Types of Autotransfomer (Step Down Autotransformer and Step Up Autotransformer)

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Types of Autotransfomer
Q. What are the two different types of Autotransformer?

      Type No.1 : Step down Autotransformer
      Type No.2 : Step up Autotransformer

  • Generally stepdown type is universally used to get a variable voltage from a supply (primary)

Relations of Primary / Secondary Voltage, Currents and Transformation Rations of Step down and Step up Autotransformer
(a) Step-down type autotransformer
Q. Write down the relation of primary and secondary currents, voltage and also transformation ratio K.

  • Primary turns N1 (Between A and B), secondary turns N2 ( Between C and B ). Primary voltage V1, secondary voltage V2, primary current I1, secondary current I2, portion CB of winding acts as secondary.
  • As seen from the diagram N2
    < N    1 hence V2 < V1.

          V2/V1 =
N2/N1      ∴ 
V2=V1 x  (N2/N1)   which is less than V1

  • So in step down autotransformer N2 is less hence secondary voltage is less. We get variable voltage by moving movable brush by handle. Variable voltage is from zero to V1 on secondary side.

(b) Step up type autotransformer
Q. Write down current/voltage relation.

  • Part CB acts as primary across CB the voltage is V1. full winding AB as secondary to which load is connected. This has N2 turns. N2 > N1
  • In this case N2 
    >  N1      ∴  V2  >  V1

    ∴  V2 
    =  V1 x (N2
    / N1)

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