Step up transformer : Comparing ratio of currents to ratio of inverse of turns

Thread Starter


Joined Mar 23, 2009
I have attached the diagram of the autotransformer and also labelled the currents so that you guys can better understand my question. The question is why is the ratio of the primary and secondary winding currents slightly different to the ratio of the inverse of turns when ideally they should be equal.

Attempt to solution :

The ratio of the current in the primary and secondary winding is not slightly different that the ratio of the inverse of turns because of the magnetizing current. Even after the voltage at the primary winding is brought to zero, there will still be some current in the secondary core because it will still be magnetized.


Last edited:


Joined Mar 6, 2009
Yes there will always be a magnetizing current component in the lower (primary connected) half of the winding of the auto-transformer

You can do a careful analysis using the winding self and mutual inductance components which would make this quite apparent.