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Thread: I need help with a current transformer diagram

  1. #1
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    Question I need help with a current transformer diagram

    I hope someone can help me out here. :smile:

    What do the black dots (red circle) represent on this diagram?

    Would a CT manufacturer call that black dot 'X1'?



    Would it be a problem if all three CTs where wired the opposite way as shown?

    Thanks in advance, Bob

  2. #2
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    Here are the CTs being used.

    H681x-V Install

  3. #3
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    The black dot is showing polarity, CT's have to be installed the right way physically or the ratios will be off. Not the same as X1, the dot is not a connection point, think of it as an arrow showing diretion of current flow.

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    I wonder if it is indicating source side of the CT?
    Just a guess..

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    Quote Originally Posted by zog View Post
    The black dot is showing polarity, CT's have to be installed the right way physically or the ratios will be off.
    OK, they are physically installed in the correct direction.

    That much I know, at least the labels are facing the right direction.

    Not the same as X1, the dot is not a connection point,
    So what is the significance of the CT company providing a white and black with a label indicating "White is X1" and where would X1 have to land on that meter diagram? :confused:

  6. #6
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    If the CT is being used to feed a single current relay or ammeter, the polarity dots are not needed. However, if the polarity dots CT's feeding a 3-phase device are not consistent then two of the CT's will be connected in an additive configuration and one in a subtractive one. This mismatch of will cause the subtractive mode CT to be 180° out of phase which means any device, like Power Factor, that depends on phase angles will be very confused. The next problem is if all three polarity dots face the same way but in a different direction than the VT's. Again this would cause an unexpected phase shift, a typical result is the KW readings being negative.
    Just because you can, doesn't mean you should.

  7. #7
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    Yes KW readings are messed up.

    This job has about 20 meters and not all are working properly, also some meters are running two sets of CTs into one set of inputs to watch the combined total of two different feeders.

    How is power factor determined in that case? Average?

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    Quote Originally Posted by iwire View Post
    So what is the significance of the CT company providing a white and black with a label indicating "White is X1" and where would X1 have to land on that meter diagram? :confused:
    The label should indicate the line and load side of the CT or there will be a painted dot on the line side of the CT opening. When current flows from the line to load side then the secondary current will flow from the white to the black wires. In your diagram the white wire is the one with the dot beside it.

  9. #9
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    Just for educational purposes, this is out of my "Electric Machinery Fundamentals" textbook:

    1. If the primary voltage is positive at the dotted end of the winding with respect to the undotted end, then the secondary voltage will be positive at the dotted end also. Voltage polariites are the same with respect to the dots on each side of the core.

    2. If the primary current of the transformer flows into the dotted end of the primary winding, the secondary current will flow out of the dotted end of the secondary winding.
    From another section of the same textbook:

    ...the dots help determine the polarity of the voltages and currents in the core without having physically to examine its windings. The physical significance of the dot convention is that a current flowing into the dotted end of a winding produces a positive magnetomotive force, while a current flowing into the undotted end of a winding produces a negative magnetomotive force. Therefore, two currents flowing into the dotted ends of their respective windings produce magnetomotive forces that add. If one current flows into a dotted end of a winding and one flows out of a dotted end, then the magnetomotive forces will subtract from each other.
    Now, a lot of that doesn't apply here since there is only one black dot on each transformer. Looks to me like they just want to make sure you get all three CTs wired with the same polarity so they are additive. Terminals I11, I21, and I31 should be connected to the line side of each phase, respectively.

    Quote Originally Posted by iwire
    How is power factor determined in that case? Average?
    As you know, the power factor is a ratio of real power to apparent power. If you were combining two circuits into one set of readings, the power factor will be the the real/apparent ratio for the combination of the two feeders. It would be a completely different value than the power factor for either of the two feeders themselves.

    As long as it took me to type all of that, someone else has probably already responded with a much more informative and helpful post in the meantime...(edit: I was wrong!)

  10. #10
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    Quote Originally Posted by iwire View Post
    I hope someone can help me out here. :smile:

    What do the black dots (red circle) represent on this diagram?

    Would a CT manufacturer call that black dot 'X1'?



    Would it be a problem if all three CTs where wired the opposite way as shown?

    Thanks in advance, Bob
    Depends on the device. A watthourmeter would indicate reverse flow.(If it is capable to do so, many are not.)

    I presume that you mean that the marked terminals would face the opposite direction, but all the wiring would remain the same and not that the unmarked leg of the A phase CT would be wired to I11.

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