STEPUP AND STEPDOWN VOLTAGE AUTO TRANSFORMER

In an auto transformer the primary and secondary of the winding is linked together both electrically and magnetically thus reducing the cost over conventional transformer
Unlike Isolation transformer which has two electrically isolated windings called Primary and Secondary an Auto transformer has only one single voltage winding which is common in both sides
Then the auto transformer has common magnetic core but has only one winding which is common to both primary and secondary circuits
The main advantage of this type of transformer design is that it can be made a lot cheaper than Isolation transformer for the same KVA rating but the biggest disadvantage of this transformer is that it does not have primary and secondary isolation and have a common neutral
An auto transformer is used mainly for the adjustments of line voltages to change its value
They are more efficient than the isolation transformer for the same KVA rating and are smaller in size as they require less copper in their construction and their loss is less compared to double wound transformer of the same KVA rating

Used as a starter to give up 50-60% of full voltage to the stator of squirrel cage induction motor during starting
It can be used as a small boost to a distribution cable to correct the voltage drop
Can be used as a voltage regulator
Electrical apparatus testing labs
For interconnecting system that are operating at threshold voltage.