Auto Transformer

Auto Transformer :-- A transformer in which a part of the winding is common for both primary and secondary circuits, is called an auto transformer .Where as in two winding transformer both windings are electrically isolated but magnetically coupled .

Auto Transformer

Two winding transformer

                               At  first sight an auto transformer appears to similar as resistance potential divider but this is not so because a potential divider can't step up the voltage but it does.In potential divider entire power to load flows by conduction but in case of auto transformer a part of power is conducted and the rest of power is transferred by transformer action. In potential divider input current must be always greater than the load current, this is not so in auto transformer.

in Auto transformer

                 input power = output power

                V1I1COS01 = V2I2COS02

If internal impedance drops and losses are neglected than

                         V1I1 = V2I2

     

     V2/v1 = I1/I2 = N2/N1  = K

  In  auto transformer  action , neglecting internal impedance drops and losees 

Transformed power / input power = 1-k

conducted power / input power = k

i.e , inductively transferred power /total power = (High voltage - Low voltage)/ High voltage

Advantage :--

Since weight of conductor in a winding is proportional to the current and number of turnes in the winding. And in case of auto transformer no of tunes in the winding is less compared to two winding transformer hence less weight compared to two winding transformer for same amount of output.

 i.e

• Weight of conductor in auto transformer = (1-k) weight of conductor in two winding transformer

 and 

• Saving in conductor material in auto transformer  = k * conductor weight in 2- winding                                                                                          transformer  

• Hence the use of auto transformer is more economical only when the voltage ratio k is more nearer to unity.

• The nearer the value of k is to unity the greater is the saving in materials.
• An auto transformer has higher efficiency than a two winding transformer of the same amount of output.
• An auto transformer has less value of leakage impedance and has superior voltage regulation than a 2-winding transformer of same output.

 Disadvantage :--
 The short circuit current in an auto transformer is higher than 2-winding transformer.
Uses:--

1. For interconnecting power systems having voltage ratios,not differing far from unity .
2. For obtaining variable output voltage.When used as variable auto transformer e.g-variac,autostat,dimmerstat etc