Question

What distinguishes an Autotransformer (Buck-Boost) connection from a standard distribution transformer connection?

Answer 1

The distinguishes an autotransformer connection from a standard distribution transformer connection are following:-

  

A ‘normal transformer’ has two (or more) sets of coils that have no connection to each other other than that they are wound around the same core. They can be step up, increasing the voltage of the a/c supply, step down, reducing the supply voltage or isolating, maintain the supply voltage but electrically separated from the supply. In all cases the volt amps (VA) is about equal in the incoming and the outgoing side. About equal due to the excitation current adding a bit of current to the supply side. The VA rating of the transformer can not be exceeded without putting the transformer at risk of damage.

An ‘auto-transformer’ also has two coils but one of the supply wires is connected to one side of both coils. They can be step up or step down but as they share a connection they can not be used for isolation. Due to the way the connections are made it is normal that the output can have a larger VA than the rating of the transformer. It is not uncommon to have a variable auto-transformer such as a ‘Variac’ (trade name) that can adjust the incoming voltage from near zero volts to often above the supply voltage.