Understanding a Transformer

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Understanding a Transformer Powered By Docstoc
					      EE 306
 TRANSFORMERS

     Instructor:
Dr. Mahamed Shwehdi

     Done By:
Ahmad Saleh Al-Owied
                       1
Transformers




     Very large three-phase network transformer.

                                                   2
Transformer
     electromagnetic device (mutual induction)
1. components:
   -- 2 adjacent coils
   -- coils not physically connected to each other




                                                     3
Transformer (cont.)
  2. operation:
    a) AC in primary coil produces D B
    b) Secondary coil receives a D in I & V
                          S
                          N




                          S
                          N

                                              4
Transformers

      Wye / Wye connection

                IA                        Ia
      A                                           a
          VAB            VAN              Van     Vab

      B     VBN      N                n     Vcn   b
                          VCN   Vbn



      C                                           c




                                                        5
  Transformers
                   Transformer                                                        Three-phase network supplies a
    Supply                                                                            load through a transformer.
                                            Load
    Network
                        Y-Y

  Vnet_AN                   VAN                   Van                Vload_an
            Xnet                                             Xtr_s
                   A
                                             Ia          a
            IA
                                        n
                                                                                       Equivalent circuit of the wye-wye
                                    N




  Supply Network                  Transformer                         Load

                       Xnet                             Xtr_s                         Single-phase equivalent circuit for phase
                                                                                      A of the three-phase wye-wye
                       IA                                 Ia                          transformer system
Vnet_AN                       VAN                 Van                      Vload_an




                                                                                                                                  6
      Wye / Delta connection

                 IA                                   Ia
       A                                                    a
                                        a   Iac
           VAB        VAN                                   Vab
                                                      Ib
                                  Iba             c
VCA    B         VBN        VCN                             b     Vca
                        N                    Icb
                                        b                   Vbc
           VBC                                         Ic
       C                                                    c




                                                                        7
Transformers
     VAN
                               Transformer
                    A IA                 Iba                   a    Ia

      AC
                              VAN            Vab                          Iab
     VBN           VAB                                             Vab
             VCA         IB              Icb             Vca        Ib
                    B                                          b
                                                                          Ica
                                                   Iac
      AC
                              VBN            Vbc                          Ibc
     VCN           VBC                                             Vbc

                    C    IC              Iac                   c    Ic

      AC                                                                 Load
                              VCN            Vca
    Supply          N




                   Wye-delta connected transformer


                                                                                8
Transformers

         Rtr_p        Xtr_p                      Xtr_s        Rtr_s
                                           spe
                                          T

                 IA                                      Ia
                          Rc   Xm   VAN          Van             Vs_an
      Vp_AN


                                          Y-D



   Single-phase equivalent circuit of a wye-delta transformer.




                                                                         9
          Delta / Wye connection

            IA                                 Ia
      A                            Vbn
                                                    a
                 IAB
      VAB                                           Vab
            IB                     n     Van   Ib
VCA   B                ICA   Vcn                    b     Vca
      VBC                                      Ic   Vbc
                 IBC
            IC                                      c
      C




                                                                10
    Delta / Delta connection
       IA                      Ia

A                                   a
       IAB         Iba
VAB                                 Vab
B            ICA         Iac
                                    b

                                    c
C



                                          11
    Transformers
              Three-phase Transformer Relations

Transformer                 Primary and Secondary Voltage Phase                             Windings Placed on the Same
                                                                           Phase Shift*
      Connection                  Relation                                                        Leg


Wye-Wye                     VAN is in phase with Van                           Zero                     A and a


Wye-Delta                   VAN is in phase with Vab                            30°                     A and ab


Delta-Wye                   VAB is in phase with Van                           –30°                     AB and a


Delta-Delta                 VAB is in phase with Vab                           Zero                    AB and ab


* Note:      Phase shift lead between line-to-line voltages, VAB and Vab, and line-to-neutral voltages, VAN and Van, and line
       currents, IA and Ia.




                                                                                                                                12
Transformer Turns Ratio
   ratio of the # of turns in the             NS
    secondary coil to the # of turns in   TR 
                                               NP
    the primary coil
   numerical factor indicating relative change in
    PD across a transformer
       TR > 1 = V (step up)
       TR < 1 = V (step down)




                                                     13
Transformer Law
    (change in voltage in a transformer)
     D in V is directly proportional to the ratio of the
     # of turns in the secondary to the # of turns in
     the primary
    Formula:
                  N S VS               NS secondary coil turns
                     =                 NP primary coil turns
                  NP VP                VS secondary voltage
                                       VP primary voltage

                 direct relationship




                                                                 14
Transformer Current
   Current in a transformer will be inversely
    proportional to the TR of the transformer
   formula: N S IP             IP primary coil current
                   =            IS secondary coil current
                NP IS
   I vs. V = inverse relationship
                VS IP
                   =
                VP IS



                                                            15
Types of Transformers
   Based on TURNS RATIO
    1. step-up
       turns ratio > 1
       VS > VP
       IS < IP


    2. step-down
        turns ratio < 1
        VS < VP
        IS > IP



                           16
Transformer Types (cont.)
Based on STRUCTURE OF COILS
1. air core
   -- inside area of coil is
        empty
   -- inefficient induction
2. open core
   -- ferromagnetic
      material inside coils
   -- B = induction



                               17
Transformer Types (cont.)
3. closed core
   -- ferromagnetic ends
      connected
   -- induction
4. shell
   -- lazy square eight
   -- overlap of coils on
       same ferromagnetic
       core

                            18

				
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