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Curent transformer spice model?

Discussion in 'Electronic Design' started by Clarence_A, Feb 5, 2005.

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  1. Clarence_A

    Clarence_A Guest

    Does anyone have a web reference to a spice model of a current
    transformer or a site where creating such a model is discussed?

    Also, I will eventually need a Saber model of the same part.


    Thanks to the serious engineers on this list!
     
  2. Clarence_A wrote...
    Current transformers have the same linear spice model as other
    kinds of common transformers, with DC winding resistances, the
    turns ratio, leakage inductance, magnetizing inductance, etc.

    .. ---Rp---, ,---+--Rs--###----
    .. | | | Lell
    .. #||# #
    .. #||# # Lm
    .. turns #||# #
    .. ratio | | |
    .. -------' '---+----------

    Not all of these parameters play a significant role, e.g. the
    magnetizing-inductance component may not be significant, and
    it's not a linear parameter anyway. You can assume Rp = 0.
    A wideband transformer (>10MHz) may also suffer from winding
    capacitance and ac copper losses, but you can ignore them.

    Generally the primary is a single turn, from the wire being
    measured going through the core. So the turns ratio N is
    the number of secondary turns. You can measure or calculate
    Lm = A_L N^2 for the secondary inductance in the spice model
    and divide that by N^2 for the primary inductance, thereby
    setting up the turns ratio for spice (isn't that awkward?).

    The leakage inductance Lell is important; it's easily measured
    by shorting the transformer, showing as k = sqr[Lm/(Lm+Lell)]
    in the classic spice model, but I prefer to leave k = 1 and
    add external inductances for Lm and Lell, so their presence
    and effect is more clear in the drawing.

    When you use a current transformer the load resistance is a
    critical item. It's transformed down by N^2 to an effective
    series resistance to the circuit being monitored, so you want
    to use a relatively low load resistance. The low frequency
    limit occurs when Lm shorts out the load, the high frequency
    limit when the Lell reactance equals the load.
     
  3. Clarence_A

    Clarence_A Guest

    "Winfield Hill" wrote

    Thanks Win.
    I was using a five winding transformer model, the Current
    transformer is a three phase unit, with a test winding and the
    sense winding. But I didn't trust the results since I have never
    seen any reference to what the model of a current transformer
    would be. Again, Thanks.
     
  4. Joerg

    Joerg Guest

    Hello Winfield,
    That cannot be emphasized often enough. Woe to those who allow that load
    resistance to fail. That can result in a serious kaboom situation
    especially when large RF currents are measured. Once I blew a coax when
    the load resistor failed. The toroid was gone, too. From then on I
    always used two load resistors so at least you have a chance to detect a
    mysterious jump in the readout and shut things down.

    Regards, Joerg
     
  5. I use MacFadyen's model for CT's, which did seem
    to give predictable results for a series of
    measurements on 1000:1 NiFe CT's, at 0-400Arms,
    over the frequency range 400-700Hz. This was
    a comparison of low-level measurements (and
    predictions) against the actual CT performance
    using a precision AC current source.

    2
    Rp (1-k )Lp Rs
    o---/\/\--))))))))--+ +---+-----+--/\/\---o
    | | | | |
    )||( ) \ \
    Np)||(Ns )Ls /Rc /Zl
    )||( ) \ \
    | | | | |
    o-------------------+ +---+-----+---------o
    2
    Inductance Ratio= k.(Lp/Ls)

    Shifting the leakage inductance over to the primary
    side puts it in series with the (constant current)
    source and allows it to be ignored.

    MacF only had an Ls, which could be assumed to be complex
    to allow for (inphase) iron losses, but I found it easier
    to add a separate Rc for core loss.

    In fact, (at 400-700Hz, on those 0.004" NiFe cores),
    iron loss was the largest single factor in the departure
    of current ratio from turns ratio. At 10Vrms across Rc,
    each 8mW of iron loss was 0.8mA of shunt current, which
    was 0.2% of Iout, or equivalent to a 2-turn error.

    ...... and the out_of_spec troubles on the CT's were
    because the core mfr was supplying (expensive!) cores
    with iron losses that could range from 3mW to 15mW.

    I did a similar exercise with 15KHz/30mArms CT's, on
    ferrite cores. MacF's model worked quite well there
    also, although there were some common-mode (capacitive)
    effects (reduced by using screened cable for the 2-turn
    primary).
     
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