auto transformer consists of only one winding .single
winding is used for both primary and secondary windings.in
this transformer windings are not electrically isolated and
in like ordinary transformer windings are electrically
isolated with each other.
due to single winding cost of this transformer is also less
and voltage regulation is also superior compared to other
normally these auto transformers is used as variac that is
for voltage variation purposes.
For example if input voltage is 220v outpu can varied in
steps of 10v 20v 30v 40v etc...........
Auto Transformer is otherwise called variable voltage
transformer. The primary voltage is constant and the
secondary voltage is variable. Its is mostly used in AC and
DC motors for laboratrical purpose
An Ordinary Transformer consists of two windings called
primary winding and secondary winding.These two windings
are magnetically coupled and electrically isolated. But the
transformer in which a part of windings is common to both
primary and secondary is called Auto Transformer. These two
windings are not only magnetically coupled but also
electrically coupled.The input to the transformer is
constant but the output can be varied by varying the
An autotransformer is an electrical transformer with only
one winding. The winding has at least three electrical
connection points called taps. The voltage source and the
load are each connected to two taps. One tap at the end of
the winding is a common connection to both circuits (source
and load). Each tap corresponds to a different source or
load voltage. In an autotransformer a portion of the same
winding acts as part of both the primary and secondary
The autotransformer has only one winding. In this special
kind of transformer a portion of the winding is common to
both the primary and secondary one.
The principle of operation of the autotransformer is the
same as the one of the common transformer, and then the
relation between input and output voltages and input and
output currents and the ratio of number of turns between
the primary and the secondary winding is the same.
The currents of the primary and secondary windings are
flowing on the opposite directions, so the total current
flowing through the common part of the winding is equal to
the difference between the current on the low-voltage
winding and the current on the high-voltage winding.
In order for an autotransformer to work properly, the two
windings should have the same winding sense.