Home Electrical Engineering Forum General Discussion what is % impedance of transformer?

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    what is % impedance of transformer? how we calculate %impedence of transformer,

    what is purpose of % impedence?



    %impedence=impedence voltage/rated voltage  *100

    %Z=IZ/Vr *100



    The normal method
    of  expressing  transformer  impedance  is  as  a  percentage  voltage  drop  in  the
    transformer at full-load current and this reflects the way in which it is seen by
    system designers. For example, an impedance of 10% means that the voltage
    drop at full-load current is 10% of the open-circuit voltage, or, alternatively,
    neglecting any other impedance in the system, at 10 times full-load current, the
    voltage drop in the transformer is equal to the total system voltage.

    Formula written by MH is perfect.

    Vz %Z


    Typical transformer impedances are around %6. Have you tried performing a short circuit test?



    thanx guys


    Simon Foster

    Another way of looking at this, and the way it's measured during factory acceptance testing is:


    Short circuit is placed on the LV connections, then simply apply a voltage to the HV connections to create Full Load current in the LV winding.

    The percentage of HV voltage required to do this is also the percentage impedance.




    600V required on HV to circulate Full Load current in the LV.


    If HV rated Voltage is 12000V


    The %Z is 5%  or 600/12000


    Hope this helps





    It is voltage drop in % of nominal voltage


    jatin333 said:

    what is % impedance of transformer? how we calculate %impedence of transformer,

    what is purpose of % impedence?

    Have you ever wondered why the name plate LV Volatge in a transformer is 433V but the name plate voltage of an LV motor is 415V?  Are they both not LV? Yes! the 433 V in a trf. name plate is the open circuit voltage or the no load voltage. But, when you load the transformer, a current would flow thro its windings. As the windings will have an impedance, this current and impedance would very definitely cause a voltage drop within the transformer windings. On emust know the magnitude of this voltage drop. The volatge drop within the transformer's LV windings, with rated load current flowing thro it, expressed as a percentage of the no load voltage of the teansformer is called its percentage impedance. 


    jatin333 said:

    what is % impedance of transformer? how we calculate %impedence of transformer,

    what is purpose of % impedence?

    Dear Mr. Jatin ;


    In some case called the impedance of transformer that is given in ” % “, but the sientific name is ” Usc  or Ucc ” that means the percentage of primary voltage ( HV ) that should be applied to have the nimonal current ” In ” on short circuited secondary.

    Normally, each transformer hase a Resistance ” Rtr ” and an Inductance ” Xtr “, where we can calculate by the following formula :

         – Ztr = Usc x U²/Sn

         – Rtr = Ptr / 3 x In²

         – Xtr² = Ztr² – Rtr²

    Noting that :

         – Ztr : the impedance of transformer

         – Usc : the voltage in percentage of transformer

         – U : the voltage between 2 phases

         – In : the nominal current for the transformer's secondary

         – Ptr : the total loss at full load


    As desiegner, we should calculate the ” Rtr  &  Xtr ” to the transformer ten add themm to the values ” R  &  X ” of cables to calculate the different values of short circuit currents.



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