "How do I earth this auto transformer to get a 120V control circuit that is bonded (or maybe "referenced" is a better word) to earth?"

    Following jon's post, depending on which two phases are out at the machine, the answer is either, "You can't"; or "You shouldn't".

    If you have A-B or C-B out there, then from your typed sketch,
    Term 1 (A or C) to ground = 120V
    Term 3 (B) to ground = 208V
    Term 2 (center tap) to ground = 120V
    Yep, that's truth. Draw a vector diagram and it will be clear.

    Now, if you have A-C , then
    A - G = 120V
    C - G = 120V
    Center tap - G = NEAR ZERO (which is not exactly zero)
    Even a few tenths of a volt could give some hellacious circulating currents.

    So, following this reasoning, the answer is, "pitch the auto transformer and get a two winding xfm."

    Hope this post helps (or, I'm all wet and still no help)

    carl
    Last edited by coulter; 03-01-07 at 01:03 AM.
    Using the code for a design guide is a sign of incompetance

  8. #8
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    While this installation may not include a branch circuit on the secondary side of the autotransformer, if it did, there would be a code vioaltion..
    210.9 Circuits Derived from Autotransformers
    Branch circuits shall not be derived from autotransformers unless the circuit supplied has a grounded conductor that is electrically connected to a grounded conductor of the system supplying the autotransformer.
    Don
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

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