Auto transformer is a transformer with only one winding. Its theory and operation are similar to the two-winding transformer. In an ordinary transformer both windings primary and secondary are connected magnetically but electrically insulated from each other but in auto transformer both primary and secondary windings are connected magnetically and also connect electrically.
Figure of auto transformer and two-winding transformer are shown below.
As shown in auto transformer figure AB is primary winding having N₁ turns and BC is secondary winding having N₂ turns. Neglecting iron losses and no-load current.
Copper Saving In Auto-Transformer
The weight of the copper is propotional to the length and area of a cross section of the conductor.
weight of T/F ∝ area of conductor ∝ I
∝ length of conductor ∝ N
weigth of T/F ∝ NI
weight of auto transformer (wa) ∝ AB + BC
therefore,
weight of two-winding transformer (wo) ∝ N₁I₁ + N₂I₂
Now, the ratio of the weight of copper in auto-transformer (wa) to the weight of two-winding transformer (also called ordinary transformer) (wo) is given as.
Saving of copper = K x weight of two winding transformer
So now it is clear that, the copper saving in auto transformer is depends on the value of K. So the auto transformer is used when the value of K is nearly equal to the unity.
Advantage Of Auto-Transformer
- Use as a Regulating transformer
- High efficiency
- Use as a booster.
- Auto transformer is used as a starter to start an induction motor.
- Low losses it is cheaper than ordinary transformer.
- Used in power transmission and distribution transformer.
- Used in railways transmission lines.
Use only in limited place where a slight variation of ther output voltage from input voltage is required.
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