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  1. #1
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    how current transformers work

    hi all. i hope my question is not out-of-topic ya.. it is more to electrical..

    well, i studied about Current Transformer, which it use almost same theory with Voltage Transformer. (i am referring to bar type Current Transformer)

    Primary winding is HV side, which is only 1 winding
    Secondary winding is output to metering devices, which i assume it have 300 windings.

    the formula is
    (AMP)p (TURNS)p = (AMP)s (TURNS)s

    i have searched the internet, but most not mentioned about secondary current proportional to primary side only. voltage is produced to drive the current only.

    what i confuse here is,
    isn't that the output should be voltage? the flux generated by primary winding, will induced voltage at the secondary winding. then the voltage (induced at secondary) will depend on the impedance of the external circuit (metering device), to determine how much current flow from CT to Meter?

    yup, i am confuse, but i dont know where is my mistake. hope expert here could point out where is my mis-concept, mistake?

    thanks a lot.

    •   Alt29th April 2009, 08:39



  2. #2
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    how do current transformers work

    voltage is produced to drive the current only
    Yes, exactly.
    isn't that the output should be voltage?
    No, you don't need a voltage, respectively not more than to drive the meter. Ideally, the meter is acting as a short and you have no voltage drop at the CT.

    I suggest a different view. The transformer has a main inductance respectively a reactance (transformed with n˛). The reactance viewn at the secondary is connected in parallel to the meter. For a minimal error, the reactance must be large compared to the meter impedance. This is the linear model of CT operation. For the nonlinear model, you have to assure, that the flux from the CT voltage drop is below the core's saturation flux. This condition also sets a limit to the meter impedance. For technical CT, a power value specifies the maximum voltage drop respectively meter impedance.

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