A variable autotransformer is a single-coil transformer in which two portions of the same coil are used as the primary and the secondary. Variable autotransformers are used for the same purposes as regular transformers, but handle lower voltages.n the autotransformer, a primary voltage is connected from the bottom to a specific winding known as a âtap,â and a secondary voltage is connected from the bottom to a separate winding.