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Transformer riddle No.45 – Transformer Magnetizing Current

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  • #533

      Which side of a loaded transformer contains the magnetizing current?
      Which side of a loaded transformer contains the magnetizing current?
      1- One side which connected to power supply (primary)?
      2- One side which connected to load (secondary)?
      3- What about window type measuring current transformers (current transformer with one turn winding in primary side)?
      4-What about transmission power transformer in power network which the direction of power flow is variable in it , dose the magnetizing current flow in one side of transformer? which side?

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    • #2018
      amir khashayar

        1)In power transformer it is absorbed from primary winding . 2)In current transformer it is absorbed from secondary winding . 3)In principle , magnetizing currenr is presented in one winding only . It causes magnetizing flux flows in core and leads to the phenomenon of coupling between two windings . 4)In power transformers to be energized from both sides , the magnetizing current is presented in both windings.


          Why? Please more explain 1-Why?, is it dependent to load power factor ( leading or lagging)? 2- Why?, is it dependent to load power factor ( leading or lagging)? 3- N1.I1+N2.I2 = (N1 or N2). Imag >0 , N1 or N2 which one? 4- Is the direction of active power flow important or reactive power flow?

          amir khashayar

            In power transformers installed in power network , in any case one side is considered generation side , and the other side despite connecting with an active element is considered load . Now , if due to conditions of network the generation winding is going to be changed into consuming winding , for a very short time the magnetizing current is absorbed from both sides. Little by little , the portion of one side approaches to zero and all magnetizing current is absorbed from generation side.


              Why is the direction of active power flow important? Why the direction of reactive power flow is not important in this regard? As you know the direction of reactive and active power flow can be opposite simultaneously in interconnection media (power transformer) of a power system.

              amir khashayar

                No load current of transformer is composed of two perpendicular components . Iw which covers iron loss and Im which is in fact magnetizing current . If as a simplifying assumption we dispense from Iw . then Im is absorbed from reactive generating winding


                  If can to assume your theory is correct, please repeat your answer to initial question.


                    When an alternating voltage is applied to one of transformer windings, generally by definition the primary, a current will flow which sets up an alternating m.m.f. and hence an alternating flux in the core. This alternating flux in linking both windings induces an e.m.f. in each of them. In the primary winding this is the ‘back-e.m.f’ and, if the transformer were perfect, it would oppose the primary applied voltage to the extent that no current would flow. In reality, the current which flows is the transformer magnetizing current. In the secondary winding the induced e.m.f. is the secondary open-circuit voltage. If a load is connected to the secondary winding which permits the flow of secondary current, then this current creates a demagnetizing m.m.f. thus destroying the balance between primary applied voltage and back-e.m.f. To restore the balance an increased primary current must be drawn from the supply to provide an exactly equivalent m.m.f. so that equilibrium is once more established when this additional primary current creates ampere-turns balance with those of the secondary. Since there is no difference between the voltage induced in a single turn whether it is part of either the primary or the secondary winding, then the total voltage induced in each of the windings by the common flux must be proportional to the number of turns. We should note that the exciting current may not be restricted to the primary winding when load current flows but may be shared by both windings and other closed electrical circuit (even transformer metallic case) which influence in linkage magnetic fluxes. Because the nature of circulating magnetizing currents and related magnetic flux are unique in physical view and we can’t consider existing of each without another, similar a body and its shade. Even in single coil we can suppose a lot of magnetizing current circuit in parallel which total current flow in it and all of them influence in produced magnetizing flux. Indeed This fact can lead to legitimate circuit models with shunt branches on either side of transformers related to importance of its technical applications. These are all mathematically equivalent since impedances can be transferred across the ideal transformer present in the circuit. We usually demonstrated this for series impedance for importance of it in transformer behavior study. What is your opinion?

                    R K Mohapatra

                      By and large I agree with Mr Hamid . Only difference is when the Primary side is long and unprortected for capacitance effect of the T/L , the power factor of the mag current will change . Also , the closed metallic case ( tank ) offers a Zero sequence path , so the behaviour will be different than what can be mathemartically deduced.

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